Soldering of Electronics Components on 3D-Printed Conductive Substrates

materials

Article Soldering of Electronics Components on 3D-Printed Conductive Substrates

Bartłomiej Podsiadły *, Andrzej Skalski and Marcin Słoma

Micro- and Nanotechnology Division, Institute of Metrology and Biomedical Engineering, Faculty of Mechatronics, Warsaw University of Technology, 8 sw. A. Boboli st., 02-525 Warsaw, Poland; [email protected] (A.S.); [email protected] (M.S.) * Correspondence: [email protected]

Abstract: Rapid development of additive manufacturing and new composites materials with unique properties are promising tools for fabricating structural electronics. However, according to the typical maximum resolution of additive manufacturing methods, there is no possibility to fabricate all electrical components with these techniques. One way to produce complex structural electronic circuits is to merge 3D-printed elements with standard electronic components. Here, different soldering and surface preparation methods before soldering are tested to ﬁnd the optimal method for soldering typical electronic components on conductive, 3D-printed, composite substrates. To determine the optimal soldering condition, the contact angles of solder joints fabricated in different conditions were measured. Additionally, the mechanical strength of the joints was measured using the shear force test. The research shows a possibility of fabricating strong, conductive solder joints on composites substrates prepared by additive manufacturing. The results show that mechanical cleaning and using

additional ﬂux on the composite substrates are necessary to obtain high-quality solder joints. The most repeatable joints with the highest shear strength values were obtained using reﬂow soldering

Citation: Podsiadły, B.; Skalski, A.; together with low-temperature SnBiAg solder alloy. A fabricated demonstrator is a sample of the Słoma, M. Soldering of Electronics successful merging of 3D-printed structural electronics with standard electronic components. Components on 3D-Printed Conductive Substrates. Materials 2021, Keywords: structural electronics; conductive composites; 3D printing; additive manufacturing; 14, 3850. https://doi.org/10.3390/ solder joint properties ma14143850

Academic Editors: Jacek Ryl and Robert Bogdanowicz 1. Introduction Additive manufacturing, also called three-dimensional printing (3D printing), is a new Received: 7 June 2021 manufacturing trend, allowing the fabrication of physical objects directly from the digital Accepted: 5 July 2021 Published: 9 July 2021 design. It is rapidly and dynamically evolving, in some cases replacing or supplementing conventional manufacturing techniques. 3D printing has many advantages like freeform

Publisher’s Note: MDPI stays neutral design, easy customization, personalization of elements, a significant automatization level, with regard to jurisdictional claims in or a lower waste of materials. Furthermore, it is also widely used in prototyping to reduce published maps and institutional affil- the time and cost of manufacturing new parts or small-quantity productions [1]. Additive iations. manufacturing has been widely applied in many industries like automotive, architecture, fashion, aerospace, biomechanical [2–5]. The term additive manufacturing has a broad meaning, and comprises many techniques that use different materials and equipment, including Stereolithography (SLA), PolyJet, Selective Laser Sintering (SLS), and Fused Deposition Modeling (FDM), with Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the last one being the most popular nowadays [6]. The FDM technique is the trendiest This article is an open access article due to its low-cost equipment, process simplicity, accessibility of low-cost materials, and distributed under the terms and acceptable printing resolution down to 50 µm [7]. Thanks to this, we are able to fabricate conditions of the Creative Commons functional elements that cannot be made with conventional methods, or we can develop Attribution (CC BY) license (https:// personalized elements with specific shapes and colors on customer demand. To achieve creativecommons.org/licenses/by/ even more complex manufacturing using 3D printing, new materials for this technique are 4.0/). continuously being developed.

Materials 2021, 14, 3850. https://doi.org/10.3390/ma14143850 https://www.mdpi.com/journal/materials Materials 2021, 14, 3850 2 of 15

A good example is a new trend in manufacturing called structural electronics. Nowa- days, most electronic devices are still manufactured using the PCB design due to the challenges of integrating new additive technology [8,9]. Structural electronics is a term used to name structures that involve electronic circuits and components in the volume of protective structures, housing elements, etc. [10–12]. Structural electronics can function as electronic circuits and components and provide the optimal mechanical and protective properties of the entire device. Additionally, reducing the volume of a conventionally fabricated device can be done by fusing additive manufacturing and electronics [13]. Due to the rapid development of this branch of manufacturing, there is a need to develop new materials with unique properties that can be used in Additive Manufacturing. Materials with excellent electrical properties like low resistivity or high dielectric constant, excellent mechanical properties, unique magnetic or thermal properties, etc., have to be developed to increase the number of structural electronics applications [14–18]. This paper presents the application of elaborated copper-based conductive filaments with a different polymer matrix for 3D structural electronics fabrication. Such filaments can be directly used in unmodified, low-cost FDM printers to fabricate electrical circuits and other functional conductive elements. To manufacture a sophisticated electrical device, we need to merge additive manufacturing methods with traditional PCB technology. Based on the approaches used so far, the only way to fabricate conductive connections on composite substrates is to use conductive adhesives. However, such connections are characterized by lower shear strength compared to solder joints [19]. Additionally, and crucial in structural electronics applications, conductive adhesives have lower electrical conductivity than solder alloys. This is because conductive adhesives are composite materials containing a non-conductive matrix that provide good adhesive properties. Only the functional phase found in the adhesives can provide electrical conductivity. In the case of solder alloys, they conduct throughout their volume [20–23]. In this paper, soldering tests of fabricated composites were performed to obtain optimal process conditions for solder connections on polymer composites. Two different soldering methods and two types of solder alloys were used in the research: reflow and iron soldering were employed, along with SnPb and low-temperature SnBiAg solder alloys, respectively. The shear strength test of the fabricated solder connections was determined. This kind of research is being reported for the first time in the literature. Additionally, the 3D-printed demonstrator incorporating LED elements is presented.

2. Experimental Procedure 2.1. Materials To fabricate a conductive composite filament for the FDM technique, two materials need to be incorporated: polymer matrix and conductive filler. The bulk copper’s conductivity is 5.96 × 107 S/m, which is only slightly lower than silver—6.14 × 107 S/m. Taking into consideration the significant weight of metal powder necessary to reach the electrical percolation threshold in the composites and the cost of such material, it was decided to choose copper micro powder as a filler. The metal powder was purchased from Makin Metal Powder (Rochdale, Lancashire, United Kingdom). According to the datasheet, the purity of copper powder was above 99%. To examine the average particle size of the filler, particle size analysis was performed on the Malvern machine. Grain size distribution and SEM picture of copper powder are shown in Figure1. The average filler particle diameter was calculated as 57 µm. Materials 2021, 14, x FOR PEER REVIEW 3 of 15

Materials 2021, 14, 3850 3 of 15

FigureFigure 1. Particle 1. Particle size size distribution distribution and and SEM micrographmicrograph of of copper copper powder. powder.

