Additive Manufacturing – Integration of Functions in EMI-Shields

Royal Institute of Technology

Master Thesis

Additive Manufacturing – Integration of Functions in EMI-shields

Author: Author:

André JOHANSSON KUCERA Alexandra LIDHOLM

Course: Course:

MG212X, Degree Project in Production SE202X, Degree Project Engineering and Management in Solid Mechanics

Supervisor KTH: Supervisor KTH:

Amir RASHID, Bo ALFREDSSON, professor in Industrial professor in Solid Production Mechanics

Examiner KTH: Examiner KTH:

Amir RASHID Bo ALFREDSSON

Supervisor Saab AB:

Mussie GEBRETNSAE

June 2019

Abstract Additive manufacturing enables a simplified production process of components with complex geometry based on computer aided three-dimensional design. The technology of creating components layer-by-layer allows an efficient process with the ability to design parts with specific properties which can be difficult to obtain when conventional manufacturing methods are used. In this master thesis, an EMI shield was analyzed where the choice of manufacturing process was of interest. Producing the shield with additive manufacturing, instead of conventional methods, and how to integrate different materials in the process were investigated. The possibility to produce the shield and its components in the same process would result in a shorter production process with less process steps and would be an effective approach for future applications. In the current EMI shield, each component has a specific function with high demands in terms of temperature resistance, weight and EMC. These requirements must be taken into account when choosing manufacturing method and suitable materials in order to obtain desired characteristics of the shield.

In the analysis of creating an electromagnetic interference (EMI) shield with multi-materials, a comprehensive literature study was conducted where different AM methods and available materials were investigated. Based on the literature research, possible concepts were generated and 3 different concepts were suggested for the final solution. A Finite Element Method software was used to verify these concepts in terms of solid mechanics, where the final design of the shield was determined based on the choice of materials in addition to optimization of the geometry.

To evaluate the function and electromagnetic compatibility of the final concepts, prototypes were manufactured and tested in an experimental setup. These results were compared to the results of the original shield in order to determine whether the concepts met the given requirements or not. Concept E showed similar EMI results as the current shielding solution, whereas concept C and D resulted in a decrease of shielding effectiveness.

Keywords: EMI-shield, Additive Manufacturing, composites, multi-material, FEM, product development, gasket, absorber.

Sammanfattning Additiv tillverkning möjliggör en förenklad produktionsprocess av komponenter med komplexa geometrier utifrån en tredimensionell design. Att skapa komponenter lager för lager möjliggör en effektiv tillverkningsprocess av komponenter som skulle varit svåra att framställa med konventionalla tillverkningsprocesser. I detta arbete har ett EMI-lock analyserats där valet av tillverkningsmetod varit av intresse. Möjligheten att framställa produkten med additiv tillverkning istället för att använda konventionella processer samt hur man kan integrera olika material i tillverkningen har undersökts. Möjligheten att tillverka locket och dess komponenter i en och samma process skulle resultera i en kortare produktionsprocess med färre processteg, vilket skulle vara ett effektivt tillvägagångssätt i tillverkningen av framtida applikationer. I nuvarande EMI-lock har varje komponent en betydelsefull funktion och höga krav i form av temperatur, vikt och EMC ställs på produkten. Dessa krav måste beaktas vid val av möjlig tillverkningsmetod och lämpliga material för att erhålla önskade egenskaper hos slutprodukten.

För att utvärdera möjligheten att tillverka ett EMI-lock med flera material har en omfattande litteraturstudie genomförts där olika tillverkningsmetoder och tillgängliga material undersökts. Baserat på litteraturstudien har generering av potentiella koncept genomförts där 3 koncept har föreslagits för den slutliga lösningen. För att verifiera dessa koncept har FEM-program används för att utvärdera hållfastheten. Baserat på dessa resultat har den slutliga designen av locket fastställts utifrån val av material och optimering av geometrin.

Funktionen hos de slutliga koncepten och dess elektromagnetiska kompatibilitet har utvärderats genom att tillverka flera prototyper och sedan testa dessa i ett experimentellt utförande. Resultaten av detta jämfördes sedan med det ursprungliga EMI-locket för att avgöra om koncepten uppfyllde de givna kraven. Koncept E visade liknande resultat som det ursprungliga EMI-locket, medan koncept C och D påvisade en minskad skärmningseffekt.

Nyckelord: EMI-lock, additiv tillverkning, komposit, multi-material, FEM, produktutveckling, packning, absorbent.

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Acknowledgement This master thesis was supported and carried out by Saab AB Surveillance in Gothenburg between January and June of 2019. The project is conducted as part of the Solid Mechanics track and Production Engineering and Management track at the Royal Institute of Technology (KTH) in Stockholm.

We would like to thank Stefan Linders and Mattias Skogsberg at Saab for guidance and support throughout the work. A special thanks to our supervisor Mussie Gebretnsae who provided us with expertise, insight and knowledge. His encouragement and dedication made it possible to accomplish this project and achieve our common goal. We also thank everyone at the Gothenburg site for the welcoming atmosphere during this time.

We are grateful to Mauricio Saldes, for giving us access to the Järfälla site, and Richard Ingman with the rest of the employees and lab technicians at the EW department in Järfälla for their help and support when it was needed.

We thank Hans Nordström for assistance of knowledge in the field of electromagnetic radiation. It was a new subject for us to learn, but his guidance and commitment made it possible for us to create an understanding of EMI which we have conveyed in this work.

We would also like to show our gratitude to our supervisors at KTH, Bo Alfredsson and Amir Rashid, for their help and dedication during this master thesis.

André Johansson Kucera Alexandra Lidholm

Stockholm, June 2019

Contents 1. Introduction...... 1 1.1 Background ...... 2 1.2 Purpose...... 3 1.3 Problem description ...... 3 1.4 Delimitations ...... 4 2. Method ...... 5 2.1 Data collection...... 5 2.2 Technology screening ...... 6 2.3 Concept development ...... 7 2.4 CAD ...... 8 2.5 FEM ...... 8 2.6 Prototype manufacturing and EMC testing ...... 9 3. EMC and EMI ...... 10 3.1 Electromagnetic radiation ...... 10 3.2 EMC and EMI ...... 10 3.3 Basic shield theory ...... 11 3.4 EMI shielding components ...... 12 3.5 EMI shield solutions on the market ...... 15 4. Additive Manufacturing ...... 17 4.1 Vat Photopolymerization ...... 17 4.2 Material jetting ...... 18 4.3 Binder jetting ...... 20 4.4 Material extrusion ...... 22 4.5 Powder bed fusion ...... 24 4.6 Sheet lamination ...... 26 4.7 Directed energy deposition ...... 27 4.8 Solid mechanics ...... 28 5. Analysis ...... 29 5.1 Analysis of concept development ...... 29 5.2 Geometrical design of components ...... 32 5.3 FEM analysis ...... 34 5.4 Material analysis ...... 36 6. Results ...... 40

6.1 FEM analyzes ...... 40 6.2 Concept C ...... 48 6.3 Concept D ...... 49 6.4 Concept E...... 50 7. Discussion ...... 52 7.1 AM technologies ...... 52 7.2 Solid Mechanics ...... 54 7.3 EMC ...... 55 8. Conclusion ...... 56

List of abbreviations

AM – Additive Manufacturing

BJ – Binder Jetting

CAD – Computer Aided Design

CNT – Carbon Nanotubes

DED – Directed Energy Deposition

EMC – Electromagnetic Compatibility

EMI – Electromagnetic Interference

FDM – Fused Deposition Modelling

FEM – Finite Element Analysis

FFF – Fused Filament Fabrication

ME – Material Extrusion

MJ – Material Jetting

MWNT – Multi-Walled Carbon Nanotubes

PBF – Powder Bed Fusion

PCB – Printed Circuit Board

RP – Rapid Prototyping

SE – Shielding Effectiveness

SL – Sheet Lamination

VP – Vat Photopolymerisation

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1. Introduction The technologies within additive manufacturing (AM) are constantly under development. The use of AM in different industries enables and facilitates construction of single products to huge quantities of components that can be individually customized. The technology build products layer-by-layer based on computer aided three-dimensional models. By using AM, a more efficient process with shorter development cycle can be obtained, which allow the products to be modified more easily. Additive manufacturing is considered less wasteful than other traditional subtractive methods and have benefits in terms of sustainability. Technical committees of American Society for Testing and Materials (ASTM) International, which is responsible for the development of AM standard, describes the technology of additive manufacturing as “AM holds the promise of creating new designs that simply can’t be made through traditional manufacturing. In sum, the vast potential for AM to change our lives is coming into full view “ (ASTM International, n.d.) .

In electronic devices, EMI or electromagnetic interference can disrupt and cause damage to surrounding equipment and systems that are used in the same application. It can result in temporary disturbance or more crucial complications like system failure or end of life. Microwave components that are functional in electronic applications must be effectively shielded from radiation emissions to avoid being disturbed or interfere with surrounding components. When designing an electronic device, it is of importance to recognize how electromagnetic radiation can cause interference in the application. The electronics can be isolated and protected from critical interference by constructing the device with shielding components. In this work, an EMI shield has been investigated which is mounted onto a printed circuit board (PCB) with a purpose to reduce the interference from the board. The current solution of the EMI shield consists of a conductive material together with gaskets and absorbers. The gasket operates as a sealing between the shield and the board to avoid leakage of emissions and to provide a conductive path. Absorbers are placed on the inside of the shield to dissipate the electromagnetic field and eliminate the radiation from the component they are placed upon. Today, these items are manufactured separately and then assembled in various process steps.

As the technologies of electronics are further developed and the size of the components are reduced, EMI shielding solutions must contribute with greater solutions of avoiding interference without compromising important characteristics such as weight, cost and mechanical properties. Therefore, alternative methods that are suitable for producing EMI shields must be identified. The possibility of creating a shield component with integrated absorbers and gasket by using additive manufacturing is the main scope of this thesis work. If AM can be used to produce EMI shields it will not only result in faster production and better control of interaction between several materials, but it would also allow a reduction of material waste which is a problem in current manufacturing methods. Since the component is used in a high demanding environment during long periods, the solution must fulfill high demands with respect to reliability and quality which must be taken into account when investigating and developing new manufacturing concepts.

1.1 Background This master thesis is carried out on the behalf of the hardware design department of Saab Surveillance which is responsible for mechanical and circuit board design, power distribution and cabling. The scope of the work is to analyze and investigate how additive manufacturing can be adapted into their production of electronic defense components.

1.1.2 Saab AB Saab AB is a global company that develops innovative, high-tech systems and solutions to increase the security for civil societies and for military defense. Depending on technologies and products, the company provides solutions with focus on different fields and their operations are divided into six business areas. Saab Surveillance is one business area that offers surveillance solutions for protection systems. This area include developing and monitoring flight, land-based and marine radar systems, air traffic control, electronic warfare and systems for signal tension and self-protection. In addition to construction work, the section is also responsible for production and issues regarding electronic components.

1.1.3 Additive manufacturing Additive manufacturing is a formalized term for what used to be called rapid prototyping (RP) or more popularly referred to as 3D printing. The technology of rapid prototyping was established in the mid-1980s as a solution for increased product development which resulted in distinct advantages such as increased product complexity, individual customization of products and reduction of waste material (Fernandez, 2017).

The basic principle of additive manufacturing is to use a three-dimensional computer-aided design (3D CAD) to create a component. It fabricates a component with a high degree of design complexity by constructing layers by layers, where each layer is a thin cross-section derived from the 3D model (Gibson, Rosen, & Stucker, 2015). This new form of technology offers benefits within the development and production of components due to significant reduction in cost and time, where it enables creation of products with complicated shapes or designs that would have been difficult to manufacture using conventional techniques. These printed components can easily be analyzed for the purpose of theoretical studies, medical procedures or other valuable processes which will benefit from rapidly producing prototypes or complex parts. Several major companies and manufacturers have recently implemented additive manufacturing as part of their production to fabricate prototypes and high-value products. Aerospace and automotive companies use AM to produce parts with less weight and reduced material waste which is usually a problem with traditional subtractive methods (Srivatsan & Sudarshan, 2016).

1.1.4 Multi-material Even though additive manufacturing makes it possible to produce products in various scales and complexity made of one material, it is not sufficient enough for certain applications since they require assembling and embedding of external components. In order to utilize the full potential of AM, the technique of creating multi-material parts must be implemented. Using several materials in one product can result in parts with different colors, appearance and mechanical properties (Fernandez, 2017). By controlling several materials in one component, characteristics like strength, corrosion resistance and environmental properties can be adapted in areas that require it the most. The materials that are currently used for production of multi-material parts are metals, polymers and ceramics. However, the multi-material technology is still under development where researchers are shifting their mindset to implement the development stage into real-world applications (Bandyopadhyay & Heer, 2018). The multi-material concept is widely used in the electronic industry where electrical conductivity between devices is the base of any circuit and is therefore of importance to control this property (Fernandez, 2017).

1.2 Purpose The purpose of this master thesis is to analyze how to integrate different materials in an EMI shield with additive manufacturing. These components have high standards with regard to reliability and quality, since these systems are usually used in demanding environments for longer periods. Investigations will be conducted to find a solution that provides the same functional requirements for an additive manufactured product as current component, which is milled out of aluminum. The current component is produced in four process steps with a complex supply chain which makes this component costly to realize. The integration of different materials in additive manufacturing is therefore desired in order to reduce the number of process steps and simplify the supply chain.

To fulfill the purpose of this work, concepts of material integration will be developed which results in characteristics that cope with the demands of the original shield. These concepts will then be realized and manufactured in order to test the function of the shield in an existing application.

1.3 Problem description The possibility to produce a component integrated with gaskets and absorbers in one process will result in a reduction of manufacturing steps. It will also provide the ability to design the product based on requirements and functionality in a more accessible approach. The research question is stated as:

How can additive manufacturing methods be used to integrate functional properties of gaskets and absorbers into an EMI shield used for a PCB?

1.4 Delimitations In this work, limitations have been made when investigating suitable manufacturing methods. The study is limited to standard additive manufacturing categories provided by ASTM F42 and methods that belong to these categories. When finding the most suitable methods for the component, the choice has been based on the availability of companies within Sweden that can use and produce parts with these processes. Analyzed materials that are considered suitable for the solution are based on potential manufacturing processes and what materials they can manufacture. The material data is taken from given material sheets provided by the companies, no material tests have been conducted in this study.

The final concepts were developed with an effort to reduce the manufacturing steps; concepts with equal amount of process steps as the current solution were not evaluated. When the prototypes were realized and manufactured, the build parameters of the machines were varied because it would be too extensive. No electromagnetic simulation was conducted on the final parts, only design rules of thumb and discussions with experts were used to justify different designs.

2. Method The timeframe of the thesis was divided in to two phases. In the first phase, data collection and concept development were conducted. In the end of that phase, the most prominent concepts were manufactured by Lasertech LSH AB and ZYYX 3D Printer. In the second phase, the EMI experiments were conducted and the results were analyzed and documented. The research methods for this thesis work were therefore both qualitative and quantitative. The initial data collection followed a qualitative approach but the EMI experiments generated hard data, hence the latter part of the project followed a quantitative approach (Blomkvist & Hallin, 2015).