To Todevelop develop composite composite filament filament that that can can be be used used in in FDM FDM printing, printing, three three different pol- polymers were chosen as the matrix material to achieve the best possible processability. All ymers were chosen as the matrix material to achieve the best possible processability. All selected thermoplastic polymers have different properties, but all are used in 3D printing: selectedacrylonitrile thermoplastic butadiene polymers styrene (ABS), have polylacticdifferent acidproperties, (PLA), and but polystyrene all are used (PS). in 3D printing: acrylonitrileTwo differentbutadiene soldering styrene alloys (ABS), were poly usedlactic to acid prepare (PLA), soldering and polystyrene tests. Sn63Pb37 (PS). is a eutecticTwo different alloy. It soldering has a melting alloys temperature were used of to 187 prepare◦C, and soldering it solidifies tests. rapidly Sn63Pb37 at one is a eutectictemperature alloy. It rather has thana melting over the temperature range, which of can 187 take °C, place and forit solidifies non-eutectic rapidly alloys. at In one the tem- peratureexperiment, rather Sn63Pb37 than over alloy the was range, used which in the formcan take of a 1-mm-diameterplace for non-eutectic wire. The alloys. second In the experiment,soldering alloySn63Pb37 used inalloy tests was was used low-temperature in the form bismuth-basedof a 1-mm-diameter soldering wire. paste. The The second solderingOM 520 alloy paste used consists in oftests 42% was tin, 57.6%low-temp bismuth,erature and bismuth-based 0.4% silver. OM soldering 520 solder paste. paste The was used because of its low melting temperature—138 ◦C—allowing it to be used in a low OM 520 paste consists of 42% tin, 57.6% bismuth, and 0.4% silver. OM 520 solder paste temperature soldering process. It is essential for soldering on thermally sensitive substrates wasnot used resistant because to high of its temperatures. low melting temperature—138 °C—allowing it to be used in a low temperature soldering process. It is essential for soldering on thermally sensitive sub- strates2.2. Conductivenot resistant Composite to high Fabrication temperatures. Metal–polymer composite filaments can be fabricated using two approaches. The first 2.2.way Conductive is the thermal Composite mixing Fabrication of polymers with metal powder. The polymer is plasticized at highMetal–polymer temperature and composite mixed with filaments metal powder can tobeobtain fabricated a homogeneous using two composite. approaches. This The method has disadvantages, because to achieve high material homogeneity, the polymer first way is the thermal mixing of polymers with metal powder. The polymer is plasticized needs to be melted at a high temperature for a long time. This process causes the thermal at highdegradation temperature of the and polymer, mixed leading with tometal the deteriorationpowder to obtain of the a composite’s homogeneous mechanical composite. Thisproperties. method has The disadvantages, second method usedbecause in this to achieve research high is a two-stage material solventhomogeneity, assisted the pro- poly- mercessing needs method.to be melted The first at stagea high of temperature this method isfor dissolving a long time. the polymer This process in a suitable causes the thermalsolvent. degradation After that, theof the metal polymer, filler is added leading and to mixed the deterioration with the polymer. of the The composite’s last step is mechanicalthe evaporating properties. of theThe solvent. second This method method used allows in this minimizing research the is thermala two-stage degradation solvent as- sistedof the processing polymer duringmethod. the The fabrication first stage of the of composite.this method After is dissolving solvent evaporation, the polymer the in a suitableobtained solvent. composite After is that, pelletized. the metal A single-screw filler is added extruder and mixed machine with was the used polymer. to fabricate The last filaments from prepared composite pellets. Composites were extruded in a hot mixing step is the evaporating of the solvent. This method allows minimizing the thermal degra- extrusion process. Most popular 3D printers use filaments with 1.75 mm diameter. Our dationlaboratory of the haspolymer extruded during filaments the fabrication with diameter of the values composite. ranging fromAfter 1.7 solvent mm to evaporation, 1.8 mm, thewhich obtained is acceptable composite for useis pelletized. in standard, A non-modified single-screw FDM extruder printers. machine A detailed was description used to fabri- cateof filaments the fabrication from ofprepared copper-based composite conductive pellets. filaments Composites and the were evaluation extruded of electricalin a hot mix- ingproperties extrusion with process. the percolation Most popular threshold 3D calculationprinters use has filaments been presented with in1.75 our mm previous diameter. Ourwork laboratory [24]. has extruded filaments with diameter values ranging from 1.7 mm to 1.8 mm, which is acceptable for use in standard, non-modified FDM printers. A detailed description of the fabrication of copper-based conductive filaments and the evaluation of electrical properties with the percolation threshold calculation has been presented in our previous work [24]. Materials 2021, 14, 3850 4 of 15

For the soldering tests, composites with the optimal amount of copper powder, ABS/Cu with 81.8 wt.%, PLA/Cu with 82.5 wt.% and PS/Cu with 85.5 wt.% of copper powder were used, respectively. All composite elements were printed on the CTC Bizer 2X 3D printer. Layer height was set to 0.3 mm, table temperature to 80 ◦C, and printing speed to 40 mm/s. The nozzle temperature value was different for each composite, depending on polymer matrix—for ABS/Cu composite, printing nozzle temperature was set to 250 ◦C, for PLA/Cu composite to 160 ◦C, and for PS/Cu composite to 150 ◦C.

2.3. Soldering Methods Two methods of soldering were tested—hot iron soldering and reflow soldering. Hot iron soldering is a soldering method characterized by low repeatability but is widely used in prototyping, small-volume production, and service repair. It is the most common and easiest method of soldering. To fabricate the solder joint, soldering iron generates the heat required to increase the temperature of the soldered elements and circuit pads, and melt the solder. On the other hand, reflow soldering is a common technique used in mass production and can be used to fabricate a large number of soldered connections at once. To produce solder connections with this method, surface-mount components and soldering paste are placed on the substrate and inserted into the reflow oven. The most important feature of this method is that the substrate and solder paste slowly heats up to a temperature slightly above the solder alloy’s melting point.

2.4. Composite Surface Preparation Methods The research results described in this article are divided into four parts. This is due to the use of two types of soldering alloys (SnPb and SnBiAg) and two methods of fabrication of soldered joints (hand soldering and reﬂow soldering). Regardless of the chosen solder and soldering method, the research was carried out on three types of composite substrates- ABC/Cu, PLA/Cu, and PS/Cu. Composite substrates were prepared for the soldering process in ﬁve different ways, according to Table1. This allows us to determine the impact of the composite surface preparation methods on the quality of soldered joints.

Table 1. List of preparation methods of the composite surfaces before soldering.