2.1 Data collection The topic for this master thesis is interdisciplinary where knowledge within EMI, AM technologies, solid mechanics and design are needed to answer the research question. An extensive initial research was conducted in order to gather information and create a knowledge foundation of the mentioned disciplines above, the data collection process was divided in to three main steps in order to capture as much information as possible; literature study, interviews and market analysis. The data collection process was continuously conducted throughout the first phase of the thesis work by alternating between a divergent and convergent approach. The reason for this was to broaden the investigation and generate as many concepts as possible without converging too early (Blomkvist & Hallin, 2015). In the beginning of the second phase, the final concepts were chosen and no additional information was gathered to support new concept generation.

The initial focus of the literature study was on the theory behind EMI and how this effect could be avoided in components that are susceptible for electromagnetic radiation. This information was beneficial because it gave key factors regarding material choice and aperture size for the EMI shielding components, which later was used in the design stage. The next step in the literature study was to gather information about additive manufacturing technologies. Initially, information regarding different technologies was of interest in order to understand the possibilities and limitations of different AM methods. Technical committees of ASTM have developed 7 different standard categories associated with this developing area in order to support the application of AM for materials and manufacturing methods (ASTM International, n.d.). The literature of each study was primarily gathered using KTH primo and Scopus which are databases containing scientific articles.

Unstructured interviews were also used to collect information about AM technologies and EMI shielding. The reason for using unstructured interviews was to have an open discussion about the subject and possibly explore new ideas within the area (Blomkvist & Hallin, 2015). Novel inputs were valuable in order to generate new concepts. Additionally, a market analysis was conducted in order to understand what materials, geometries and processes are used today for production of an EMI shield. This knowledge was also of importance in order to generate possible solutions.

2.2 Technology screening AM methods are newly developed production technologies where each method has difference in performance with individual drawbacks and benefits. The selection of appropriate methods was a nontrivial task but needed in order to find the process which was most suitable for the application. Based on the 7 ASTM additive manufacturing categories mentioned before, a technology screening was conducted. This screening was done to evaluate the potential of each process and to categorize which manufacturing processes are considered most suitable for the production of an EMI shield.

Potential methods were identified where important properties of each approach were analyzed. These are considered as crucial factors to take into account when the choice of manufacturing process is implemented and are derived from the specified requirements of the component. These characteristics can be seen in Table 1. The final choice of methods was based on the best results of each analyzed property.

Table 1: Analyzed properties used in the choice of manufacturing process.

Selection criteria Explanation Printable materials What material group can be printed: ceramics, polymers and/or metals Successful material integration in research Printing several materials together have been successfully conducted at research level Printing a composite A material that is composed together but each substance/element can be distinct - carbon fiber etc. Print several materials in one setup Two or more types of materials can be integrated in the same process Print walls thinner than 0.9 mm Output measure derived from the thinnest geometry of the original component Structural components The process can manufacture a component with structural properties

2.3 Concept development The concept development process had to consider both the manufacturing process and the function of the final product. In order to generate concepts that were feasible from both perspectives, it was important to have a structured approach in the concept development process. In addition, a systematic approach would generate more concepts and the possibility of neglecting potential concepts would therefore be reduced. Based on this, the method used was the five-step method developed by Ulrich et.al and the schematics of that process can be seen in Figure 1 (Ulrich & Eppinger, 2012).

Figure 1: Schematics of the concept development process.

In the first step, the function of the EMI shield was divided in to sub functions. This was done so that the purpose of every component was clear and these components could then be studied in isolation. In this research, the whole system consisted of three components; shield, gasket and absorber. The sub-functions were studied from two different perspectives, internally and externally. The external search was the one described under data collection. In this step, market analysis, interviews and literature were used in order to gather existing information about EMI solutions and AM methods used for this purpose. Several brainstorming seminars were also conducted as a part of the internal search. During these sessions, different ideas were treated as potential solutions in order to cover as many possibilities as possible, even those that seemed impossible at the time. With both novel concept ideas and existing solutions for each subsystem, new system solutions were explored. The most promising concepts were then further investigated in terms of production feasibility, mechanical strength and number of process steps required to finalize the product. The concept generation process described above is illustrated as a sequential process, in practice an iterative approach was used but all five steps was still conducted.

2.4 CAD The CAD software that was used to model the shield and gasket was Creo Parametric 3.0®, a multi- platform for three-dimensional interactive design that creates and simulates 3D-parts with a parametric feature-based approach.

For the concept were the gasket and shield was manufactured in one setup, gasket geometries was designed as an integrated part of the wall of the shield. These gaskets were modelled with sufficiently small dimensions in order to obtain a functional purpose in the shield while still having an approved geometry to be produced with additive manufacturing. The design was generated in regards to the functionality of a gasket with incentive from traditional gaskets made of solid materials. Inputs from mechanical designers at Saab AB Surveillance were also used for the idea generation to design appropriate models. The gaskets and the final shield were designed and modelled and then exported to a .step file format which was used in the FEM software for the simulations. In order to compare these products appropriately, they were modeled with the same length and height for equal evaluation of the geometry. For the final concepts, the gasket was modelled on to the shield.

2.5 FEM The shield is exposed to external forces and several FEM analyzes of the component were conducted to ensure that the geometry can withstand the impact. The software ANSYS® Workbench 19.2 was used for implementation of mechanical analyzes of the component. The software creates finite element models of structures and simulates interaction of attributes and disciplines of physics by modelling different phenomena.

When the CAD-files of the 6 gaskets were exported to correct file format, they were imported into ANSYS Workbench where structural analyzes were carried out. The first analysis that was performed was on the whole component without current gasket. The material in the shield was set to aluminum which it is originally made of. This analysis was done in order to evaluate how the actual shield reacts to external forces and to benchmark the current solution of the component for comparison with further studies. From given values of forces that are applied on the shield, the pressure could be calculated and simulations of stress, deformation, plastic- and elastic strains were performed in order to determine which gasket to proceed with. In next step, simulations of the most suitable geometry of the gasket were carried out with different materials. The gasket can only deform a certain distance and the correct force that resulted in this deformation could be found for each material. Known boundary conditions were applied in every simulation. When the most suitable material and geometry of the gasket were found, final analyzes of the entire shield were performed and compared to the original solution.

Based on the analyses that were conducted in ANSYS, there had to be modifications of the geometry in order to improve the results. When printing this component in softer materials than metals, the mechanical properties can be deteriorated. To enhance the stiffness of the shield, the thickness of the walls and the bottom plate were increased. This change resulted in utilization of maximum allowed space in the component, both in height and width, before the shield comes in contact with closely located components. When the most suitable geometry of the gasket was found, the design was optimized to utilize all available surfaces in the component in order to provide sufficient quality when printing.

2.6 Prototype manufacturing and EMC testing Several prototypes were manufactured during this thesis. Two prototypes were manufactured at the Saab site in Järfälla in order to test the feasibility of printing small structures with the fused deposition modelling (FDM) technique. The conclusion from this experiment was then used in order to decide which 3D-printing technology to proceed with in the final concepts. Three final concepts were then printed in order to verify and test the function of the EMI shield. The final concepts consisted of different designs and two experiments were conducted in order to verify and falsify the electromagnetic compatibility. These experiments were performed using both the original PCB and the shield as a benchmark. By using this approach, it was possible to get the relative effectiveness of the concepts compared to the current EMI shield solution. The experimental setup is illustrated in Figure 2.

Figure 2: Experimental setup of EMC tests.

In experiment 1, a waveguide was used in order to radiate EM waves at the shield. The amplitude and frequency span were kept constant for all three concepts and a fixture was used in order to keep the distance between source and shield constant. The signal from the receiver was measured with a network analyzer and then plotted in order to evaluate the results. Experiment 2 was conducted to measure the power attenuation between the two cavities in the middle of the shield. A signal of specified amplitude was transmitted to one of the cavities and the magnitude of the remaining signal was then measured using a network analyzer. The values were later plotted and analyzed in a similar manner to experiment 1.

3. EMC and EMI Initially this section gives a brief introduction to electromagnetic radiation and the concept of EMC and EMI. Further on, basic shield equations are presented to give the reader an understanding of important parameters to consider for reflection and absorption of EM waves. The components of the current shield solution are then introduced and lastly, shield solutions on the commercial market are presented.

3.1 Electromagnetic radiation Electromagnetic radiation can be explained as the energy that is produced from electric and magnetic disturbance. When electrically charged particles are travelling through vacuum or other substances, they are considered as bundles of energy that are acting as harmonic waves travelling with the speed of light. These electric and magnetic waves have different properties depending on their wavelength, frequency and amplitude (Patel, Vo, & Hernandez, 2019). As indicated by the name, an electromagnetic wave consists of two elements, both an electric wave and a magnetic wave. These waves are orthogonal to each other and to the propagating direction of the wave. The electric field is denoted with E and the magnetic field is denoted with H, the ratio E/H is called the wave impedance. The impedance of free space is often used in EM related calculations and the constant value is 377 Ohm (Geetha, Satheesh Kumar, Chepuri, Vijayan, & Trivedi, 2009).

As the wavelength increase, the frequency of the wave decreases and vice versa. When the energy level increases, electromagnetic energy is released resulting in a decreased frequency and wavelength. The wavelength and frequency determine what kind of electromagnetic radiation there is in the electromagnetic spectrum (Patel, Vo, & Hernandez, 2019). The EM waves considered in this thesis work belong to the microwave spectrum with a frequency between 1-10 GHz.

Electromagnetic waves, as other types of waves, have the ability to combine and interact with other waves. This phenomenon is called interference and when the waves interact with each other, they produce either higher or lower amplitude which is classified as constructive and destructive interference respectively (Patel, Vo, & Hernandez, 2019). When a number of waves are added together, they produce an intensity maximum of the resultant wave which is called constructive interference. When the waves instead produce intensity minimum, destructive interference occur (Chakravorti, 2015).

3.2 EMC and EMI Electronic devices can be disturbed by electromagnetic radiation from other components or disturb other applications by the same principle. In radio and electronic systems, it is common that components are overlapping on the same electromagnetic spectra and interfere with each other (Tsaliovich, 2010). EMC explains the ability of a device or equipment to function properly in an electromagnetic environment without causing unacceptable disturbances. The device is compatible in that surrounding and no EMI is caused by emitting levels of electromagnetic energy to other devices in vicinity areas (FDA, 2018).

Interference between waves in an electronic system may not only lead to operator annoyance and distortion of transmitted information but also other consequences like equipment malfunction (Tsaliovich, 2010). If levels of electromagnetic energy exceed the resistance of the device, it can be vulnerable and result in complex problems regarding the technical standpoint but also in terms of public health and safety issues (FDA, 2018). To avoid these kinds of problems, it is of importance to consider the theory of EMC and that right directives are taken into account at an early stage in the design to comply with existing laws and regulations (Reenaas, 2017).

In order for an EMI situation to occur there is a need for both emission sources and receptors. Many electronic systems are considered both an emission source and a receptor since they contain both receivers and transmitters. In order for EMI to occur, a source of electrical noise, a coupling path and a victim receptor must be present. Electromagnetic interference refers to any unwanted, conducted or radiated signal from an electronic device that can result in degradation in performance of the equipment. All components within the system must ensure electromagnetic compatibility and comply with required specifications to avoid interference issues (Eushiuan, 1999).

3.3 Basic shield theory To avoid electromagnetic interference in a system, it is essential to control the emissions by using efficient types of EMC shielding. The effectiveness of any technique or strategy, that is used to reflect or absorb the electromagnetic field in a certain product, highly depends on the material, shield topology and characteristics of the application. The complexity of the product is associated with the shape, what kind of source that is present among others. Shielding effectiveness (SE) is commonly used to quantify the shielding performance of the product and it is measured in dB. Electromagnetic shielding represents a way to improve the EMC performance of certain application, devices or systems. By using EMC shields, the electromagnetic emissions of the device can be reduced and the electromagnetic immunity against external fields can be increased (Salvatore, Rodolfo, & Giampiero, 2008).

The shielding performance relies on two main parameters; absorption and reflection. In this section, equations based on Schelkunoff´s approach will be presented to give an explanation of which parameters that are important for absorption and reflection of EM radiation. Loss is stated as the result of generated heat in the shield material and the reflection is due to the difference in wave impedance between incident wave and shield material. The equations consider a plane sheet material, hence the calculations does not consider the geometry of the component. The results are therefore useful to determine the relative effectiveness of different shielding materials and not the geometry (Henry W.Ott, 2009). The total shielding effectiveness, SE, can be calculated with the general equation:

SE=A+R+B (1)

A is the loss of absorption, B is the multiple reflection correction factor and R is the reflection loss. Factor B considers multiple reflections in thin shields which are described later in this chapter. The absorption loss is the same for plane waves, electric fields and magnetic fields. When the shield is located at a greater distance than λ/2π (λ is the wavelength) from a point source, it is considered to be the far field. The general expression for the absorption loss is given by

퐴 = 3.34푡√푓휇휎 (2)

Where f is the frequency in Hz, t is the shield thickness in inches, 휇 is the relative permeability and 휎 is the relative conductivity of the shield material (Henry W.Ott, 2009). Indicated by equation (2), the absorption loss will increase with the frequency. It is also worth mentioning that high permeability and high permittivity are properties that are desirable for high frequency EM elimination of both the magnetic and electric component (Dixon, 2004). The reflection loss however depends on the field type and the distance. Therefore, the expression will depend on if the point source is located in the far field, if it is a magnetic field or an electric field (Henry W.Ott, 2009). The general reflection loss, R, formula is given by equation 3 where r is the distance in meters and C, m and n are constants dependent on the certain field type:

휎 1 푅 = 퐶 + 10 log ( ) ( ) (3) 휇 푓푛푟푚

From equation (3), the reflection is a function of the conductivity divided with the permeability of the shield material. High conductivity is therefore important for reflection loss. Another important parameter to consider in shielding situations is the skin depth. One skin depth corresponds to the thickness required to reduce the current in the material by 1/e which is roughly 37%, two skin depths reduce the current by 1/e2 and so on. Skin depth is usually denoted with 훿 (Evans, 1997) and depends on both the frequency and the material properties as seen in equation 4:

2.6 훿 = ( ) 0.0254 (4) 푓휇휎

The correction factor B, which was mentioned earlier, will now be described in greater detail. When an electric field hits a thin metal barrier, the primary reflection occurs at the first surface because of the difference in wave impedance, multiple reflections can therefore be neglected for electric fields. For magnetic fields however, the primary reflection occurs at the second surface creating a magnetic field within the shield material. This reduces the SE of the shield and it has to be considered for thin shields. The factor B for the correction is described equation 5, seen below (Henry W.Ott, 2009).

퐵 = 20 log(1 − 푒−2푡/훿) (5)

The important implication from equation (5) is that the result is a negative value which indicates a reduction of SE for thin shields compared to thick shields. Another parameter that will reduce the SE is apertures in the shield, especially for high frequency applications. For slot shaped apertures, a rule of thumb is to have the largest dimension smaller than λ/20 (Fenical, n.d.).