Surface Section ABCDE Symbol Surface RF800 ﬂux Surface Surface mechanically No surface A method of mechanically applied on chemically cleaned with sandpaper preparation before surface preparing cleaned with 400 surface before cleaned with and RF800 ﬂux applied soldering grit sandpaper soldering solvent before soldering

3. Results and Discussion 3.1. Solder Wettability Test Wettability is the ability of a material, typically a liquid, to spread over another material. It is crucial for soldering because it plays an essential role in providing a good connection between the solder element and the substrate. One of the solder wettability measurements on different substrates is measuring the contact angle (θ). It has been proven that the smaller the value of the contact angle, the better the wettability [25]. Good wettability has a direct impact on solder joint properties, like mechanical strength and electrical resistance. The contact angle depends on many factors, like the soldering method, soldering alloy, substrate surface preparation, etc. In the ﬁrst part of this research, the SnPb solder alloy and hot iron soldering method were applied. Standard SMD 1206 resistors were used throughout the research, with dimensions as shown in Figure2. Materials 2021, 14, x FOR PEER REVIEW 5 of 15 Materials 2021, 14, x FOR PEER REVIEW 5 of 15 Materials 2021, 14, 3850 5 of 15

Figure 2. The schematic of SMD resistor with dimensions. FigureFigure 2.2. TheThe schematicschematic ofof SMDSMD resistorresistor withwith dimensions.dimensions. HotHot ironiron soldering soldering is is characterized characterized by by the the fact fact that that the the temperature temperature of theof the substrate substrate is Hot iron soldering is characterized by the fact that the temperature of the substrate increasedis increased only only locally locally (at (at the the area area where where the th jointe joint is formed), is formed), and and the the process process itself itself can can be is increased only locally (at the area where the joint is formed), and the process itself can carriedbe carried out out very very quickly, quickly, within within a few a few seconds seconds (around (around 4–6 4–6 s), s), which which reduces reduces the the impact impact be carried out very quickly, within a few seconds (around 4–6 s), which reduces the impact ofof highhigh temperature on on the the composite composite substrat substratee reducing reducing the the risk risk of ofthermal thermal damage. damage. On of high temperature on the composite substrate reducing the risk of thermal damage. On Onthe theother other hand, hand, SnPb SnPb solder solder alloy alloy makes makes it necessary it necessary to use to a use soldering a soldering temperature temperature value the other hand, SnPb solder alloy makes it necessary to use a soldering temperature value valuemuch much higher higher than the than softening the softening temperature temperature of polymers of polymers used in used composites. in composites. This is This due much higher than the softening temperature of polymers used in composites. This is due isto due the relatively to the relatively high melting high melting temperature temperature of the SnPb of the soldering SnPb soldering alloy—187 alloy—187 °C. Observa-◦C. to the relatively high melting temperature of the SnPb soldering alloy—187 °C. Observa- Observationstions show that show it is that possible it is possible to make tosuch make connections such connections thanks to thanks the mechanical to the mechanical pressing tions show that it is possible to make such connections thanks to the mechanical pressing pressingof the solder of the particles solder particles into the intocomposite the composite structure. structure. On the other On the hand, other there hand, was there no waspos- of the solder particles into the composite structure. On the other hand, there was no pos- nosibility possibility of fabricating of fabricating repetitive repetitive solder solder joints. joints. The process The process of hot of iron hot ironsoldering soldering with with high sibility of fabricating repetitive solder joints. The process of hot iron soldering with high hightemperature temperature causes causes further further damage damage to each to each composite composite (Figure (Figure 3). In3). the In thecase case of these of these con- temperature causes further damage to each composite (Figure 3). In the case of these con- connections,nections, it is itimpossible is impossible to determine to determine the cont theact contact angle. angle.As a “rule As aof “rule thumb”, of thumb”, the selection the nections, it is impossible to determine the contact angle. As a “rule of thumb”, the selection selectionof the solder of the joints solder for joints the further for the test further was testbased was on based the preliminary on the preliminary mechanical mechanical strength. of the solder joints for the further test was based on the preliminary mechanical strength. strength.If resistors If resistorswere attached were attachedto the composite to the composite substrate substrate and did not and detach did not during detach transpor- during If resistors were attached to the composite substrate and did not detach during transpor- transportation,tation, additional additional experiments experiments were perform were performed.ed. The soldered The soldered joints jointsmade madeon all on three all tation, additional experiments were performed. The soldered joints made on all three threetypes types of composite of composite substrates substrates fulfilled fulﬁlled this ru thisle. However, rule. However, it should it should be noted be that noted the that ob- types of composite substrates fulfilled this rule. However, it should be noted that the ob- thetained obtained soldered soldered joints joints have havea very a irregular very irregular and unrepeatable and unrepeatable shape. shape. The substrate’s The substrate’s prep- tained soldered joints have a very irregular and unrepeatable shape. The substrate’s prep- preparationaration method method does doesnot affect not affect the quality the quality of the ofjoints, the joints,which whichis caused is causedby partial by damage partial aration method does not affect the quality of the joints, which is caused by partial damage damageto the composite to the composite substrate substrate during the during soldering the soldering process. process. to the composite substrate during the soldering process.

Figure 3. Soldered joints on different composites (from left: ABS/Cu, PS/Cu, PLA/Cu) fabricated FigureFigure 3.3. SolderedSoldered joints on different composites (from left: ABS/Cu ABS/Cu,, PS/Cu, PS/Cu, PLA/Cu) PLA/Cu) fabricated fabricated with hot iron soldering method and Sn63Pb37 solder alloy. withwith hothot ironiron solderingsoldering methodmethod andand Sn63Pb37Sn63Pb37 soldersolder alloy.alloy. Materials 2021, 14, x FOR PEER REVIEW 6 of 15

Materials 2021, 14, 3850 6 of 15

In the case of the SnPb alloy, it was not possible to perform reflow soldering on com- positeIn substrates. the case of In the our SnPb conductive alloy, it composites, was not possible ABS, PLA, to perform and PS reflow are used soldering as matrix. on compositeThese polymers substrates. have In a ourmuch conductive lower melting composites, temperature ABS, PLA, than and the PS melting are used point as matrix. of the Thesesolder polymers alloy. In havecontrast a much to hot lower iron melting soldering, temperature where a thanhigh thetemperature melting point is applied of the solder selec- alloy.tively In at contrastthe spot towhere hot ironthe soldered soldering, joint where is ma ade, high it temperatureis necessary to is heat applied the entire selectively volume at theof the spot substrate where the during soldered reflow joint soldering is made, in it a is convection necessary tooven heat in the which entire the volume elements of theare substratesoldered. during Therefore, reflow composite soldering samples in a convection in the convection oven in which oven heated the elements to the temperatures are soldered. Therefore,above the melting composite point samples of the inPbSn the solderin convectiong alloy oven are heated destroyed to the what temperatures unable to prepare above thethe meltingsoldered point joints of using the PbSn this technique. soldering alloy are destroyed what unable to prepare the solderedAnother joints type using of this soldering technique. alloy used during the research was the low-temperature solderingAnother alloy type SnBiAg. of soldering In contrast alloy to used the duringpreviously the researchconsidered was PbSn the low-temperaturealloy, the melting solderingtemperature alloy of SnBiAg.the soldering In contrast alloy SnBiAg to the previously is lower: 138 considered °C. The use PbSn of a alloy, low-temperature the melting temperaturesoldering alloy of the makes soldering it possi alloyble SnBiAgto significantly is lower: reduce 138 ◦C. the The soldering use of a low-temperaturetemperature and solderinghas the advantage alloy makes of reducing it possible the to risk significantly of damage reduce to composite the soldering substrates temperature during andboth hassoldering the advantage processes. of reducing the risk of damage to composite substrates during both solderingAgain, processes. attempts have been made to solder SMD resistors to composite substrates using twoAgain, soldering attempts techniques–hot have been made iron to and solder reflo SMDw soldering. resistors toTests composite show that substrates using the using hot twoiron solderingmethod with techniques–hot low-temperature iron andSnBiAg reflow soldering soldering. paste Tests makes show it impossible that using to the obtain hot ironsatisfying method results. with low-temperatureThe preparation method SnBiAg solderingof the composite paste makes surface it impossiblehas no direct to impact. obtain satisfyingThe wetting results. of the The composite preparation surface method is not ofgood the enough, composite and surface solder hasalloy no sticks direct to impact. the hot Theiron wetting preventing of the the composite formation surface of the issolder not gooding joint. enough, The surface and solder energy alloy ofsticks fusedto metal the hotalloys iron is preventing not compatible the formation with the ofsurface the soldering energy of joint. polymer The surface substrate, energy creating of fused obstacles metal alloysin the isproper not compatible wetting of with thethe alloys surface on pr energyinted ofcomposited polymer substrate, during the creating soldering obstacles process in the[26]. proper The greater wetting the of substrate the alloys surface on printed energy, composited the better during the wettability. the soldering The process surface [26 en-]. Theergy greater of polymers the substrate is approximately surface energy, two orders the better of magnitude the wettability. lower The than surface metals energy [27–29]. of polymersThose material is approximately properties result two orders in significantly of magnitude better lower wetting than of metals the soldering [27–29]. iron Those tip materialthan the propertiescomposite resultsubstrate in significantly and make it better impo wettingssible to offabricate the soldering solder ironjoints. tip Soldering than the compositeattempts were substrate carried and out make on all it three impossible composit to fabricatees without solder significant joints. positive Soldering results attempts (Fig- wereure 4). carried out on all three composites without significant positive results (Figure4).

FigureFigure 4.4. ResultsResults of of hot hot iron iron soldering soldering with with low-temp low-temperatureerature SnBiAg SnBiAg solder solder paste paste on different on different compositescomposites (from(from top:top: PLA/Cu,PLA/Cu, ABS/Cu, Ps/Cu). Ps/Cu). Materials 2021, 14, x FOR PEER REVIEW 7 of 15 Materials 2021, 14, 3850 7 of 15

Reflow soldering makes it possible to use the temperature closest to the melting point Reflow solderingof soldering makes paste. it possibleThanks to to that, use the the lowe temperaturest possible closest temperature to the necessary melting to obtain a point of solderingsoldering paste. connection Thanks to is that,used thein this lowest process. possible Moreover, temperature additional necessary heating of to the substrate obtain a solderingon which connection the solder is used connection in this process. is fabricated Moreover, helpsadditional improve the heating wetting of theof the soldering substrate on whichalloy. the The solder soldering connection process was is fabricated prepared helpsin the improvereflow oven, the wettingand the reflow of the temperature soldering alloy.profile The soldering is shown processin Figure was 5. Low-temperat prepared in theure reflow SnBiAg oven, paste and was the applied reflow with the dis- temperature profilepenser is shownto ensure in Figurethe same5. Low-temperature volume of paste SnBiAgis applied paste on waseach applied sample. with Thanks to using the dispenser tolow-temperature ensure the same soldering volume paste, of paste the is maximu appliedm ontemperature each sample. used Thanks during tosoldering was using low-temperatureset to 140 soldering °C, which paste, is a temperature the maximum that temperature does not cause used significant during soldering damage to the com- ◦ was set to 140 positeC, which substrates is a temperature used. that does not cause significant damage to the composite substrates used.

Figure 5.FigureTemperature 5. Temperature proﬁle usedprofile in used reﬂow in reflow soldering soldering process process in a convection in a convection oven. oven.

Tests of reflowTests soldering of reflow have soldering shown have that itshown is possible that it is to possible make repeatable to make repeatable joints joints sol- soldered on alldered three compositeon all three substrates composite using substrates the SnBiAg using low-temperaturethe SnBiAg low-temperature solder alloy solder alloy with the reflowwith soldering the reflow technique. soldering All technique. SMD resistors All SMD soldered resistors to composite soldered to substrates composite substrates were attached towere the substrate,attached to so the they substrate, were taken so they into were account taken in furtherinto account tests ofin mechanicalfurther tests of mechan- properties of solderedical properties joints. of soldered joints. After reflow solderingAfter reflow tests, soldering we noticed tests, that we PS/Cu noticed composite that PS/Cu substrates composite had substrates been had been deformed in thedeformed process in (Figure the process6). Other (Figure composites 6). Other composites have not shown have not such shown behavior. such behavior. The The deformationdeformation of PS/Cu of elements PS/Cu elements can be duecan be to due the to internal the internal stresses stresses in the in material.the material. Thermal Thermal stressesstresses can emerge can emerge after the after printing the printing process process and be and relieved be relieved in the process in the process of slow of slow heat- heating up. Polystyreneing up. Polystyrene has a lower has Young’s a lower Young’s modulus modulus compared compared to PLA to and PLA ABS. andThis ABS. This causes causes PS/Cu compositesPS/Cu composites to be theto be easiest the easiest to process, to process, especially especially at higher at higher temperatures. temperatures. Internal Internal stressesstresses occurring occurring in the in material the material can deform can deform it while it while relieving. relieving. To avoid To avoid such such defor- deformations duringmations the during soldering the soldering process onprocess polystyrene on polystyrene matrix compositematrix composite substrates, substrates, selec- selective solderingtive soldering may be necessary.may be necessary. During During the reflow the reflow soldering soldering process, process, the entire the entire volume ◦ volume of the sampleof the sample was heated was heated to about to about 140 C,140 causing °C, causing its deformation. its deformation. The The use ofuse of another another selectiveselective soldering soldering method, method, such as such wave as soldering wave soldering or laser or soldering, laser soldering, should should make make it pos- it possible to obtainsible to soldered obtain soldered joints without joints without damaging damaging the substrate. the substrate. Another Another possible possible way to way to avoid suchavoid deformations such deformations may be may to optimize be to optimize the 3D the printing 3D printing process process with PS/Cuwith PS/Cu compo- composite to preventsite to prevent an accumulation an accumulation of thermal of thermal stresses stre on onesses sideon one of theside element. of the element. This This opti- optimization canmization be achieved, can be achieved, for example, for example, by randomly by randomly choosing choosing the place the where place where the the depo- deposition of thesition layer of starts.the layer At starts. this moment, At this moment, the fabrication the fabrication of every of layer every begins layer inbegins the in the same same X-Y spot.X-Y This spot. causes This an causes accumulation an accumulation of stress of on stress one side on one of the side element. of the element. With each With each layer layer started instarted a different in a spot, different deformation spot, deformation should not should appear not due appear to the due even to distribution the even distribution of of stress throughoutstress thethroughout element. the element. Materials 2021, 14, 3850 8 of 15 Materials 2021, 14, x FOR PEER REVIEW 8 of 15