3.4 EMI shielding components This section describes the components that are included in the current EMI shield solution; shield, gasket and absorber. The function of these components are then presented and exemplified with research literature within the topic.

3.4.1 Shield The purpose of the shield is to reduce the electromagnetic field strength in the considered environment. The shield is either designed to contain electromagnetic radiation from the noise source or protect components within the shield from outside radiation. If entire systems are

considered, it is usually a better practice to shield to noise source to avoid unwanted effects on adjacent equipment. As previously descried, electromagnetic radiation is reduced by two main phenomena once in contact with the EMC shield, reflection and absorption. A shield that is homogeneously conductive and provides a perfect seal is easy to analyze. However, in reality the shields have imperfections from manufacturing, inhomogeneous seams and the need for apertures which adds complexity to the EMI problem (Archambeault, Brench, & Ramahi, 2001). The following paragraphs will give a brief introduction shield materials, metallic coating techniques and conductive gaskets.

EMC shields are often made of ferromagnetic or metallic materials. Ferromagnetic materials are often used for their mechanical properties rather than their magnetic nature and the main concern is that the shield material should be electrically conductive. A possible solution for nonconductive materials is to apply a thin film coating. These coatings are usually varying between 1 ힵm and tens of ힵm in thickness which gives advantages in terms of component weight and cost. Thin coatings give acceptable shielding efficiency typically for frequencies above 10 MHz. One important factor to consider when evaluating a thin film coating is the surface resistivity of the conductive layer. There are several coating technologies available and the 4 main approaches are electroless plating, conductive painting, vacuum metallizing and electrolytic deposition (Salvatore, Rodolfo, & Giampiero, 2008).

Electroless plating is usually used on plastic components and consists of two layers. The first layer should be adherent to the internal material, usually copper, and the external substrate, often made of nickel, should protect the internal substrate from oxidation and impacts. The thickness of the coating is typically only a few ힵm with a uniform coating distribution which results in lightweight component. Another advantage with this technology is that there is no need for surface treatment before the coating is applied. The conductive paint is usually based on silver and the thickness of the coating can easily be varied. Disadvantages with this technology are that the surface requires pretreatment and that it can be difficult to paint areas of the component that are not directly accessible. Vacuum metallizing is commonly performed by using aluminum or copper with a nickel surface and the layer thickness is in the order of a few ힵm. The limitation with this process is that it can be difficult to coat deep cavities in the component. Electrolytic deposition offers greater shielding effect than the previous mentioned techniques and the thickness of the coating can reach 25 ힵm (Salvatore, Rodolfo, & Giampiero, 2008).

3.4.2 Gasket Electromagnetic-interference gaskets are installed in the seam between two panels with a purpose to seal the enclosure in terms of electromagnetic radiation. The performance of an EMI gasket can for example be evaluated in terms of mechanical strength, EM shield efficiency and chemical resistance. As indicated above, there are several factors to consider when choosing a gasket and the importance of these might vary depending on the application (Salvatore, Rodolfo, & Giampiero, 2008). The primary function of an EMI gasket is to provide a conductive path between the shield and the ground on the PCB. Effective conductivity between the components will reduce the impedance in the seam resulting in a more efficient EMI shield. It is important to remember that the gasket works mainly by providing an electrically conductive path and not by filling the gap (Henry W.Ott, 2009).

As mentioned previously there are several factors that affect the selection of EMI gaskets depending on the application. In harsh environments where water, salt or other chemical substances comes in 13

contact with the gasket, the electrical conductivity can be drastically decreased. This might occur even if the metal particles are embedded in other materials and will affect the shielding efficiency of the shield. Another aspect to consider if metal enclosures are used is the galvanic process between the metals (Koledintseva, Chandra, Drewniak, & Lenn, 2007). The galvanic process occurs when dissimilar metals are used and there is moisture or water in-between resulting in a galvanic couple. This creates a voltage difference between the metals and the amplitude depends on their relative position in the galvanic series (Henry W.Ott, 2009). There are a wide variety of materials and gasket solutions depending on the component which the gasket is applied to and type of application. The most common gasket materials however are rubber, stainless steel, beryllium copper, aluminum and electrically conductive polymer composites (IEEE Std, 2009).

3.4.3 Absorber There are several solutions and techniques of RF absorbers with different practical implementations. It is of importance to differentiate between free space and cavity absorbers. Free space absorbers are resonant at a narrow frequency band because they absorb best at a thickness of ¼ of the wavelength, consequently they are only effective for certain frequencies. Cavity absorbers on the other hand absorbs over a broad frequency range because of inherent properties of the material (Dixon, 2004). The following section will briefly describe cavity resonance and the materials used to absorb. This is of interest for the considered component during this work.

Cavity resonance is an issue when circuit boards are shielded with metal enclosures or material with significantly higher wave impedance than free space. The problem of cavity resonance is especially noticeable in higher frequencies like microwaves or millimeter waves and the resonance can disturb the electronics within the cavity. The issue can be solved by redesigning the circuit board elements, but this is often both costly and complex. Because of this, cavity absorbers are used inside the cavity which eliminates or reduces the resonant modes. In order to be an efficient broadband absorber, the material should have high permeability and permittivity. The most efficient broadband absorbers are magnetic absorbers but purely dielectric materials can be used as well even though not as effective (Dixon, 2004).

As mentioned in section 3.3, absorbed energy is converted into heat within the material and therefore, thermal conductivity is also of interest in EM absorbers. Thermal conductivity in EMI absorbing materials can be achieved by having thermally conductive fillers. Conductive absorbers that contain several materials have been developed where the composite has one EMI-absorbing material, for example Fe particles, together with a thermally conductive material, like ceramic particles. Each material is suspended with a matrix of an elastomeric material, such as thermoplastics or thermosets. The matrix material is made of a material that does not compromise the absorptive action of the absorbers and must therefore be substantially transparent to electromagnetic energy. A material having a loss tangent less than 0,1 and a dielectric constant lower than 4 is considered to be transparent to electromagnetic interference (Tong, 2011). Common metal materials can also be used for absorbing purposes, in a study conducted by Geetha et. al was it stated that aluminum and magnesium could be used when the main shielding effectiveness based on absorption is desired (Geetha, Satheesh Kumar, Chepuri, Vijayan, & Trivedi, 2009).

3.5 EMI shield solutions on the market The following chapter will exemplify current EMI shielding solutions available on the market with benefits and disadvantages for each solution. The focus will be on metal shields, metallized shields and composite shields.

3.5.1 Metal shields Metal shields are often used as EM shields because they offer good electrical and heat conductivity. The material used to produce metal shields depends on the application and for low frequency shielding; a material named Mu-metal alloy is usually used that has high permeability. It consists of 79.5% Ni, 1.5% Cr, 5% Cu and 14% Fe (Geetha, Satheesh Kumar, Chepuri, Vijayan, & Trivedi, 2009). Metal shields are usually soldered in place on the PCB and can be produced as either one piece or two piece shields. Two piece shields consists of one base which is fastened on the PCB and a cover which seals the enclosure, these shields are used in applications where it is necessary to have access to the PCB components without removing the entire EMI shield. Metal shields are conventionally used as EMI shields but the weight of the components, difficulty of tuning the SE and corrosion are some of the current disadvantages with metal shields (Geetha, Satheesh Kumar, Chepuri, Vijayan, & Trivedi, 2009).

3.5.2 Metallized Shields Metallized shields are shields that are made out of a plastic core and are then coated with metal in order to make the surface electrically conductive. There are several technologies regarding how to apply metallic coatings to plastic surfaces, where the most common ones were described in chapter 3.4.1. Electroless plating is one common method of plating and it is the most cost effective solution when coating plastics. When this method is used, there is no need for appliance of electric potential to the component and a wet chemical process is used instead (Ishikawa, Kato, Takeyasu, Fujimori, & Tsuruta, 2017). The disadvantage with the metallizing processes is that it adds further processing steps and equipment into the manufacturing cycle of the component (Geetha, Satheesh Kumar, Chepuri, Vijayan, & Trivedi, 2009). The following paragraph will describe an existing metallized plastic shields that is available on the market and its performance compared to conventional metal enclosures.

An application produced by the company Gore, located in Delaware, US, is Gore Snapshot EMI shield. This shield consists of thermoformed plastic with a 5 ힵm of Sn coating. The component design is reconfigurable, it is lightweight and the SE is similar or better compared to conventional metal enclosures. A benchmarking was performed were the metallized shield was compared with two perforated EMI shields of metal, one with a two piece design and one standard perforated cover. The result showed that the metallized plastic shield had improved shielding effectiveness, especially for high frequencies (Gore snapshot, 2015).

3.5.3 Composite Shields As described previously metallic EM shields exhibits shielding effects due to reflection because of the electrical conductivity and high impedance of metal. However, metal shields suffer from their weight and they are not resistant to corrosion. To save weight, several plastic alternatives with metal coatings can be used but this is costly since it adds additional processing steps in the manufacturing process. As a result, extensive research has been conducted on other potential solutions and polymer composites with graphene and carbon nanotubes have been one solution that gained researchers

interest. Polymer composites with carbon fillers have several properties which make them interesting for EMI solutions, such as lightweight, corrosion resistance, good electrical, thermal and mechanical properties. Researchers within the area claim that carbon composites will be important for EMI shielding in the near future (Nazir, et al., 2018).

Parker Corporation, a company from Ohio, US, offers a thermoplastic composite with conductive fillers. The conductive filler is either nickel plated carbon or nickel-graphite powder depending on the desired shielding performance. The conductive filler is included in the injection mold process which results in less manufacturing steps resulting in lower manufacturing costs. The attenuation for both absorption loss and reflection loss can reach 85 dB and the weight is significantly reduced when comparing to aluminum shields. The reflection loss is less compared to metal shields but the absorption loss is greater due to the high permeability of the composite (Parker Hannifin Corporate, 2018).

4. Additive Manufacturing The following chapter describes the 7 categories defined by ASTM and their degree of material integration among other significant characteristics. The result is summarized in chapter 5, which later is used to screen potential AM technologies in order to choose the most appropriate methods for manufacturing the shield. The most appropriate methods are Binder Jetting, Material Extrusion, Material Jetting and Powder Bed Fusion which are described more detailed, the rest of the methods are briefly presented. The 7 classified manufacturing categories within this standard are stated below (ASTM International, n.d.).

• Vat Photopolymerisation (VP)

• Material Jetting (MJ)

• Binder Jetting (BJ)

• Material Extrusion (ME)

• Powder Bed Fusion (PBF)

• Sheet Lamination (SL)

• Directed Energy Deposition (DED)

4.1 Vat Photopolymerization Manufacturing process This manufacturing method involves a vat of liquid photopolymer resin where the model is created onto a moving platform. Desired geometry is built by modulating an image mask onto the polymer that corresponds to the fabricated geometry, as the vertical platform moves upwards. UV light or photo polymerization are used to cure or harden the resin where the curing process converts liquid polymer into a highly crosslinked structure (Davoudinejad , et al., 2018). Three technologies within the process of VP are Stereolithography (SLA), Digital Light Processing (DLP) and Continuous Liquid Interface Production (CLIP) (Badiru, Valencia, & Liu, 2017).

Multi-materials There has been some research conducted of the approach to use several materials in different VP processes and how to integrate polymers with fibers in order to create structural components. Short carbon fibers have been reinforced in photopolymers used for VP which is applicable for all processes within the category. The resins could also be filled with material that improves the electrical conductivity, like silver-filled epoxies (Hofstätter , Pedersen, Tosello, & Hansen, 2017). There is also a developed technology on SLA processes that use multiple vats positioned on a rotating device, where each one contains different photopolymers. When a layer of one material is constructed, the platform can be raised out of the current vat, rotated and submerged into another vat with a different material (Choi, Kim, & Wicker, 2011).

4.2 Material jetting Manufacturing process Objects of photopolymers are created and wax material is used as support objects, see Figure 3. Material is deposited onto a platform through a moving nozzle in form of droplets which are released from a pressure change occurring in the nozzle. These droplets are later solidified to create layers on layers which later are cured by heating or photo curing. Material can be jetted in two ways, continuous or “drop on demand” (DOD) approach (Gibson, Rosen, & Stucker, 2015). To facilitate jetting, the materials must be in liquid state with a certain viscosity. This means that materials that are originally solid at room temperature must be heated before jetting and for high viscosity fluids, the viscosity must be lowered before jetting. During the process, liquid material is converted into small discrete droplets which later are ejected from a nozzle. By varying the velocity and droplet size, the deposition pattern can be controlled. The quality and viscosity of the droplets must also be controlled to avoid spreading of material on larger areas than intended. Small changes to the material, like addition of particles, can affect the droplet forming and thereby changing the physical setup (ibid). When droplets are ejected on a surface, a leveling roller flattens the droplets into a thin film which later is cured by UV light into a solid layer. This method repeats the solidification process and builds 3D structures in a layer-by-layer manner (Rouhollah, 2018). A support structure made of a secondary material, that are dispensed from a parallel ink-jet print head, can be used to stabilize the main component, which usually is made of a material that can be removed easily without damaging the part (Gibson, Rosen, & Stucker, 2015). Two common technologies that are used in material jetting are continuous stream (CS) and drop-on-demand (DOD) where the possible modes of expulsion are the distinction between them. In the first technology, a steady pressure is applied and causing a column of fluid that is ejected from one nozzle. The stream is then transformed into droplets by forced vibration or other functions that control the formation rate. CS approach has a high throughput rate and has the ability to produce small droplets at high frequency. In DOD method, the droplets are directly produced from the nozzle by applying individual pressure pulses caused by actuators like thermal or piezoelectric among others. Thermal actuators heat the liquid until a bubble is occurring which force the droplet out from the nozzle. When comparing these two main approaches, it can be concluded that DOD have higher placement accuracy and smaller drop size than CS (Gibson, Rosen, & Stucker, 2015).

Figure 3: Schematic view of Material Jetting process (Loughborough).

Materials Polymers are the only material group that is commercially used in material jetting because they need to undergo polymerization in order to solidify (Rouhollah, 2018). Since polymers consist of a wide range of materials, they represent different mechanical properties and applications depending on what polymers are used and how the process is structured. Changing variables like print head speed or frequency and velocity of the droplet, the quality of the deposit can differ and thereby affecting the properties of the component. There has been research conducted on potential materials that are promising for future applications with the use of ceramics, metals and other types of polymer composites. However, there have been complications in this field of research where polymerization of the material happened in different stages and resulted in clogging of the nozzles. Ceramic suspensions have also been investigated for creation of structures with multiple layers, where a mixture of zirconia powder with additives was used. In the field of metals, most of the research work has been on manufacturing electronic applications, especially by printing solder droplets. Metals that have been printed successfully are aluminum, copper, and mercury among others. Even though metals are considered as appropriate building material for structural components, there are significant challenges with depositing metals due to their high melting point which can cause damage to the printing system (Gibson, Rosen, & Stucker, 2015).

Multi-materials Material jetting can manufacture components made of multi-material in a single printing process, which allows production of complex parts with different performance and functionality. Since material jetting manly manufacture polymers, thermoplastics and elastomers are the most common materials to combine (Rouhollah, 2018). The integration of several materials in material jetting has been demonstrated, using DOD approach where it allows rapid exchange of building materials with different ink jet actuators (Yang, et al., 2017). XJet, an Israel-based company, has recently come up with a technology called Nano-particle jetting (NPJ), where a mix of metal and ceramics are used by integrating solid nanoparticles in a liquid suspension. The building material is jetted together with the supporting material which fills out all cavities that occurs during the process, instead of creating a separate supporting part. Since the technology use nanoparticles, high accuracies and complex details can be achieved because the small particles are controlled during the process (XJet, 2018). There has been research on the manufacturing of electronic composite materials using material jetting printing techniques. Production of micro electronic devices by using deposition techniques is common and widely used due to high throughput and high resolution. When it comes to EMI shielding of printed nanocomposites, there is still ongoing research within that field (Rouhollah, 2018). Polymer composites containing silver particles may be printed by material jetting approach to achieve a final component with high electrical conductivity. Conductive carbon-based nanomaterials, like carbon nanotubes (CNT), are also highly promising as ink for material jetting when producing devices with good electric properties. As mentioned earlier, a major challenge with this approach is controlling the viscosity when having nanoparticles in the liquid. It is also important to relate to potential damage of the component, such as shear stresses or surface tension, that comes with the jetting process (Koumoulos, Gkartzou, & Charitidis, 2017).

ACEO, a German printing company, together with their chemical group WACKER created an electrically conductive silicone rubber material for material jetting, especially designed for the DOD approach. This material was announced autumn 2018 and is considered to be a suitable material for modern medical or functional automotive parts. The material has high performance characteristics like temperature tolerances up to 200°C, electrical resistance of approximately 10 Ωcm with unchanged conductivity up to 25% elongation (ACEO, 2018).

Advantages/disadvantages Material jetting is a technology that can produce high quality products with complex, accurate and smooth structure with impressive resolution. It can be incorporated with wide variety of materials into a single component depending on desired characteristics. Even if there are many advantages with this method, there is also some limitations that must be taken into account. One major challenge with material jetting is the formulation of the material. As mentioned earlier, it must be of liquid state which can lead to increased process steps where the material must be dissolved or melted before entering the print head. The speed of the process can easily be modified by just increasing the number of dispensing nozzles. However, a significant limitation with material jetting is the viscosity of the material which is difficult to control, especially when having a mix of several materials. Since the nozzles are quite small, they often clog and prevent liquid from being dispensed. The deposition of droplets must also be controlled, and they will interact differently depending on what material the previous layer is made of. Therefore, this must be considered in the process in order to avoid delamination or damage of the ground layer (Gibson, Rosen, & Stucker, 2015).

4.3 Binder jetting Manufacturing process The process normally uses two materials, usually a solid powder material of which the part is to be made of and a liquid binder material that function as a glue between the layers. Binder liquid is ejected through a nozzle onto a powder bed on a movable platform, where the binder form droplets that create spherical agglomerates in contact with the powder particles (Gibson, Rosen, & Stucker, 2015). This results in bonding of the materials and a layer is constructed where the binder solidification mechanism is dependent on the interaction between the powder and binder particles (Rouhollah, 2018). Once a layer is created, the platform is lowered, and a counter-rotating mechanism is recoating with a new layer of powder onto the bed and the process with the binder liquid is repeated and building layer by layer until the part is completed. When all the layers of the product are completed, a “green part” is finished which subsequently have to be pre-processed to get a completely dense and final structure (Gibson, Rosen, & Stucker, 2015). Post-processing for binder jetting parts involves curing, sintering and infiltrating it with another material to accomplish desired mechanical properties. Some of these post-processes can sometimes take longer time than the actual printing, especially sintering the product which can result in an increased cost (Gokuldoss, Kolla, & Eckert, 2017). A schematic view of the process can be seen in Figure 4.

Figure 4: Schematic view of Binder Jetting process (Loughborough).

Materials The choice of the powder and binder are highly affecting the product that is created. High packing density of the powder is a desirable characteristic in order for the part to achieve a high-volume fraction and stiff mechanical properties. Today, polymers and metal powders are commercially used where the type of infiltrant determine the use of the final product. Polymer parts are usually considered as visual prototypes but can be used for functional purposes when having an elastomeric infiltrant (Gibson, Rosen, & Stucker, 2015). However, since binder jetting use binder as adhesive, there is a possibility for presence of porosities in the sintering process which can cause less suitable material characteristics when constructing structural applications (Gokuldoss, Kolla, & Eckert, 2017). Metal components created with binder jetting are also available as prototypes or as structural components depending on choice of metal powder. When processing metal powders, the green part is usually subjected to furnace treatments to burn of the binder and to lightly sinter the metal particles (Gibson, Rosen, & Stucker, 2015).

Multi-materials By introducing additional extractable powder materials in the binder jetting process, a desired percentage of several materials in one component can be made. Multiphase composites have been produced by infiltrating the created part with another material. When the part undergoes the sintering step, different levels of porosity is obtained where those porosities can be filled with another functional material to achieve a more structural component. This can be done in some processes by immersing the sintered part into a molten metal bath. However, the sintering temperature of the ceramic part has been discussed since it affects the density and volume fraction of the metal phase and thereby the final microstructure of the component. The submerging time is also a crucial parameter in the process where the amount of metallic material entering the ceramic part will as well have a big impact on the final characteristics (Toursangsaraki, 2018). Similar, powders can be used together with different types of infiltrating substances like acrylate, water or wax, depending on desired properties of the application (Gibson, Rosen, & Stucker, 2015). Besides creating composites with good structural properties, binder jetting can also produce products with electric and thermal conductivity. A material that has been used widely in the electronic industry is aluminum nitride which a covalently bonded ceramic semiconductor material used in electronic packaging applications (Diaz, et al., 2018).

Advantages/disadvantages Most of the powder materials that are commercially available are recyclable in unprinted form. One other significant advantage of binder jetting is the production of components without the need of support structures. This process can also be economically scaled by using several ejection nozzles in the same set up, where it is possible to achieve a high deposition speed at low cost since the process do not require any high-powered energy sources (Gibson, Rosen, & Stucker, 2015). Another advantage is that the process allows a two-material approach by using different powder-binder combinations and this gives the ability to control the mechanical characteristics by just changing the powder-binder ratio. Moreover, as mentioned earlier, since there is no melting involved in the process, it can result in a porous structure and the volume and size of these pores may differ within the same material batch (Gokuldoss, Kolla, & Eckert, 2017).

4.4 Material extrusion Manufacturing process Material extrusion is the most common AM process with a large number of available commercial fabricators. The method of material extrusion is to preheat the material and then force it through an extrusion nozzle. Material can be forced through the nozzle in several ways where one approach is to use a tractor feed system. If the pressure and nozzle speed is constant, the diameter of the extruded material will be constant. The material properties of the finished product are dependent on the adhesion between deposited layers which traditionally leads to anisotropic material characteristics. Basic method steps for all heat-based extrusion AM processes are shortly described below (Gibson, Rosen, & Stucker, 2015) and can be seen in Figure 5.

• Raw material storage is loaded into the machine • Material liquefaction • Applied pressure to feed material trough an extrusion nozzle • Scanning a predefine pattern in the XY plane to create one layer • Adhesion between extruded layer and previously printed layer due to residual heat • Support structures to allow complex geometries

Material extrusion is the common name for a collection of AM processes, where Fused Deposition Model (FDM) and Fused Filament Fabrication (FFF) are examples of two common commercialized processes. FDM is a technology that is patented and developed by the American company Stratasys. The process is accomplished by using a pre-extruded wire feedstock which is placed in the AM machine. The feedstock is heated above its glass-liquid transition temperature and the material is extruded into thin layers which are repeated until a 3D object is produced (Stratasys, 2019).

Figure 5: Schematic view of Material Extrusion process (Loughborough).

Materials There are a wide range of commercial materials available for material extrusion processes, but the most common materials are amorphous thermoplastics. ABS and PLA are examples of common filament materials and they are used due to their high viscosity properties which makes them suitable for material extrusion (Bourell, et al., 2017).The reason why amorphous polymers are better suited for material extrusion compared to crystalline polymers is due to their melting properties. Amorphous polymers have no distinct melting point which makes it possible to heat the material to a high viscosity liquid (Gibson, Rosen, & Stucker, 2015).

Multi-materials Material extrusion allows creation of multi-material parts by either printing preformed composite feedstocks or alternating between different feedstocks to produce a laminate composite. ME technologies have shown the ability to integrate several materials and create simple electronic equipment in one setup (Kutuniva, Mäkikangas, Mustakangas, Rautio, Kumpula, & Mäntyjärvi, 2018). The ability to print electronical equipment using material extrusion methods makes it highly relevant technology for EMC applications. Composite parts usually consist of a thermoplastic matrix reinforced with short fibers of a metallic, ceramic or carbon. Recent developments in the FDM process have resulted in manufacturing of commercial machines that are able to print continuous fiber by utilizing dual extrusion nozzles. However, composite filaments are limited on the commercial market and one key element in printing these materials is to prepare them manually. The process of preparing a composite involves steps of drying polymers in an oven following by creating a mixture containing the polymers and a filler to achieve desired wt. %. The final step is to extrude the composite mixture using an extruder. In order to increase the homogenous distribution of filaments, a second extrusion could be incorporated (Farahani, Therriault, Dubé, Bodkhe, & Mahdavi, 2015).

It has been reported that the use of carbon nanotubes as filler (CNT) is one of the most promising ways to increase the electrical conductivity of polymer material. A certain wt.% of conductive filler makes it possible to achieve a conductive path within the polymer material which is called percolation threshold. In a study conducted by A. Dorigato 2017, et.al, the electrical conductivity of a 3D printed ABS polymer nanocomposite was investigated. It was determined that the printing direction of the part influenced the electrical conductivity and that compression molded parts of similar wt.% achieved a lower electrical conductivity (Dorigato, Moretti, Dul, Unterberger, & Pegoretti, 2017). Another study by Manzoor et.al was conducted to investigate the possibility to use 3D printed conductive composites in EMI shield applications. The research team utilized a FDM process because it is easy to create multilayer carbon-based structures with high resolution. By

controlling the material structure, the dielectric properties of the produced part could be altered where high dielectric loss and high conductivity results in efficient shielding. The volume concentration of the filler was in the range of 20-35% and the results showed that carbon-polymer composites have high potential as an alternative solution for metal shielding (Manzoor , Ghasr, & Donnell, 2018).

In a study conducted by Kutuniva et.al, 2018, a dual extrusion machine named Duo XL 3D, produced by the company Prenta, could manufacture conductive and flexible structures. A PLA-based graphene was used to print conductive paths and the flexible parts were made of a polyurethane-based material. The study resulted in a successful material integration of flexible and electrically conducting materials in the same machine setup, which proves the ability to integrate several materials using a FDM process. Similar to previous researches, it was found that printing properties of the composite compound where important to avoid complications in the extrusion process (Kutuniva, Mäkikangas, Mustakangas, Rautio, Kumpula, & Mäntyjärvi, 2018).

Advantages/disadvantages The variety of available materials and the ability of material integration are the main advantages of the FDM process. In addition, FDM are usually a cheaper technology compared to traditional manufacturing processes. A general disadvantage with the FDM process is that the geometry of the part is limited to the nozzle diameter of the machine. More specific, the minimum edge radius of the part will be equal to the nozzle diameter. Another disadvantage is the resulting anisotropic properties of the printed parts, where the mechanical strength in the Z-direction is usually less compared to the X-Y direction. This should be considered when designing structural parts and can be avoided by aligning the X-Y plane in same direction as resulting stresses (Gibson, Rosen, & Stucker, 2015).

4.5 Powder bed fusion Manufacturing process PBF is the common name for a wide range of additive manufacturing technologies. Common terminologies for processes within the PBF technologies are Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Direct Metal Laser sintering (DMLS) and Electron Beam Melting (EBM). Similar for all technologies and the main concept is the usage of a heat source that melts powder on a powder bed in a predefined path (Yang, et al., 2017). The machine usually consists of a powder supply mechanism, re-coater mechanism, build chamber, laser, mirror and a moving build-platform. The schematic of a PBF system is shown in Figure 6. Prior to the printing step, the powder bed is preheated according to the specifications of the powder supplier. Several parts can be integrated and printed in the same setup in order to optimize the utilization of the machine. After the process is completed, the powder and the printed parts are cooled down and then removed from the build chamber. Powder that is not sintered can be recycled, but there are limitations depending on the material that is used due to decreased material properties upon reuse (Brandt, 2017). It is of importance to control the thermal environment in the printer in order to manage the properties of the finished part. The powder bed temperature should be in between the recrystallization temperature and melting point of the material, so that the molten material remains in liquid state and in thermal equilibrium with the powder. If the temperature is controlled in such manner, the residual stresses in the build material will be minimized.

Another aspect that will influence the quality of the printed product is the feedstock where parameters such as flow ability and grain size are of importance. As described previously in this section the recycling management is of great importance in order to produce products with good mechanical properties (Bourell, et al., 2017).

Figure 6: Schematic view of Powder Bed Fusion process (Loughborough).

Materials In theory, any material that can be in powder form could be processed using the PBF technology. For production of parts with strong mechanical properties and high tolerances, the materials on the markets are limited. This is due to complex process parameters, the physical properties of the materials and the thermal properties of the material (Brandt, 2017). Powder bed fusion processes can process both metals and thermoplastics. In general, semi-crystalline polymers are best suited for PBF but some amorphous plastics are suitable for the process as well, for example polystyrene. For metal PBF, a rule of thumb is to use metallic alloys that can be welded or used in casting (Bourell, et al., 2017).

The market of commercially available metals is currently limited, but it is expanding due to extensive research within the topic. Common for all metals produced with PBF technologies is that large residual stresses are introduced to the build substrate because of large thermal differences between the melt pool and the surrounding material. General trends can be distinguished when it comes to AM parts and fatigue. These trends show that a rough surface finish results in stresses and increased nucleation growth which reduced the fatigue performance compared to conventionally machined parts. In order to control these effects, different post processing methods can be implemented like heat treatment processes using pressure mechanisms (Bourell, et al., 2017).

Multi-materials In metal PBF processes where laser is the heat source, it is possible to create multi-material parts with alternating materials in the building direction. However, the process has to be conducted manually since no current technology supports this automatically (Anstaett & Seidel, 2016). In a research conducted by Christine Anstaett et.al, 2016, the current state of multi-material processing using PBF technologies was investigated and an implementation methodology was also suggested. In the research, properties within all phases were considered like pre-process parameters, building process parameters and post processing parameters. The long-term scope of the research is to implement material changes in every building direction altering between e.g. metals and polymers. In the pre-process, the CAD file is prepared and it is necessary define the materials that will be integrated in this stage. If the material change is discrete, it is currently not a problem to define

several materials in the current software. If the change is graded however, this function needs to be integrated in the software tool.