FigureFigure 6.6. Soldered joint joint fabricated fabricated with with reflow reﬂow soldering soldering on on different different composites: composites: (a) ABS/Cu, (a) ABS/Cu, (b) PLA/Cu, (c) PS/Cu. (b) PLA/Cu, (c) PS/Cu.

WhileWhile thethe useuse ofof aa low-temperaturelow-temperature SnBiAgSnBiAg solderingsoldering alloyalloy andand reflowreflow soldering soldering mademade itit possiblepossible toto obtainobtain solderedsoldered jointsjoints withwith satisfyingsatisfying qualityquality andand highhigh repeatability,repeatability, contactcontact angleangle measurementsmeasurements werewere carriedcarried outoutdepending depending onon the the preparation preparation method method of of thethe composite composite surface. surface. TheThe contactcontact angle angle measurement measurement resultsresults show show that that the the appropriate appropriate preparationpreparation ofof thethe surface surface is is necessary necessary to to achieve achieve good good quality quality solder solder connections. connections. ItItis is notednoted thatthat thethe mechanicalmechanical cleaningcleaning ofof thethe surfacesurface hashas a a significant significant impact impact on on the the contact contact angle.angle. ItIt isis necessarynecessary toto soldersolder toto thethe coppercopper grainsgrains inin the the composite, composite, not not to to the the polymer polymer matrix,matrix, in in order order to to formform aa correctlycorrectly soldered soldered joint. joint.The Theprepared prepared samples samples are are characterized characterized byby thethe factfact thatthat duringduring thethe printing printing process, process, they they are are covered covered with with a a layer layer of of polymer polymer that that mustmust be be removed removed to to expose expose as as much much of of the the copper copper grains grains as possibleas possible in the in placethe place where where the solderedthe soldered joint joint is formed. is formed. After After the mechanical the mechanical removal removal of polymer of polymer film, thefilm, solder the solder alloy canalloy be can soldered be soldered to the copperto the copper particles particles situated situated below. below. This is This clear is for clear the for measured the measured angle valuesangle forvalues samples for samples A and D. A Moreover, and D. Moreover, using mechanical using mechanical cleaning together cleaning with together additional with fluxadditional allows fittingflux allows the criteria fitting of the acceptable criteria of wetting acceptableθ < 55 wetting◦, according θ < 55°, to Klein-Wassinkaccording to Klein- [30]. TheWassink differences [30]. The in thedifferences observed in wetting the observed angle valueswetting depending angle values on depending the composite on the surface com- preparationposite surface are preparation presented in are Table presented2 on the in example Table 2 ofon ABS/Cu the example composite of ABS/Cu substrates. composite substrates.

Table 2. Contact angle measurements results of SnBiAg solder correlated with the surface preparation of ABS/Cu composite.

Surface Sec- A B C D E tion Symbol A Method Surface mechani- RF800 flux applied Surface chemically Surface mechanically No surface prepara- of Surface cally cleaned with on surface before cleaned with sol- cleaned with sandpa- tion before solder- Preparing 400 grit sandpaper soldering vent per and RF800 flux ing Materials 2021, 14, 3850 9 of 15

Table 2. Contact angle measurements results of SnBiAg solder correlated with the surface preparation of ABS/Cu composite.

Surface Section MaterialsMaterialsMaterialsMaterialsMaterialsMaterials 2021 20212021 2021,,2021 , ,14 14202114,, , ,x x, x 14x 14FOR FOR, FOR FOR14,, xx, FOR xFORPEER PEER PEER PEERFOR PEERPEER REVIEW REVIEWPEER REVIEWREVIEWABCDE REVIEWREVIEW REVIEW 999 ofofof 99 15 15159ofof of 1515 15 Symbol Surface RF800 ﬂux Surface Surface mechanically No surface A Method of mechanically applied on chemically cleaned with sandpaper preparation Surface Preparing cleaned with 400 surface before cleaned with andappliedappliedapplied RF800appliedappliedapplied before before ﬂuxbefore beforebefore appliedbefore sol- sol-sol- sol-sol- sol- applied before sol- before soldering grit sandpaper soldering solvent beforederingderingdering solderingderingderingdering

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Measured MeasuredMeasuredMeasuredMeasuredMeasuredMeasured 57◦ 74◦ 78◦ 50◦ 83◦ ContactAngle ContactContactContactContactContactContact 57°57°57° 57° 57° 57° 74°74°74° 74° 74° 74° 78°78°78° 78° 78° 78° 50°50°50° 50° 50° 50° 83°83°83° 83° 83° 83° AngleAngleAngleAngleAngleAngle This is the first time such experiments measuring the contact angle on conductive compositesThisThisThisThisThis is Thisisis have the the theisis is the firstthe firstbeenfirstthe firstfirst time firsttimetime performed. timetime timesuch suchsuch suchsuch suchexperiments experimentsexperiments The experimentsexperiments experiments literature measuring measuringmeasuring measuring doesmeasuring measuring not the thethe provide the contactthe contactcontactthe contactcontact contact similar angle angleangle angleangle angle wettingon onon conductive onconductiveonconductive on conductiveconductive angleconductive resultscompositescompositescompositescompositescompositescomposites for composite have havehave havehave have been beenbeen substrates,beenbeen been performed. performed.performed. performed.performed. performed. so itThe TheThe is The The necessary literat literatTheliterat literatliterat literatureureure ure uretodoes doesdoesure refer doesdoes doesnot notnot to not notprovide provide standardprovidenot provideprovide provide similar similarsimilar metal similarsimilar similar wetting substrates.wettingwetting wettingwetting wetting angle angleangle Itangleangle angle mustresultsresultsresultsresultsresults alsoresults for forfor be compositefor forcompositecomposite for noticed compositecomposite composite that substrates, substrates,substrates, substrates, usingsubstrates, substrates, Pb-free so soso it it soitso is soisis it solderingitnecess necess necessit isis is necessnecess necessaryaryary alloysary toary totoary refer refer refertoto to alwaysreferrefer refer to toto standard standard standardtoto to resultsstandardstandard standard metal metalmetal in metal highermetal metal substrates. substrates.substrates. substrates.substrates. contactsubstrates. It ItIt ItIt It anglemustmustmustmustmust must valuesalso alsoalso alsoalso be bealsobe compared noticed benoticednoticedbe be noticednoticed noticed that thatthat with that that using usingthatusing SnPb usingusing using Pb-free Pb-freePb-free soldering Pb-freePb-free Pb-free solder soldersolder solder alloyssolder solderinginging ing alloysing [alloysalloys31ing – alloysalloys33 alloys always]. alwaysalways Another alwaysalways always results resultsresults conditionresultsresults results in inin higher higher higherinin in thathigherhigher higher contact contact madecontact contactcontact contact itangleangleangle challengingangleangleangle values valuesvalues valuesvalues values compared compared tocompared fabricatecomparedcompared compared with withwith good-quality withwith SnPbwith SnPbSnPb SnPbSnPb SnPb soldering solderingsoldering solderingsoldering soldersoldering alloys alloysalloys joints alloysalloys alloys [31–33]. [31–33].[31–33]. on [31–33].[31–33]. polymer[31–33]. Another AnotherAnother AnotherAnother Another composite condition conditioncondition conditioncondition condition substrate that thatthat that that made madethatmade ismademade made that higher soldering temperatures positively influence contact angle values [34,35]. In the ititit challenging challengingchallengingitit it challengingchallenging challenging to toto fabricate fabricatetofabricateto to fabricatefabricate fabricate good-quality good-qualitygood-quality good-qualitygood-quality good-quality solder soldersolder soldersolder solder joints jointsjoints jointsjoints joints on onon polymer onpolymeronpolymer on polymerpolymer polymer composite compositecomposite compositecomposite composite substrate substratesubstrate substratesubstrate substrate is isis isis is presented research, the lowest possible soldering temperature was used in order to not thatthatthatthat thathigher higherhigherthat higherhigher higher soldering solderingsoldering solderingsoldering soldering temperatures temperaturestemperatures temperaturestemperatures temperatures positively positivelypositively positivelypositively positively influence influenceinfluence influenceinfluence influence contact contactcontact contactcontact contact angle angleangle angleangle angle values valuesvalues valuesvalues values [34,35]. [34,35].[34,35]. [34,35].[34,35]. [34,35]. In InIn the thethe InIn In the the the damage the composite substrate. Contact angle values from 30◦ to 58◦ are reported in the presentedpresentedpresentedpresentedpresentedpresented research, research,research, research,research, research, the thethe the lowestthe lowest lowestthe lowestlowest lowest possible possiblepossible possiblepossible possible sol solsol dering deringsolderingsol solderingderingdering temperature temperaturetemperature temperaturetemperature temperature was waswas waswas used wasusedused usedused used in inin order orderinorderin in orderorder order to toto not nottonotto to not not not literature for bismuth-based soldering paste, Cu substrate and optimal soldering tempera- damagedamagedamagedamagedamagedamage the thethe the compositethe compositecomposite the compositecomposite composite substrate. substrate.substrate. substrate.substrate. substrate. Contact ContactContact ContactContact Contact angle angleangle angleangle angle values valuesvalues valuesvalues values from fromfrom fromfrom from30° 30°30° 30°to 30° toto 30° 58° 58° 58°toto to 58° are58° areare 58° arereported are reportedreported are reportedreported reported in inin the the theinin in the the the ture [32,36,37]. Values in this range were obtained on all tested composite substrates after literatureliteratureliteratureliteratureliteratureliterature for forfor bismuth-basedfor forbismuth-basedbismuth-based for bismuth-basedbismuth-based bismuth-based soldering solderingsoldering solderingsoldering soldering paste, paste,paste, paste,paste, paste, Cu Cu CuCu substrateCu substrate Cusubstratesubstrate Cu substratesubstrate substrate and and andand and and optimal optimal optimaloptimaland optimaloptimal optimal soldering soldering solderingsoldering solderingsoldering soldering temper- temper- temper-temper- temper-temper- temper- appropriate substrate preparation (A and D). However, it was observed that the application atureatureatureatureature ature[32,36,37]. [32,36,37].[32,36,37]. [32,36,37].[32,36,37]. [32,36,37]. Values ValuesValues ValuesValues Values in inin this this this inin in this thisrange range rangethis rangerange range were werewere werewere wereob obobtainedtained tained obob obtainedtainedtained on onon all onallallon on tested tested alltestedall all testedtested tested composite compositecomposite compositecomposite composite substrates substratessubstrates substratessubstrates substrates after afterafter afterafter after of mechanical cleaning and suitable flux, with chemical cleaning of the soldered substrate, appropriateappropriateappropriateappropriateappropriateappropriate substrate substratesubstrate substratesubstrate substrate preparation preparationpreparation preparationpreparation preparation (A (A(A and (Aand(Aand (A and and D). D).andD). D). However,D). However,However, D). However,However, However, it itit was waswas itit it waswas observed observedwasobserved observedobserved observed that thatthat that that the thethatthe the applica-the applica-applica- the applica-applica- applica- reduced the average contact angle for reflow soldering significantly. It also resulted in the tiontiontiontiontion of oftionof mechanical mechanicalmechanical ofof of mechanicalmechanical mechanical cleaning cleaningcleaning cleaningcleaning cleaning and andand and andsuitable suitablesuitableand suitablesuitable suitable flux, flux,flux, flux,flux, with flux, withwith withwith chemical with chemicalchemical chemicalchemical chemical cleaning cleaningcleaning cleaningcleaning cleaning of ofof the thethe ofof of thesoldered the solderedsoldered the solderedsoldered soldered sub- sub-sub- sub-sub- sub- highest-quality solder joints that fit the criterion of acceptable wetting mentioned earlier. It strate,strate,strate,strate,strate,strate, reduced reducedreduced reducedreduced reduced the thethe theaverage the averageaverage the averageaverage average contact contactcontact contactcontact contact angle angleangle angleangle angle for forfor refl for reflforrefl for ow reflreflowow refl owsoldering ow solderingsolderingow solderingsoldering soldering significantly. significantly.significantly. significantly.significantly. significantly. It ItIt also alsoalso ItIt It alsoalso resulted resultedalsoresulted resultedresulted resulted is worth mentioning that the contact angle value does not vary significantly with different ininin the theintheinin the highest-qualitythe highest-quality highest-qualitythe highest-qualityhighest-quality highest-quality solder soldersolder soldersolder solder joints jointsjoints jointsjoints joints that thatthat that that fit fitthatfit the thefitthefit fit the criterionthe criterion criterionthe criterioncriterion criterion of ofof acceptable acceptableofacceptableof of acceptableacceptable acceptable wetting wettingwetting wettingwetting wetting mentioned mentionedmentioned mentionedmentioned mentioned polymer matrix materials in the composite substrate. The most important is the process of earlier.earlier.earlier.earlier.earlier.earlier. It ItIt is isis It Itworth worth worth It isis is worthworth worth mentioning mentioningmentioning mentioningmentioning mentioning that thatthat that thatthe thethethat thecontac the contaccontac the contaccontac contactt tangle angleanglett angleanglet angle value valuevalue valuevalue value does doesdoes doesdoes not does notnot notvary not varyvary not varyvary significantly vary significantlysignificantly significantlysignificantly significantly with withwith withwith with surfaceearlier. preparationIt is worth mentioning before the soldering.that the contac Tablet 3angle presents value the does average not vary contact significantly angle values with differentdifferentdifferentdifferent polymerpolymer polymer polymer matrixmatrix matrix matrix materialsmaterials materials materials inin the thein in the compositecomposite the composite composite substrate.substrate. substrate. substrate. TheThe The most mostThe most most importantimportant important important isis thethe is is the the dependingdifferentdifferentdifferent polymer polymer on polymer the compositematrix matrix matrix materials materials typematerials and in in the surfacethein compositethe composite composite preparation substrate. substrate. substrate. method. The The Themost most Each most important important average important value is is the the is the processprocessprocessprocess ofof surface surfaceof of surface surface preparationpreparation preparation preparation beforebefore before before thethe the solsol the dering.dering.sol soldering.dering. TableTable Table Table 33 presentspresents 3 3presents presents thethe the averageaverage the average average contactcontact contact contact wasprocessprocess calculatedprocessprocess of of surface surfaceof of fromsurface surface preparation preparation the preparation measurementpreparation before before before before resultsthe the solthe sol thedering. of dering.sol sol 10dering.dering. samples. Table Table Table Table 3 3 presents presents 3 3presents presents the the averagethe average the average average contact contact contact contact angleangleangleangleangleangle values valuesvalues valuesvalues values depending dependingdepending dependingdepending depending on onon the ontheonthe on thecomposite the compositecomposite the compositecomposite composite type typetype typetype and andtype andand and and surface surface surfacesurfaceand surfacesurface surface preparation preparation preparationpreparation preparationpreparation preparation method. method. method.method. method.method. method. Each Each EachEach EachEach Eachav- av- av-av- av-av- av- erageerage value value was calculatedwas calculated from from the measurement the measurement results results of 10 of samples. 10 samples. Table 3. Average contacterageerage angleerageerageerage value value measured valuevalue value was was waswas calculated wascalculated values calculatedcalculated calculated for from SnBiAg from fromfrom fromthe the soldering themeasurementthe measurement the measurementmeasurement measurement alloy and results results reflow resultsresults results of solderingof 10 10ofof ofsamples. 10samples.10 10 samples.samples. technique. samples.