Several changes have to be conducted in the delivery strategy of the powder so that at least two powder materials can be dispersed. The researchers proposed three different delivery strategies where a photoconductor, a nozzle or a coater were used. The utilization of a photoconductor to transport electrically charged powder particles is a promising technology, but it is still in early research. A promising technology already explored by SLM solutions is the use of two material chambers. In this way, the material can be altered in the building direction. The post processing of a PBF part also needs development in order to fully operate with different materials. One challenge with post processing is the need of heat treatment since a multi-material part can be in need of different heat treatment strategies (Anstaett & Seidel, 2016).

Similar to the material extrusion process, it is possible to print polymer composite materials using the PBF technology. There are extensive literature on how carbon fillers can be used to improve the electrical conductivity, thermal conductivity and tensile strength. In an experiment conducted by Koo et.al, it was shown that a PA11 polymer matrix with 3 wt. % MWNT filler could be successfully printed. It was also concluded that the addition of MWNT resulted in a significant decrease in resistivity among the printed samples. These findings suggest that composite materials manufactured with the powder bed fusion technology can be a possible candidate for unconventional EMI shielding (Koo, Ortiz, Ong, & Wu, 2016).

Advantages/disadvantages As previously stated, one disadvantage with parts produced with powder bed fusion is the fatigue performance. In terms of metal additive manufacturing, this is usually due to the rough surface finish and residual stresses along the building direction. Similar to material extrusion processes, a notch is usually present in-between the layers and the notch effect favors delamination between the interconnecting layers (Goodridge & Ziegelmeier, 2016). However, the anisotropic behavior of PBF parts can be tampered with in order to create parts with specific properties which can be utilized to tailor the part for its purpose. Research suggests that the fatigue properties are better when the loading is perpendicular to the building direction (Brandt, 2017). Another advantage is the high resolution of PBF technologies which enables thin walls and highly detailed parts. Material integration in the building direction is possible in theory and the literature suggests several methods how to implement it. However, there are currently no commercially available machines that are capable of multi-material integration which is the main disadvantage with this process.

4.6 Sheet lamination Manufacturing process SL is a group of additive manufacturing methods where sheets of material are laminated together using adhesive material. The process can be categorized in two main processes; Laminated Object Manufacturing (LOM) and Ultrasonic Consolidation. To bond the sheets together the machine uses heat combined with pressure and the sheets are either cut to shape before the bonding or after the bonding. LOM is often used for large scale prototypes and it faces several difficulties, for example anisotropic material properties, poor surface finish and resolution (Kim, Lin, & Tseng, 2018). Ultrasonic consolidation is similar to LOM except for differences in feedstock materials. In order to obtain acceptable part tolerances and surface structures, the process is combined with milling in a hybrid manufacturing setup.

One main drawback is layer delamination for products produced with this technology. Layer delamination occurs when the layers are separated from each other which can result in failure of the structure. As a result of delamination in addition to anisotropic characteristics, components made with the sheet lamination process can be argued to not be used for structural applications (Sames, List, Pannala, Dehoff, & Babu, 2016).

Multi-materials In LOM, several materials such as polymers, paper, ceramics and composites can be used. Most typically is the production of paper prototypes. Ultrasonic consolidation only uses metal and metal alloys as feedstock. During this process there is a possibility to combine several metallic materials in the Z-direction to create a multi-material part (Kim, Lin, & Tseng, 2018).

4.7 Directed energy deposition Manufacturing process DED refers to an additive manufacturing category where focused energy melts a feedstock. All processes within this category are similar in nature, what differentiates them is the type of feedstock and energy source that are used. The components that are produced with a DED process obtain a near-net shape with a rough surface finish. The feedstock can be either a wire or powder and the heat source can be laser, electron beam or electric arc. The most prominent DED process is laser engineered net shaping, which use a powder feedstock and a laser heat source. These methods can be combined with subtractive manufacturing technologies, often referred to as hybrid manufacturing (Sames, List, Pannala, Dehoff, & Babu, 2016).

Multi-materials Metals are commonly used as feedstock material. There is currently no method available to process polymer materials using laser DED processes (Sahasrabudhe, Bose, & Bandyopadhyay, 2018). However, multi material parts consisting of metal alloys can be produced using a pre-alloyed feedstock (Toursangsaraki, 2018).

4.8 Solid mechanics This chapter presents the theory within solid mechanics that was used for this thesis project and the importance of the mesh during the simulations.

4.8.1 Finite Element Analysis To predict the behavior of the geometry during the moment when forces are applied, a numerical modelling activity is required. The Finite Element Method can be used to simulate such behavior where it discretizes the system by creating second-order tetrahedral elements of the structure. These elements are subdivisions or sub-regions of the domain where each element has defined nodes at which the unknown field variables are to be determined (Ramamurty, 2011). In the field of engineering analysis, FEM is used as a tool for solving and analyzing complex problems like structures with extreme geometrical irregularity, combined material characteristics and complicated loading patterns. One significant application of this method is the analyzation of structural mechanics (Rohit & Uttar, 2018).

4.8.2 Mesh The number of elements that are present in an analytical model that are to be analyzed can be defined as mesh density, where each node is connected to another and creating a type of mesh that is applied to the component. Depending on the geometry, several types of mesh implementation are possible to use based on special requirement of the analysis (Rohit & Uttar, 2018). The nodes are connected together with adjacent nodes to create triangular or quadrilateral elements. By increasing the number of nodes, the elements become finer which results in an increased density of the mesh and higher level of discretization. A model with higher mesh density gives more distinct result which converges towards a reliable solution that correlates with original site condition of the component. However, finer mesh analysis takes significantly longer time for results to be generated and is not desirable in every simulation. A solution to that problem can be the use of a coarser meshing that results in shorter solving process and minimal percentages of errors. The use of a coarser mesh can unfortunately lead to variation far from actual results and collapse of the structure. When creating the mesh of structures to be modelled, the advantages of mesh density in terms of least computing time must be taken into account in order to maintain a significantly high level of proximity compared to the actual results (Rohit & Uttar, 2018).

In ANSYS Workbench, an automatic mesh generator can be used for meshing the component. This generator usually produces elements that are less well-shaped and with lower amount of nodes. To improve the mesh, a smoothing technique or a size function can be used on a specific surface or on the whole solid part where it provides an option to control the number of elements, angles between adjacent mesh elements and gradation between maximum and minimum sizes based on growth rate.

5. Analysis This chapter describes the analyses that were conducted during the thesis work. Firstly, the concept development phase is described in detail. Further on, the geometrical modifications of the gasket and the shield are presented. Lastly, the FEM and material analyses are described.

5.1 Analysis of concept development The current EMI shield solution is made out of bulk aluminum which is milled down to the correct dimensions and the current weight of the whole component 27,5 g. The shield is then coated and an EMI gasket is dispensed to the component. In the last step, the cavity EMI absorber is glued to the aluminum shield. The shield is used in a demanding environment both in terms of dimensional tolerances and temperature which has to be considered when designing the shield. Since integrating several steps in the manufacturing process was of main concern, the amount of process steps has been considered when the concepts were evaluated. Concepts with an equal amount of process steps as the current solution have therefore not been considered as a viable solution. The current manufacturing process steps are visualized in Figure 7 below.

Figure 7: Manufacturing steps in current process.

The first step in the concept development phase was to clarify the problem at hand and a function analysis was conducted. The function of each ingoing component was broken down and the requirement of these parts was also evaluated. This analysis made it possible to understand which property was the most important for each sub-problem and these properties were then considered during the external and internal search. The function analysis can be seen in Figure 8 below:

Figure 8: View of the function analysis.

These functions were then considered during the external and internal search in order to select relevant manufacturing methods, materials and designs. The external search was conducted using both interviews and literature and the important literature was presented in section 4. From the literature, an initial technology screening was conducted were the most prominent technologies for the application was determined. The considered technologies followed the ASTM classification and were weighted towards several important selection criteria. These selection criteria were determined after discussion with experts within the topic in addition to literature research. The reason for choosing the manufacturing technologies prior to the concept development phase was to understand the manufacturing possibilities/limitations. However, it was important to have an open mind in order to generate concepts thinking “outside the box” but it was also important to have a realistic understanding of the manufacturability of the concepts. The selection criteria and the technologies can be seen in Table 2 below.

Table 2: Manufacturing technologies and selected properties.

Selection matrix of methods

Photopolymerization

Technologies Bed Fusion Powder SLM/DMLS/SLS EBM VAT SLA CLIP DLP Jetting Binder Jetting Material CS DOD Extrusion Material Lamination Sheet UAM LOM Energy Directed Deposition WAAM EBF3 LENS

Printable No No No Yes No No No No Yes No No No material: - - (6) (6) (6) (9) (9) (9) (15) (1) (18) (21) (1) (1) ceramic Printable Yes No Yes Yes Yes Yes Yes Yes Yes No Yes No No No material: (1,4) (1) (6) (6) (6) (9) (9) (9) (4) (1) (18) (21) (1) (20) polymer Printable Yes Yes No No No Yes Yes Yes No Yes No Yes Yes Yes material: (1,4) (1,4) (6) (6) (6) (9) (9) (9) (15) (1) (18) (21) (10) (1) metal Printing a Yes Yes Ye Yes Yes Yes Yes Yes No Yes No No Yes - composite (5) (7) (7) (7) (9) (11) (11) (16) (1) (18) (22) (1) (20) Printing several No No Yes No No Yes Yes Yes Yes Yes No Yes materials (5) (5) (8) (8) (8) (9) (13) (13) (17) (1) - (22) - (20) in one setup Successful Yes Yes No Yes Yes Yes Yes Yes Yes No No Yes material - - (2) (8) (7) (7) (10) (12) (12) (16) (1) (18) (22) (20) integration Walls thinner Yes Yes Yes Yes Yes Yes No No No No Yes - - - than 0.9 (3) (3) (9) (14) (14) (9) (15) (15) (21) (21) (1) mm Structural Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes parts (3) (3) (6) (6) (6) (9) (9) (9) (9) (15) (15) (1) (1) (1) References 1 (Sames, List, Pannala, Dehoff, & Babu, 2016); 2 (Chen, Wan, Li, & He, 2018) ; 3(Brandt, 2017) ; 4 (Bourell, et al., 2017); 5 (Anstaett & Seidel, 2016); 6 (Davoudinejad , et al., 2018); 7 (Hofstätter , Pedersen, Tosello, & Hansen, 2017); 8 (Choi, Kim, & Wicker, 2011); 9 (Gibson, Rosen, & Stucker, 2015); 10 (Toursangsaraki, 2018); 11 (Rouhollah, 2018); 12 (XJet, 2018); 13 (Yap, Wang, Sing, Dikshit, Yeong, & Wei, 2017); 14 (Sculpteo, 2019); 15 (Yang, et al., 2017); 16 (Farahani, Therriault, Dubé, Bodkhe, & Mahdavi, 2015); 17 (Kutuniva, Mäkikangas, Mustakangas, Rautio, Kumpula, & Mäntyjärvi, 2018); 18 (Kim, Lin, & Tseng, 2018); 20 (Sahasrabudhe, Bose, & Bandyopadhyay, 2018); 21 (Williams, Martina, Addison, Ding, Pardal, & Colegrove, 2016); 22 (Hoefer, Haelsig, & Mayr, 2017)

The sub functions that were determined initially were now used to generate possible solutions for each sub-problem. In order to keep an open mind and create creative solutions several brainstorming sessions between the authors were conducted. Possible concepts were then evaluated at seminars with mechanical engineers and electronic engineers within the company in order to validate the concepts. The validation of each concept considered both manufacturability and EMC capability. The selection criteria were determined with help both from literature and discussion with engineers within the company. As previously mentioned it was important to reduce the amount of process steps compared to the current product. If this was not the case, additive manufacturing would not benefit the production of the part. All concepts with associated explanation are shown in Table 3 and the selection matrix is shown in Table 4. Based on the content in Table 2 above, it is evident which concepts that were the most prominent and these were further analyzed in order to determine which manufacturing technology, geometry and material to proceed with. The materials considered in this research are all commercially available and they are described further in section 5.4.

Table 3: Explanation of each concept.

Concept Description A Integrate magnetic absorbent, elastic electrically conductive gasket and metallic shield material in one setup. B Metal shield with integrated geometrical gasket + cavity absorbers. C Electrically conductive composite shield with integrated geometrical gasket. D Electrically conductive composite shield with metallized elastic gasket. E Metallized plastic shield with an integrated geometrical gasket + cavity absorbers. F Metal shield with an elastic and electrically conductive gasket + cavity absorbers. G Plastic shield with elastic gasket. Metallize shield and gasket + cavity absorbers.

In concept A, all sub-functions would be integrated in one setup using some of the seven additive manufacturing technologies. A magnetic absorbent material was investigated because it is the most frequently used material when absorbing broadband EMI frequencies. The shield would be printed in a metal material and the gasket would be electrically conductive and elastic. In Concept B, metal materials were investigated because of their high conductivity. The idea was to make a flexible structure of metal and use that as the gasket in order to avoid printing an additional gasket material. The absorber would then be applied in each cavity. Concept C was chosen due to existing research within the topic of composite materials as EMI shields. The research showed that composites with electronically conductive fillers could be successfully used as EMI shields and that the radiation was absorbed rather than reflected. Because of this, the cavity absorber could in theory be neglected which would benefit the reduction of process steps. The gasket would be similar to concept B where a flexible structure would be used instead of a separate elastic gasket material. Concept D is similar to the previous concept with the exception that a separate gasket material is used instead of a geometric gasket. Since no elastic electrically conductive material that could be exposed to high temperatures was found for any of the additive manufacturing technologies, the gasket would need to be metallized afterwards in order to be electrically conductive. In Concept E, the geometrical flexible structures would be used as a gasket which is similar to concept B. However, the shield would be made out plastic and an electrically conductive coating would be needed. The main reason of using plastic is the reduction in density which is of importance in the application. Concept F would be similar to the current solution except that everything would be manufactured using additive

manufacturing. The last concept would be manufactured using a stiff plastic material as the shield and the gasket would be printed in an elastic material. In order to make it electronically conductive, the shield would be metallized and the cavity absorber would then be used to absorb EM radiation.

Table 4. Selection matrix of concepts.

Selection Criteria Concepts A B C D E F G Number of process + + + + + 0 0 steps Temperature - + 0 0 0 + - resistance Manufacturability - - + + + - - Density - - + + + - + Gasket flexibility 0 - 0 0 0 0 0 Sum +´s 1 2 3 3 3 1 1 Sum -´s 3 3 0 0 0 2 2 Sum 0´s 1 0 2 2 2 2 2 Score -2 -1 3 3 3 -1 -1 Continue No No Yes Yes Yes No No

In the concept selection matrix a zero indicates that the selection criteria is similar to the reference criteria, a minus indicates that this concept performs worse for that specific criteria and a plus indicates that it performs better. The reference value for amount of process steps is four since that is the amount of the current solution. The reference value for the temperature is the service temperature for the environment in which the component will be used. The manufacturability is measured as a plus if a supplier was found who claimed that they could manufacture the component for that concept, consequently it would be a minus if no possible supplier or AM expert claimed that it would be possible. The density is compared to the density of the existing component. Lastly the flexibility of the gasket is compared to the existing gasket. From this it was determined that concepts C, D and E would be investigated in more detail.