TableTableTableTableTableTable 3. 3.3. Average AverageAverage 3.3. 3. AverageAverage Average contact contactcontact contactcontact contact angle angleangleSurface angleangle anglemeasured measuredmeasured measuredmeasured Preparationmeasured values valuesvalues valuesvalues values for Methodforfor Sn forSnforSn forBiAgBiAg BiAg SnSn SnBiAgBiAg soldering BiAg solderingsoldering solderingsoldering soldering alloy alloyalloy alloyalloy and alloy andand andandreflow reflow reflowand reflowreflow reflow soldering solderingsoldering solderingsoldering soldering Matrix Polymer technique.technique.technique.technique. ABCDEtechnique.technique.technique.technique. ABS 57.82◦ ± 0.85◦ 74.48◦ ± 0.82◦ 78.07◦ ±SurfaceSurfaceSurface0.81SurfaceSurfaceSurface◦ Preparation PreparationPreparation PreparationPreparation Preparation50.27◦ ± Method MethodMethod0.95 MethodMethod◦ Method 83.74 ◦ ± 1.15◦ ◦ MatrixMatrixMatrixMatrixMatrix◦ Matrix Polymer PolymerPolymer PolymerPolymer Polymer ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ PLA 57.24 ± 0.85 74.09 ± 0.98AAA AAA 78.18 B±BB 0.79BB B 50.48CCC CC C ± 0.91 DDD DDD 84.02 ± 1.19EEE EE E PS 57.57◦ ± 1.11◦ 74.34◦ ± 0.76◦ 78.48◦ ± 0.77◦ 50.55◦ ± 0.81◦ 84.11◦ ± 1.18◦ ABSABSABSABSABS ABS 57.82°57.82°57.82°57.82°57.82°57.82° ± ±± 0.85° 0.85°0.85° ±± 0.85°±0.85° 0.85° 74.48° 74.48° 74.48° 74.48° 74.48° 74.48° ± ±± 0.82° 0.82°0.82° ±± 0.82°±0.82° 0.82° 78.07° 78.07° 78.07° 78.07° 78.07° 78.07° ± ±± 0.81° 0.81°0.81° ±± 0.81°±0.81° 0.81° 50.27° 50.27° 50.27° 50.27° 50.27° 50.27° ± ±± 0.95° 0.95°0.95° ±± 0.95°±0.95° 0.95° 83.74° 83.74° 83.74° 83.74° 83.74° 83.74° ± ±± 1.15° 1.15°1.15° ±± 1.15°±1.15° 1.15° PLAPLAPLAPLAPLA PLA 57.24°57.24°57.24°57.24°57.24°57.24° ± ±± 0.85° 0.85°0.85° ±± 0.85°±0.85° 0.85° 74.09° 74.09° 74.09° 74.09° 74.09° 74.09° ± ±± 0.98° 0.98°0.98° ±± 0.98°±0.98° 0.98° 78.18° 78.18° 78.18° 78.18° 78.18° 78.18° ± ±± 0.79° 0.79°0.79° ±± 0.79°±0.79° 0.79° 50.48° 50.48° 50.48° 50.48° 50.48° 50.48° ± ±± 0.91° 0.91°0.91° ±± 0.91°±0.91° 0.91° 84.02° 84.02° 84.02° 84.02° 84.02° 84.02° ± ±± 1.19° 1.19°1.19° ±± 1.19°±1.19° 1.19° A briefPSPSPS PS PS summaryPS 57.57°57.57°57.57°57.57° of57.57°57.57° the± ±± 1.11° 1.11°1.11° quality±± 1.11°±1.11° 1.11° 74.34° 74.34° 74.34° evaluation 74.34° 74.34° 74.34° ± ±± 0.76° 0.76°0.76° ±± 0.76°±0.76° 0.76° of 78.48° 78.48° 78.48° soldered 78.48° 78.48° 78.48° ± ±± 0.77° 0.77°0.77° ±± joints 0.77°±0.77° 0.77° 50.55° 50.55° 50.55° depending 50.55° 50.55° 50.55° ± ±± 0.81° 0.81°0.81° ±± 0.81°±0.81° 0.81° on 84.11° 84.11° 84.11° the 84.11° 84.11° 84.11° solder-± ±± 1.18° 1.18°1.18° ±± 1.18°±1.18° 1.18° ing alloy used, the soldering method, and the surface preparation is presented in Table4. FurtherAAA brief testsbriefbriefAAA briefbrief ofbriefsummary summarysummary the summarysummary summary joints’ of ofof mechanicalthe theoftheof of the qualitythe quality qualitythe qualityquality quality propertiesevaluation evaluationevaluation evaluationevaluation evaluation were of ofof soldered solderedofsolderedof carriedof solderedsoldered soldered joints outjointsjoints jointsjoints only joints depending dependingdepending fordependingdepending depending specimens on onon the ontheonthe on that the sol-the sol- sol-the sol-sol- sol- deringderingderingderingderingdering alloy alloyalloy alloyalloy alloyused, used,used, used,used, used, the thethe thesoldering the solderingsoldering the solderingsoldering soldering method, method,method, method,method, method, and andand and andthe thetheand thesurface the surfacesurface the surfacesurface surface preparation preparationpreparation preparationpreparation preparation is isis presented presentedpresented isis is presentedpresented presented in inin Table TableTable inin in TableTable Table 4.4.4. Further FurtherFurther4.4. 4. FurtherFurther Further tests teststests teststests oftests ofof the theoftheof of the joints’the joints’ joints’the joints’joints’ joints’ mechanical mechanicalmechanical mechanicalmechanical mechanical properties propertiesproperties propertiesproperties properties were werewere werewere were carried carriedcarried carriedcarried carried out outout out outonly onlyonlyout onlyonly foronly forfor forspecimens forspecimensspecimens for specimensspecimens specimens thatthatthatthat that passed passedthatpassed passedpassed passed the thethe the “rulethe “rule “rulethe “rule“rule “rule of ofof thumb”. thumb”.ofthumb”.of of thumb”.thumb”. thumb”. Soldering SolderingSoldering SolderingSoldering Soldering joints jointsjoints jointsjoints joints that thatthat that that did didthatdid diddid not notnotdid not notpass passpassnot passpass thepass thethe the “rulethe “rule “rulethe “rule“rule “rule of ofof thumb” thumb”ofthumb”of of thumb”thumb” thumb” werewerewerewerewere were classified classifiedclassified classifiedclassified classified as asas not notasnotas as not notmeeting meeting meetingnot meetingmeeting meeting the thethe the requiremthe requirem requiremthe requiremrequirem requirementsentsentsentsents and entsandand and and were wereandwere werewere were not notnot not notconsidered considered considerednot consideredconsidered considered to toto be be betoto to soldered besolderedsolderedbe be solderedsoldered soldered jointsjointsjointsjointsjointsjointsjoints of of ofof acceptable acceptable acceptable acceptableofof of acceptableacceptable acceptable quality. quality. quality.quality. quality.quality. quality. Materials 2021, 14, 3850 10 of 15