5.2 Geometrical design of components The shape of the flexible structures was determined based both on current EMI gaskets and after discussing with mechanical engineers at the company. The geometries were then modelled in CAD and simulated using FEM in order to determine which geometry that was the most flexible. The geometries of existing EMI gaskets are often complex with both sharp angles and thin walls, therefore the feasibility of manufacturing was investigated with the supplier. This process was iterative in order to find a solution that would be possible to manufacture and still be flexible enough for the application. The geometries that were investigated in this research can be seen in Figure 9.

Figure 9: Analyzed geometries of gaskets.

As seen in Figure 9, the force is applied vertically and the purpose of the gasket is to be flexible in the direction of the force. In reality the strucutres above will be deformed perpendicular to the applied force as well. This had to be considered when designing the geometries so that the top of the gasket still would be in contact with the circuit on the PCB, see circuit in Figure 11. Otherwise the electrical circuit would not seal the area around the shield which would casause an EM leakage. The gasket would be aligned along the same surface as the previouse gasket, which is on the top surfaces of the walls in the shield. There was a need for a gap in the corners of the gasket to make it flexible, the gaps were designed to follow the EMI design rules according to section 3.3. In concept D and E metallization was needed in order to provide a conductive surface layer and therefore a skin depth calculation was conducted to find the minimum thickness of the coating. According to an electronic design engineer at the company, the required thickness is at least one skin depth which was considered when choosing the correct thickness of the coating.

5.2.1 Optimization of geometry A modification of the geometry for the specific gasket was first conducted by designing a sharp edge at the bottom of the structure. Since the component is manufactured layer by layer according to the shape of the CAD model, overhanging features can be difficult to manufacture due to deficiency of support like the overhanging circular shape at the bottom of the gasket. Usually, a support structure must be deposited in the same process when having overhanging structures. However, these techniques can sometimes be difficult to implement and to facilitate such problems the geometry can be modified to avoid an overhanging geometry. Designing a sharper edge of the gasket was done in order to simplify the design prior to the printing process.

Another change of the gasket geometry was an increase of the thickness and height in order to utilize maximum allowed space it may have before it comes in contact with adjacent components or critical surfaces on the PCB. In order to avoid overlapping circuits located on the PCB when the gasket is compressed, the shape of the top was also changed where a part of the geometry has been removed. The gasket still allows equal contact area with the PCB during attachment as before the removal of the geometry. Figure 10 demonstrates the different modifications of the geometry of the gasket where the gasket to the left is the first generation.

Figure 10: Modifications of the gasket geometry.

The geometry of the shield was also updated. Just as previous modifications, the maximum area around the shield has been utilized in all directions. The height has been increased, which means an increase of the thickness of the bottom plate, as well as extended thickness of the walls. To stabilize the bulged circular surfaces on 2 sides of the component where the screw holes are located, the height has been increased so that they have equal height as the rest of the shield.

5.3 FEM analysis The EMI shield is attached onto a PCB to provide protection for enclosing components. It is fastened with 8 threaded screws that compress the card and the gasket until the bulged circular surfaces around the screw holes comes in contact with the surface of the board. When this process is completed, the shield is attached tightly onto the surface while the gasket provides a complete sealing, see Figure 11. During the attachment to the PCB, the shield is subjected to a pressure which results in stresses and deformation of the component. This is mainly present around the screw holes and in the walls that are connected to the gasket, but also in the center of the shield which is a common problem in the current product.

Figure 11: EMI-shield attached onto a PCB, the PCB is the green part

When new concepts with certain materials were evaluated, the geometry had to be simulated in order to investigate whether the structure can withstand the load or not. The FEM analysis was carried out step by step, starting with simulation of gaskets in order to find the geometry to continue with and what pressure the deformation of each gasket corresponds to. Furthermore, these results formed the basis for further analyzes. The materials and geometries that gave the best results when the gaskets were simulated were used in continued simulations of the entire shield.

5.3.1 Analyzed components The first FEM simulation that was conducted was of the original component without the rubber gasket and absorbents. The gasket is originally made of a patented rubber material which consists of two different constituents with unknown material properties. Since the properties are not made public, a decision was to proceed with the analysis of the shield without a gasket, based on the assumption that the entire force is absorbed by the gasket and transferred into the wall structure of the shield. Simulation of the whole shield was done to benchmark the existing concept and to evaluate exactly how the shield reacts to applied load when the shield is attached onto the PCB in current application. The material in the shield was set to aluminum, which is a standard engineering material in Ansys Workbench.

In the next analysis, geometries of the 6 different gaskets were evaluated in order to determine which geometry yields the best result and are the most suitable one for the final application. The length and height of each gasket were the same and they were modelled on a surface where each one was separated and placed at an equal distance from next adjacent gasket, see Figure 15. First, the material was set to aluminum for every gasket. This choice of material was done to evaluate how the gaskets would perform if they were manufactured with the same material as in the current shield. Later on, several polymeric materials were also analyzed, all of which commercially available materials for additive manufacturing.

The next simulated component was the specific gasket that provided the best results from previous analyses and was the one that this work chose to proceed with for the final concept. The geometry consisted of same dimensions as before in the first implementation of analyses. In order to evaluate the optimization of the geometry, the gasket with increased thickness and height was analyzed. One simulation was conducted of the geometry with the sharp edge on the bottom of the gasket and another without, see Figure 10 above. The same evaluation of correct pressure as described above was conducted in this step as well. When the final geometry of the gasket was found through previous analyses, it was modelled onto the shield and a final simulation of the entire component was conducted with the materials which the shield will consist of when it is realized.

5.3.2 Mesh of the components The components were imported into Ansys Workbench and meshing of each geometry was conducted. To ensure accurate prediction in the analyses, a large number of elements and nodes were applied over the whole structure of each analyzed component. A more significant increase in elements was done by applying a sizing function in correspondence of surfaces where large deformation was expected. This was mainly around screw holes and on larger surfaces on the back of the shield that have a tendency to create buckling, see Figure 12 for this specific mesh of the shield. Increased elements of the gaskets were implemented on the short sides and at the inner surfaces which will constitute the greatest impact. This mesh of each gasket can be seen in Figure 15.

5.3.3 Conducted analyses Several analyses were performed of each geometry that was investigated where the choice of material and loads were varied. These analyzes gave results of deformation, stresses (according to Von Mises), plastic- and elastic strain of each component and simulated material. Based on known dimensions and torque of threaded screws, the required force and what pressure that corresponds to could be calculated which was applied in the simulations. The applied load in the simulation of the 6 gaskets was the original force which is present in the real application. Since they have different geometries and contact surfaces where the force is applied, the correct pressure had to be calculated for each gasket based on the area of its top surface.

When evaluating the most suitable geometry of the gasket, the original pressure could not be applied since the stiffness of the evaluated materials differs from aluminum, from which the original pressure is calculated from. In reality, the gasket will stop deforming before the top part comes in contact with the bottom part during the attachment to the PCB.

The gasket will only deform 0,32 mm before the screw fasteners of the shield are aligned with the surface of the PCB and thereby stop the deformation. Therefore, it was of importance to evaluate what pressure that corresponds to when simulating different materials in order to conduct a comparison of total pressure when the gasket consist of different materials.

When the entire shield was analyzed, it was of interest to evaluate how the shield is deformed when loads are applied. Earlier, when the shield was used in real applications, there was a tendency for the shield to bulge in the middle of the bottom plate because there is no screw attachment located there. This had to be further examined for all alternative materials that are considered as appropriate to use in the final concepts. The deformations of the geometrical gaskets were also of importance. How it deforms is the most crucial feature since the gasket must be flexible and able to deform under pressure.

Due to the applied load, stresses in the component are present which can lead to failure of the structure. This was analyzed for each material where the results were compared to their tensile strength which indicates that plastic deformation can be present after that point. Plastic deformation is not desirable, especially not in the gasket since it must be able to deform back to original form when removing the shield from the PCB. Based on the results of the stresses, a modification of the geometry could be conducted to improve these areas which had the most critical stresses. Plastic- and elastic strains were the final analyses that were implemented. Since the gasket must provide elastic properties, it was of interest to evaluate what strains the applied load results in and if these strains lead to a critical plastic behavior.

5.4 Material analysis Current EMI shield has certain requirements that must be met in order to function adequately in the application where it is located. Since the shield is exposed to loads, the material in the shield must obtain properties like high modulus, high specific strength, increased stability, excellent wear resistance and the ability to withstand elevated temperatures. It must also be electrically conductive to create a leading path to the gasket.

The gasket must provide sealing between the two surfaces and must be compressed to generate a pressure-tight barrier in order to protect the inside components. It must be of a material that can easily be deformed and adequately fills irregularities. An important characteristic of the gasket material, in addition to flexible properties, is electrical conductivity.

The last and third component, the absorbent, should be made of a material with electromagnetic absorbing characteristics. Usually, these materials are composites with magnetic fillers in a silicone matrix.

5.4.1 Evaluated materials In order to find alternative materials for the production of these components, the analysis was limited to those 2 methods that were considered most suitable for production of an EMI shield; FDM and SLS. When examining properties of known materials these processes can manufacture, these were categorized based on the characteristics of the materials in the current components where the new materials need to provide equal or better properties than the original materials. These materials were later analyzed and simulated in FEM in order to evaluate their performance in the component and to conclude which materials to proceed with. In the concepts where the shield and the gasket

will be integrated into one component, the gasket will have a geometry that allows to be deformed under high pressure. The final properties of the gasket must therefore be flexible, which is necessary when the shield is attached onto the PCB. However, it must also obtain high mechanical characteristics since the shield will be manufactured from the same material.

Analyses of several materials were conducted. These materials have characteristics that are considered suitable to function as a shield material as well as gasket material, see Table 5. The data of properties of each material can be found in Appendix 4.

Table 5: Analyzed materials and their functions in the shield.

Analyzed material Suitable function Process AlSi10Mg Shield + gasket SLS

PA1101 Shield + gasket SLS PA2200 Shield + gasket SLS Alumide Shield + gasket SLS PC Shield + gasket FDM ULTEM1010TM Shield + gasket FDM ZYYX proCarbonTM Shield FDM PI-ETPU85TM Gasket FDM

AlSi10Mg is an aluminum alloy and was chosen because the shield is currently manufactured in aluminum. This is a common material that is used for additive manufacturing which is implemented in a range of different machines. Low-cost alloying elements (Si and Mg) are added to produce complex strengthening phases in the aging process. The final properties of the printed product are high specific strength, fracture toughness, high fatigue resistance combined with electrical conductivity (Martin, Yahata, Hundley, Mayer, Schaedler, & Pollock, 2017). However, aluminum does not acquire elastic properties which are necessary for the functionality of a gasket.

PA1101 and PA2200 are two polymeric materials with similar properties. They are made of polyamide, commonly known as nylon, used in a widely range of applications. Nylon has characteristics like high strength, abrasion resistance and high toughness and is a common material used for military applications (Gong, Chen, & Zhou, 2018). PA1101 and PA2200 are considered as multipurpose materials with high strength and detail resolution combined with excellent long-term constant behavior. Both materials are appropriate for construction of functional parts with high quality. They have a density that is lower than aluminum, which is one of the most important requirements this component must meet. When it comes to flexible characteristics, both PA1101 and PA2200 have a tensile modulus, tensile strength and hardness that are much lower than aluminum. In addition to those properties, they obtain high values of strain at break.

Alumide is an aluminum-filled polyamide with characteristics such as high stiffness, high temperature performance with a well-balanced ratio of density and stiffness and excellent dimensional accuracy. The polymer matrix is made of PA2200. This is a material that is suitable for applications that must have a low weight combined with high mechanical properties. It has more than twice as high tensile modulus compared to ordinary polyamide materials due to reinforcement of aluminum particles but lower strain at break. This material was chosen since it obtains properties from both aluminum and

polyamide which results in electrical and flexible characteristics. According to electrical engineers at Saab Surveillance, Alumide can also obtain absorbing properties. It was therefore worth investigating this material further to find out if these characteristics are good enough for the final application.

PC (polycarbonate) is a thermoplastic with high durability and stability, commonly used for applications within the automotive and aerospace industry. It has mechanical properties that are superior over most thermoplastics and is a suitable for functional prototyping, production parts and manufacturing tools. In addition to flexible properties, PC can withstand high temperatures and great impact but do not obtain any electrical characteristics.

Ultem1010TM is a polymeric material, produced by the company Stratasys. It offers high tensile strength with excellent thermal and chemical resistance, commonly used in aerospace and automotive applications. Ultem1010TM provides flexible properties with a no brake value of compressive ultimate strength. However, it has no electrically conductive properties and no absorbing capability.

ZYYX proCarbonTM is a material that is produced by the company ZYYX 3D Printer. The filament is made of nylon, reinforced with 20% carbon fiber strands. This material is appropriate for structural and functional parts due to high mechanical properties, which is a result from the reinforced fibers. It has low density and high distortion temperature combined with electrically conductive characteristics.

PI-ETPU85TM is made of Thermoplastic PolyUrethane (TPU) which is an elastic plastic. Main advantage of a TPU material is high abrasion resistance combined with high load bearing capacity and conductive properties. However, a reduction of the mechanical characteristics can be distorted when the material is present in temperatures above 70°C (K.DE & White, 2001). Even though TPU do not meet all requirements that the function of a gasket must fulfill, it is still considered interesting to proceed with this material since it will demonstrate the possibility to combine several material groups within the same process when using additive manufacturing.

5.4.2 Coating material In concept E, the component must be coated with a material that provides conductive properties. Copper is a noble metal with improved mechanical properties such as fatigue resistance, high strength in addition to high thermal and electrical conductivities. Copper is widely used as coating or as solid components for regulation of electrical characteristics in many types of applications due to extremely high conductive properties. However, a main disadvantage is low resistance to oxidation of certain copper alloys, which results in hydrogen embrittlement and corrosion of the metal (Davis, 2001).

5.4.2 Materials in Ansys Workbench These evaluated plastic materials are not a part of the standard material database in Ansys Workbench. New materials had to be created where their material properties have been taken from given material sheets which belong to companies that manufacture these materials. These sheets are missing values of important characteristics in order to fully evaluate the plastic behavior. Due to this, an alternative method has been used when entering the material properties called bilinear isotropic hardening. That is a plasticity material model used in large strain analyses. It uses an isotropic hardening assumption coupled with the von Mises yield criteria to define a stress-strain curve of the material based on the yield strength and the tangent modulus. The value of Young´s modulus represents the slope of the first segment in the curve and the tangent modulus is the slope of the second segment. The behavior is described only by two lines, hence the name bilinear (Gaertner, 2006). In all simulations of these plastic materials, all necessary values for this material model were stated in the material sheets accept for the tangent modulus, which was set to 1/10 of the value of Young´s modulus.

Another important aspect when simulating these materials in Ansys Workbench is that the components are assumed to be solid in all simulations, which will not be the case in the prototypes. Depending on the choice of manufacturing method and how the process is carried out, air pockets can be created between the layers or porosity can be present which will result in a non-solid product. These factors are directly correlated with processing parameters of the AM process, like scan speed, power input and melt pool characteristics. The orientation of the printing has also an impact on the strength of the product.

6. Results This chapter covers the findings from the conducted analyses. The FEM results are first described in detail and the final three concepts are then presented.

6.1 FEM analyzes The following chapter describes the results of the FEM analysis provided by Ansys Workbench. The simulations resulted in several figures where the most relevant results are shown below. Additional figures and results can be found in the Appendix 1 – 3.

6.1.2 Original shield The first simulation that was conducted was of the original shield without the rubber gasket and absorbing sheets. The material in the shield was set to aluminum. See Figure 12 of the mesh.

Figure 12: Meshing of the shield with original geometry.

Boundary conditions of fixed support were applied on the surfaces around the screws which will be in contact with the PCB during attachment, see Figure 13.

Figure 13: Applied boundary conditions.

The force was calculated from given values of dimension and torque of threaded screws and resulted in 1680 N, which is the original force used in the current shield. This force was applied on the surfaces of the shield where the gasket was attached, corresponding to the surface that will be in contact with the PCB during attachment. The screw fasteners will thus be in contact with a flat surface of the PCB the whole time and the surface where the gasket is located is subjected to mechanical load. The analyses that were conducted were directional deformation, equivalent stress

and equivalent elastic strains. No plastic strains were present in this simulation. See Figure 14 for results of deformation. Maximum values for each result can be seen in Table 6. Rest of the values can be found in Appendix 1.

Figure 14: Results of deformation.

Table 6: Resulted values from simulation of the original shield.

Analysis Maximum value Directional deformation (Z-axis) 0,739 mm Equivalent stress (von-Mises) 790,090 MPa Equivalent elastic strain 1,135 %

6.1.3 Gasket geometries When the potential gaskets geometries were simulated, the material was set to aluminum for each of them. The meshing of gasket number 1 – 4 and 6 were equal with increased sizing on their short sides. Gasket number 5 had a complex geometry and could only be meshed with triangular elements. The mesh of each one can be seen in Figure 15.

Figure 15: Meshing of 6 gasket geometries.

The boundary conditions that were applied for each gasket were fixed support on the bottom surface. When these gaskets are modelled into the shield, the bottom surface will be the surface that is integrated into the walls of the shield. Figure 16 shows the boundary condition of gasket number 3.

Figure 16: Boundary condition applied on the bottom surface.

The correct pressure that was applied on each gasket was calculated from the original force of the shield in addition to the top surface area of each gasket. The original force was used in this simulation only to evaluate a difference in deformation of the gaskets. An assumption was made that the pressure comes in contact with the entire top surface in order to simplify the analysis. These values can be seen in Table 7.

Table 7: Surface area and applied pressure of each gasket.

Gasket Top surface Pressure 1 49,06 mm2 1,14 MPa 2 17,51 mm2 3,19 MPa 3 32,71 mm2 1,71 MPa 4 32,71 mm2 1,71 MPa 5 31,50 mm2 1,78 MPa 6 21,33 (2 surfaces) mm2 2,62 MPa

The conducted analyses were directional deformation and equivalent stress, see Figure 17-18. The color bar is inverted due to positive definition of the Z-axis. The maximum values from the simulation can be seen in Table 8.

Figure 17: Results of deformation.

Figure 18: Results of equivalent stresses (von Mises).

Table 8: Resulted maximum values from simulations of the different gaskets.

Analysis Maximum value Directional deformation (Z-axis) 106,9410-4 mm Equivalent stress (von-Mises) 199,78 MPa

These simulations resulted in most deformation of gasket number 3 and 4. The largest stresses can be found in gasket number 4, see Figure 19. Based on these results, gasket number 3 was chosen for further evaluations.

Figure 19: Equivalent stresses (von Mises) of gasket number 3 (left) and 4 (right).

6.1.4 Choice of gasket When the most suitable gasket was found, which was number 3, several simulations with varying materials and pressures were carried out. The meshing of the gasket number 3 can be seen in Figure 20.

Figure 20: Meshing of gasket number 3.

Fixed support was applied on the bottom surface as boundary condition. This was equally done as in the simulation of the 6 different gasket geometries.

The pressure was applied onto the top surface as seen in Figure 21. Same assumption was made as in previous simulations; the entire force is applied on the whole top surface. The original pressure is calculated from the original force of 1680 N and resulted in 1,71 MPa.

Figure 21: Applied force on gasket number 3.

PA1101, PA2200, Alumide, PC and ULTEM1010TM were analyzed. When each material was simulated, the specific pressure that corresponds to a deformation of approximately 0,32 mm of a point on the top surface was evaluated. From each simulation, values of directional deformation, equivalent stress, equivalent elastic- and plastic strain were analyzed. The results from these simulations were later compared to each other in order to evaluate which material resulted in highest flexibility at lowest force.

The materials that gave the best results were PA1101, Alumide and ULTEM1010TM. Results of directional deformation and equivalent stresses for these materials when specific pressure was applied on gasket number 3 can be seen in Appendix 2. The best result of both deformation and

stresses at lowest pressure was PA1101, seen in Figure 22. The specific pressure and its proportion of the original pressure can be seen in Table 11, in addition to the results of deformation and equivalent stress of each simulation. It can be seen in the figure of the stress simulation that the color bar only changing in color up to 48 MPa which is the value of the yield strength of PA1101. Each value above this is represented by grey color in the figure, which indicates of stresses that are present in the plastic region.

Figure 22: Results of deformation (left) and equivalent stresses (Von Mises) (right) of PA1101.

Table 9: Resulted values from simulations of 3 different materials.

Material Directional Corresponding Max value - Max value - deformation of pressure directional equivalent stress top surface deformation (von-Mises) (Z-axis) PA1101 0,316 mm 0,752 MPa (44%) 0,428 mm 63,608 MPa Ultem1010 0,317 mm 0,761 MPa (44,5%) 0,425 mm 67,955 MPa Alumide 0,315 mm 1,026 MPa (60%) 0,437 mm 96,649 MPa

Based on these results and information regarding the FDM process found in the conducted research, it was decided that PA1101 and Alumide will constitute the chosen materials for the final concepts.

6.1.5 Updated geometry – with and without a sharp edge When the geometry was updated, new pressures had to be found that results in the same deformation of 0,32 mm in both optimizations of the geometries. PA1101 and Alumide were analyzed and simulations of directional deformation, equivalent stress, equivalent elastic- and plastic strain were conducted. The result of the simulations can be seen in Appendix 2 where differences in material and geometries are stated. The change in directional deformation of the geometry with and without the sharp edge can be seen below. Figure 23 shows the gasket in PA1101, which resulted in a correct deformation at lowest pressure. The corresponding pressure, which resulted in a deformation of approximately 0,32 mm when both geometries were simulated, can be seen in Table 12. The increase of previous pressure can also be seen in the table.

Figure 23: Results of deformation of PA1101, without sharp edge (left) and with sharp edge (right).

Table 10: Results of pressure in both geometries.

Material Pressure – without sharp edge Pressure – with sharp edge PA1101 19 MPa (x25 of previous) 22 MPa (x29 of previous) Alumide 28 MPa (x27 of previous) 35 MPa (x34 of previous)

The updated geometry of the gasket without a sharp edge was implemented in the shield for the final concept.

6.1.6 Final geometry – gasket and shield When correct materials and geometry of the gasket were found, the entire shield had to be simulated in order to evaluate the final results. The gasket was modelled into the shield, where the geometry of the shield was also updated. Figure 26 shows the new geometry of the shield with the integrated gasket.

The mesh of the shield was increased compared to the simulation of the original shield due to increased geometry, see Figure 24.

Figure 24: Mesh of the shield with the final geometry.

The boundary conditions were same as previous, fixed support on the surfaced around the screws, facing downwards. The force was the original force of 1680N and was applied on the top surfaces of the gaskets. The original force was used to perform a comparison between the new shields and the original shield in order to evaluate the differences when the same pressure is applied.

Two simulations of the shield were conducted, one with PA1101 and one with Alumide. These simulations gave results of directional deformation, equivalent stress, equivalent elastic- and plastic strains. Figure 25 shows the difference in directional deformation of both materials. The rest of the results can be seen in Appendix 3.

Figure 25: Results of deformation of PA1101 (left) and Alumide (right).

6.2 Concept C The following section will present the design of the final concepts that were generated during the thesis work. A detailed description covers the manufacturing technology, process steps, material and design considerations for each concept. In concept C and E, the aperture geometry of the gasket was assumed to be a slot and the size of the maximum size of the aperture was calculated from the rules of thumb described in the literature section. The calculation showed that the maximum length of the slot for 10 GHz is 1,5 mm which later was considered in the design of both concepts. The EMI results are displayed in Appendix 5 and will be covered in the discussion.

Concept C is an EMI shield manufactured with Alumide. This material was chosen because of its high relative permittivity and partial electrical conductivity. As mentioned in the literature survey, both electrical conductivity and permittivity are desired characteristics in order to successfully attenuate EM radiation. It is also mentioned that aluminum has previously been used in casted shields and that aluminum should be used when absorption loss is required. Instead of using a separate material that absorbs the cavity resonance like the current solution the idea was to integrate this function in the shield material itself. For this reason, it is of interest to test if Alumide have sufficient EM absorbing properties so that the need for a separate absorber can be eliminated. The geometrical gasket was used in this solution in order to further reduce process steps and the result is a shield produced in just one process step. The CAD results for concept C can be seen in Figure 26 below.

Figure 26: The shield in concept C.

The design of the gasket was in close collaboration with the manufacturer and the result is a tradeoff between FEM results and manufacturing requirements. Focus was mainly on the gasket geometry because this was the geometry that would be the most challenging to manufacture. The design was revised several times and two prototypes were then produced in order to verify the manufacturability and function of the design. The machine used to produce the final concept was EOS P396 and the layer thickness was set to the minimum value for Alumide which was 0,15 mm. The dimensions of the gasket can be seen to the left in Figure 27 and the theoretical layer thickness has been illustrated to the right to give the reader an understanding of the resolution.

Figure 27: Dimensions and layer thickness of the gasket.

The weight of the produced Alumide shield is 23,5 g which is a reduction of 4 g compared to the current shield solution. The gasket geometry was successfully printed and the tolerance of the screw holes was sufficient in order to fasten the shield in the fixture. However, the small holes in each cavity was not successfully printed because the holes were clogged with sintered powder. Figure 28 shows the gasket geometry and the screw holes of the manufactured prototype.

Figure 28: Produced prototype in Alumide.

6.3 Concept D This concept consists of a shield which is made out of ZYYX proCarbonTM which is a carbon fiber composite. This material was chosen for its stiffness, thermal resistance and electrical conductivity. As suggested on several occasions in the literature study, carbon filled polymer composites showed promising results in high frequency EMI shield applications. According to the experiments in the literature study the shielding effectiveness of these composites showed to attenuate EM radiation solely by absorption. Because of this, the same argument that was used in concept C was also used for this concept; the shield itself absorbs the incident radiation. ZYYX proCarbonTM can only be printed using the FDM technology resulting in worse resolution compared to the SLM process, additionally the material is to stiff and brittle to function as a geometrical gasket. Therefore, an alternative gasket was designed which used the maximal available area. The gasket for this concept is an electrically conductive TPU gasket which can be printed on top of the carbon composite.

Figure 29: Concept D, CAD picture to the left and manufactured prototype to the right.

The weight of the produced component is 12,5 g which is a reduction of 15 g compared to the current solution. Seen in Figure 29, the resolution of the manufacturing process is not adequate which results in a deformed gasket, small holes and a skewed geometry. Therefore, additional post processing was needed in order to fasten it to the fixture.

6.4 Concept E This shield is made out of PA1101 which is a flexible plastic material and is not electrically conductive. The geometry of this EMI shield is identical to concept C with the same geometrical gasket and aperture size. Theoretically, this concept needs to have additional EMI absorbers in the cavities of the shield since the difference in wave-impedance between air and copper is large. Because of this, the majority of the intrinsic wave will reflect upon the copper surface. In order to avoid this phenomenon, there is a need for EM absorbers in this concept. The absorber is applied as a separate process step, which results in a total of three process steps when manufacturing this concept. Initially, the geometry is 3D printed using the SLS technology where the inside is later coated with copper and absorbers are attached to the inside of the cavities in the final step. The theoretical thickness of the copper coating was determined from equations 4 and 5, the graph for the correction factor can be seen in Appendix 4. Concept E is visualized in Figure 30 where the theoretical numbers of layers are also displayed.

Figure 30: CAD picture of concept E and illustration of layer thickness.

The concept was produced with EOSP396 machine with the minimum layer thickness available for PA1101 which is 0,120 mm. Because of this, the resolution of the gasket is improved by two layers compared to the Alumide shield. The weight of concept D is 22,5 g which is a reduction by 5 g compared to the current solution. However, the geometry of the absorber is not optimized in the concept, thus there is a potential for further weight savings. The dimensional accuracy of the manufactured part was sufficient in order to attach the part to the test fixture. All holes were created, even the small holes in the cavity. The coating thickness of the prototype is 4-6 µm, resulting in a ratio of 6 between T and 훿. Consequently, factor B is approximately zero and can be neglected. The coating was successfully applied to all surfaces, except the inside of the screw holes and the back surface of the shield. Figure 31 shows the manufactured prototype of concept E.

Figure 31: Produced prototype of concept E.

7. Discussion Integration of 3 materials with desired characteristics proved to be a challenge when using AM methods. As a result, two of the concepts tried to integrate the function of a gasket even though the material was not changed. The gasket design is a result of 6 potential solutions that were evaluated in terms of manufacturability and flexibility. Further evaluation of additional designs could be conducted for future work but was limited in this work due to time constraints. Novel additive manufacturing methods, found mainly in research papers are briefly described in theory. However, one important criterion for the company was to manufacture prototypes of each concept that were chosen for the final solution. Therefore, an important contributing factor to the technology screening was to find a manufacturer in Sweden. Further work could therefore be conducted in order to find other potential technologies and manufacturers. The following section will consider and discuss some important factors regarding additive manufacturing, where the focus will be on the final four technologies. The result from the FEM analysis and EMC experiments will later be discussed.

7.1 AM technologies In terms of material integration, there are several promising manufacturing technologies within the category of material jetting. As stated in the literature chapter, there are existing solutions that are capable of integration several polymers in one setup. Even electrically conductive elastic materials that are suitable for gasket can be implemented using the jetting technology. ACEO silicone is one example which might be suitable as a substitute of the original EMI gasket. One drawback of this technology is the thermal characteristics of the majority of the materials that were found during this research, which made them inapplicable as a solution. However, the research for new materials is extensive and electrically conductive composites and silicones might occur in the future at the commercial market. Because of the multi-material capability and research for new electrically conductive materials, one could argue that material jetting might provide an alternative solution for EMI shields and gaskets in the future. Likewise, binder jetting showed to be a promising technology for electrically conductive composites. Nonetheless, the density of the materials found in this research was considerably higher than the density of aluminum which made it inapplicable. Additionally, binder jetting is a powder bed process and because of this, the technology suffers from the same limitations as PBF which makes it problematic to integrate several materials in the printing process.

Extrusion processes are frequently occurring in multi-material applications which were evident from both literature and experts within AM. Available materials offer limited electrical conductivity and usually consist of a polymer matrix with carbon filler. Concept D is an example of two commercially available FDM materials with carbon in order to make the product electrically conductive. It is challenging to theoretically predict the adhesion between the two materials and this knowledge is more about “learning by doing”. Therefore, experimentation is needed in order to understand the compatibility of different materials. In concept D, the adhesion is sufficiently good for the application but no experiments were conducted to determine the force required to tear the materials apart. Such experiments would however be necessary to classify the materials and use them in real applications. Furthermore, the wt. % of carbon in the material is a critical factor for electrical conductivity and dielectric properties. In commercially available materials, this is not a parameter that can be altered with and experiments within the company could therefore be beneficial for further work. Literature suggests that a high wt. % is beneficial for EMI applications but it also

complicates the printability of the material, since it is more prone to clog in the extrusion nozzle. Similarly, the complexity of printing the TPU gasket is proportional to the hardness of the material and a soft material increase the complexity. Further research within the topic is needed to find a tradeoff between manufacturability and EMI properties of the materials.

Literature suggests that integration of multiple materials should be possible in the future of PBF technologies. Currently, this is not possible for several reasons regarding both machining and post processing. Therefore, the geometrical gasket was an attempt to integrate the function of the gasket without adding an extra material. The SLS process was chosen for the resolution, the availability of manufacturers and the material properties. Seen in Figure 30, the gasket was designed to be printed with a layer thickness of 120 µm. This was applicable to the gasket with PA1101 but not with Alumide since the layer thickness needs to be increased. It is possible to alter the machining parameters and this was also discussed with the manufacturer, but the conclusion was to follow the guidelines for layer thickness. After all, future work might experiment with the possibility to print with a thinner layer thickness to investigate if it is possible to reach a finer resolution with Alumide. Both concept C and E was printed so that the force applied to the gasket is perpendicular to the printing direction, because theory found in the literature suggested that this would have a positive effect on the fatigue properties. Neither of the shields are post processed which results in a relatively rough surface. It was determined that a rough surface was desired to maximize the adhesion of the coating and reduce the process steps further. However, this is a tradeoff since a rough surface increases the amount of nucleation points which reduce the fatigue properties of the component.

In concept D, it can be seen that the quality of the component is poor, both in terms of the gasket and the shield. The gasket is made with a new electrically conductive TPU which has a hardness of 85 Shore A and is still under development. Therefore, the manufacturer had no printing parameters for this material which is arguably one of the reasons that the gasket was misshaped. Further on, the material is softer than the TPU that is commonly used and as mentioned earlier it is usually harder to print softer materials. The geometrical accuracy is generally lower for composite materials because a nozzle with increased diameter is used. Because of this, the dimensional accuracy of the holes and walls were poor. Another issue with the material integration was the software. It has no function that allows the operator to predefine which layer to stop and change material and as a result, this process had to be done manually. Another aspect to consider is that the material can only be changed in the printing direction and a result of this was that the gasket material was also printed on top of the screw holes. Yet again, this is due to limitations in the software for this particular printer. All the concepts were then visually inspected by the authors using a microscope to determine the geometrical accuracy. In order to improve the internal validity of the geometrical analysis, a laser scanner could be used to scan the components and then compare the scanned product with the actual CAD model.

Another aspect to consider is the impact of additive manufacturing on the supply chain. All of the concepts presented in this study showed a reduction in both the number of process steps and the number of suppliers. In concept C and D, the EMI shield is produced in just one process step with a supplier which is located in Sweden. Hence the supply chain and logistics of these concepts are significantly simplified compared to the original shield. In general, it can be seen that the supply chain is positively affected by AM with a reduction in lead time and a supply chain that is easier to manage.

7.2 Solid Mechanics When the results from the first FEM analyze was evaluated, it was concluded that aluminum is not considered appropriate to use as material when producing the gasket and shield in same material. In the simulation when pressure was applied, the deformations of the gaskets were barely noticeable when they were made of aluminum due to the mechanical properties of the material. Based on these results, it was decided to not proceed with aluminum and instead evaluate other suitable materials. However, aluminum has a low density that is valuable for this component. A metal with lower density than the density of aluminum and additionally allows production in an AM process was difficult to find. Since polymers are common AM materials, the focus was shifted to find appropriate plastic materials that can be in accordance with given requirements and desirable properties of the final component. As mentioned in chapter 5, the material properties that were used Ansys Workbench were derived from material sheets. There are some values that are not stated in these sheets which are required for a reality-based evaluation of the material behavior in the simulations. Further on, the model in Ansys Workbench is considered completely solid which does not correspond to the structure of a 3D-printed product. With regard to these aspects, it must be made clear that the results from Ansys Workbench will not fully comply with the behavior of the components that are manufactured in the reality. In order to obtain a more consistent result, tensile tests must be performed on test roads made of same AM materials where correct stress-strain curves can be evaluated and later used in the simulations. All materials that have been analyzed, seen in chapter 5, have properties that considered suitable for an EMI shield. Despite this, not every material obtains characteristics that can meet the exact requirements of the specific EMI shield, especially when it comes to operating temperature. The shield must be able to withstand a wide temperature range, possess a low density and withstand high mechanical stresses which are material properties that rarely are combined. These materials were still chosen to be analyzed due to the fact that they may be suitable for alternative components which do not require this specific combination of properties.

When the final choice of gasket was simulated with PA1101 and Ultem1010, it could be concluded from the simulations that Ultem1010 resulted in higher stresses than PA1101 when correct pressure was applied. When evaluating these results and information regarding the FDM process found in section 5.2, it was decided to not continue the study with an FDM approach due to poor quality during the printing process. Therefore, it was concluded that PA1101 and Alumide were chosen for further analyses since they can be manufactured by a SLS process and both of these materials resulted in correct deformations at low pressures. When the gasket was simulated in PA1101, the force which was required to deform the gasket 0,32 mm was the lowest force of all materials. This indicates a high degree of flexibility of the gasket, which is desired in this solution.

Nonetheless, Alumide resulted in an increase of pressure and higher stresses than PA1101. Since it consists of a mixture of PA2200 and aluminum, these stiff properties can be dominant in the final component. An alternative approach to increase the elastic properties of the material is to instead use a mixture made of PA1101, which has more flexible properties than PA2200. Since this approach propose creation of a new material, this would result in an experimental process where the correct amount of each material has to be determined in order to obtain flexible behavior in addition to conducting properties. This mixture must also constitute an approved composition that can be safety classified for this type of application. This solution was not implemented in this work due to lack of time but is of interest for future studies.

From the results of the simulations of the updated geometry, it can be seen that the geometry with the sharp edge was much stiffer and required higher pressure compared to the geometry without the sharp edge. However, a sharper geometry will result in a simplified production process which is important to consider, especially when it comes to small dimensions which is present in this EMI- shield. The geometry without the sharp edge was however chosen because of the increase in flexibility that this geometry offers.

7.3 EMC In both EMI experiments, the total attenuation is plotted in dB on the Y axis and the frequency is plotted on the X axis. In experiment 1, it is the difference between the X axis and the graph that indicates the shielding efficiency. It can be seen that concept C, which is the Alumide shield, shows the least amount of attenuation for the majority of the frequency spectrum. Concept D showed an increase in attenuation compared to Alumide, but it performed worse than the original shield. Concept E showed similar efficiency to the current solution, especially at higher frequencies. This result is also in compliance with the theory that states that thin shields attenuate more at higher frequencies. However, it can be seen that the difference in attenuation is relatively small, even compared to the data were no shield was used. This result indicates that there are some other components on the PCB which influence the measurement. In order to eliminate possible background variables, another test setup should be used. Instead of the PCB, a solid metal plate with no electrical components could be used. It will entail that the actual shielding effectiveness of the shield could be measured in isolation without any disturbances. The result of the second experiment displays similar results as the first. However, the shielding efficiency of Alumide and ZYYX are similar in performance and it is hard to differentiate them. Concept E showed similar performance as the current shield and once again, the performance increased with the frequency. It can be concluded that the results for the experiment without a shield shows similar results to all concepts as well, therefore it is hard to distinguish any prominent difference. According to engineers at the company the difference between the results with no shield and the results with the original shield should be somewhere around 100 dB. As a conclusion of this, it could be argued that the experimental test results are affected by other elements within the PCB. Again another experimental setup should be used in order to eliminate external factors and increase the internal validity of the experiment.

The coating of the shield in concept E is made of copper and as mentioned in section 6.4, copper has a tendency to oxidize in contact with oxygen. This can cause surface irregularities and thus loss of shielding. In order to determine if the materials can withstand temperature, moisture and vibrations, further environmental tests must be performed. It is also of interest to test further coating materials in future studies to create a wider choice of potential materials based on the type of application.

8. Conclusion In this thesis work, three concepts were successfully realized using two different AM methods. In concept C and E, the function of the gasket was directly integrated into the shield using the SLS manufacturing process. The design was optimized based on two parameters; the manufacturability and the flexibility of the gasket, which was evaluated and verified using FEM analysis. In concept D, two carbon composites were integrated using the FDM manufacturing process. The functionality of these concepts was later tested using an experimental setup with a similar environment as the current application. Concept E showed the most promising EMI results, whereas the other concepts showed less promising results. Despite this, additional experiments have to be conducted in order to evaluate and determine a more realistic result of the shielding effectiveness of each material. As a conclusion, this report presents three concepts that integrate the functions of an EMI shield in additively manufactured components where one concept yielded potential results which are worth investigating further in future studies.

The integration of both EMI absorbers and gaskets are not possible with the technologies found in this research. There are several promising AM technologies for printing elastic materials but the electrical conductivity in these materials is limited and are therefore not considered suitable as EMI gaskets. For the cavity absorber, a magnetic rubber material is usually desired and no such material was identified for any AM process. However, there are several plastic materials with dielectric properties, like Alumide or ZYYX. Additional dielectric materials could therefore be tested to determine the attenuation of several composite materials. Further on, the potential of integrating gasket geometry directly into the shield with AM was shown and the gasket was successfully printed in two different materials.

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APPENDIX 1 - Simulations of original shield

Materials: Aluminum

Conducted analyzes: directional deformation, equivalent stress, equivalent elastic- and plastic strains

Applied force: 1680 N

Results of directional deformation (mm): Results of equivalent stress (von Mises) (MPa):

Results of equivalent elastic strains (mm/mm):

Simulation of shield with original geometry Material Max value - directional Max value - Max value – Max value – deformation equivalent stress equivalent elastic equivalent plastic strains strains Aluminum 0,739 mm 790,090 MPa 0,011 -

APPENDIX 2 – simulations of gasket number 3: original and updated geometry

Gasket number 3 – original geometry:

Materials: PA1101, Ultem1010, Alumide

Conducted analyzes: directional deformation, equivalent stress, equivalent elastic- and plastic strain

Applied pressure: resulted in a deformation of a point at the top surface of approximately 0,32 mm

Simulation of gasket number 3 Material Directional Corresponding Max value - Max value - Max value – Max value – deformation force directional equivalent equivalent equivalent of top surface deformation stress elastic plastic strains strains PA1101 0,316 mm 0,752 MPa 0,428 mm 63,608 MPa 0,044 0,111 Ultem1010 0,317 mm 0,761 MPa 0,425 mm 67,955 MPa 0,034 0,127 Alumide 0,315 mm 1,026 MPa 0,437 mm 96,649 MPa 0,027 0,131

Gasket number 3 – updated geometry:

Materials: PA1101, Alumide

Conducted analyzes: directional deformation, equivalent stress, equivalent elastic- and plastic strain

Applied pressure: resulted in a deformation of a point at the top surface of approximately 0,32 mm

Optimization of geometry: with sharp edge and without sharp edge

Simulation of gasket with updated geometry – with sharp edge Material Directional Corresponding Max value - Max value - Max value – Max value – deformation of force directional equivalent equivalent equivalent top surface deformation stress elastic plastic strains strains PA1101 0,321 mm 22 MPa 0,626 mm 163,080 MPa 0,105 0,673 Alumide 0,316 mm 35 MPa 0,618 mm 289,640 MPa 0,078 0,592

Simulation of gasket with updated geometry – without sharp edge Material Directional Corresponding Max value - Max value - Max value – Max value – deformation of force directional equivalent equivalent equivalent top surface deformation stress elastic plastic strains strains PA1101 0,320 mm 19 MPa 0,574 mm 133,050 MPa 0,086 0,521 Alumide 0,322 mm 28 MPa 0,521 mm 219,540 MPa 0,059 0,423

APPENDIX 3 – simulations of final shield geometry

Materials: PA1101 and Alumide

Conducted analyzes: directional deformation, equivalent stress, equivalent elastic- and plastic strain

Applied force: 1680 N

Simulation of shield with updated geometry Material Max value - directional Max value - Max value – equivalent Max value – equivalent deformation equivalent stress elastic strains plastic strains PA1101 50,799 mm 332,290 MPa 0,215 1,663 Alumide 21,394 mm 331,570 MPa 0,090 0,698

iii

APPENDIX 4 – Material data & Correction factor for skin depth

Material data:

Material Density Tensile Tensile Yield Elongation Softening Melting Electric Hardness (kg/cm3) modulus strength strength at break temperature temperature conductive (MPa) (MPa) (MPa) (%) (°C) (°C) AlSi10Mg(1) 2670 60 – 410 – 240 – 4 – 6 - 570 – 660 Yes 199 ± 5 75·103 440 265 (HBW) PA1101(2) 990 1600 48 - 30 – 45 - 201 No 75 (shore D) PA2200(3) 930 1650 42 – 48 - 4 – 18 - 176 No 75 (shore D) Alumide(4) 1360 3800 48 - 4 - 176 Yes 76 (shore D) PC(5) 1200 1944 57 40 2,5 – 4,8 139 - No - ULTEM 1270 2200 – 48 – 81 41 – 64 2 – 3,3 214 - No - 1010TM (6) 2770 ZYYX pro- 1200 - 118 - 5 160 - Yes - CarbonTM (7) PI- 1280 12 23 22 >700 - 230 Yes 85 ETPU85TM (8) (shore A) References 1 (EOS ALSi10Mg, 2018); 2 (EOS PA1101, 2018); 3 (EOS PA2200, 2018); 4 (EOS Alumide, 2018); 5 (Stratasys PC, 2019); 6 (Stratasys ULTEM, 2019); 7 (ZYYX proCarbon, 2019) 8 (Palmiga Innovation PI-ETPU85, 2019)

Correction factor for skin depth:

Correction factor B - Copper 12

6 dB

4 Correction factor B

0 0 2 4 6 T/훿

APPENDIX 5 – EMI experiments

Experiment 1 10.00

0.00

-10.00 Original -20.00 PA1101

dB Alumide -30.00 ZYYX -40.00 Without shield

-50.00

-60.00 GHz

Experiment 2 10

-10 Original

-20 PA1101 dB Alumide -30 Without shield -40

-50 GHz