passed the “rule of thumb”. Soldering joints that did not pass the “rule of thumb” were classiﬁed as not meeting the requirements and were not considered to be soldered joints of acceptable quality.

Table 4. Impact of the soldering alloy and soldering method on the quality of the soldered joints. Materials 2021, 14, x FOR PEER REVIEW 10 of 15 Fit the Criteria of Acceptable Solder Alloy Soldering Method Description Wetting (θ < 55◦)

Table 4. Impact Possibleof the soldering to fabricate alloy and soldered soldering joints method on all on testedthe quality of the soldered joints. composite substrates; the method of surface SnPb Hot iron soldering preparation does not affect the quality of joints; not yesFit the Criteria of Solder Alloy Soldering Methodpossible to determine the wetting Description angle; irregular Acceptable Wet- shape of the obtained soldered joints ting (θ < 55°) Possible to fabricate soldered joints on all tested composite sub- Not possible to fabricate soldered connections due strates; the method of surface preparation does not affect the to the high melting point of the soldering alloy SnPb ReflowSnPb soldering Hot iron soldering no yes (significantlyquality of exceeding joints; not the possible softening to determine temperature the wetting angle; ir- of the polymerregular matrixes shape in all of testedthe obtained composites) soldered joints Not possible to fabricate soldered connections due to the high Not possible to fabricate solder joints due to the melting point of the soldering alloy (significantly exceeding the SnPb Reflow solderingsignificant difference in wettability between the no SnBiAg Hot iron soldering compositesoftening substratestemperature and of thethe solderingpolymer matrixes tip; in all tested com-no soldering alloy sticks to the solderingposites) tip and does notNot depositpossible on to thefabricate composite solder substrate joints due to the significant difference in wettability between the composite substrates and the SnBiAg Hot iron solderingPossible to fabricate repeatable soldered joints; no soldering tip; soldering alloy sticks to the soldering tip and does quality of soldered joints depends mainly on how SnBiAg Reflow soldering the composite substrate’snot deposit surface on the composite is prepared; substrate yes minorPossible impact ofto thefabricate type ofrepeatable composite soldered matrix joints; on quality of soldered joints depends mainly on how the composite substrate’s SnBiAg Reflow soldering the quality of joints yes surface is prepared; minor impact of the type of composite matrix on the quality of joints 3.2. Shear Test According3.2. to Shear the wettabilityTest test results, two soldering methods were used to fabricate samples for shearAccording tests: hot ironto the soldering wettability with test Sn63Pb37results, two solder soldering alloy methods and reflow were solderingused to fabri- with OM 520cate solder samples paste. for 1206SMDshear tests: resistorshot iron solder wereing soldered with Sn63Pb37 on each solder of the alloy composite and reflow substrates, ABS/Cu,soldering PLA/Cu, with OM 520 and solder PS/Cu, paste. using 1206SMD both resistors methods. were The soldered schematic on each diagram of the com- of the laboratoryposite stand substrates, for investigating ABS/Cu, PLA/Cu, the maximumand PS/Cu, shearusing both force methods. of the solder The schematic joints is diagram of the laboratory stand for investigating the maximum shear force of the solder joints presented in Figure7. The thorn applies the shear force on the soldered element until the is presented in Figure 7. The thorn applies the shear force on the soldered element until component, joint,the component, or substrate joint, is or damaged. substrate is The damage maximumd. The maximum value of value the shearof the shear force force used used during the testduring was the measured. test was measured. For all tested For all components, tested components, the connection the connection was damagedwas damaged between the surfacebetween of the the surface composite of the and composite the solder. and Therethe solder. were There no cases were in no which cases thein which SMD the component orSMD composite component paths or werecomposite damaged. paths were The measurementdamaged. The measurement results are presented results are in pre- Figure8. sented in Figure 8.

Figure 7. SchematicFigure representation 7. Schematic of representation shear test machine. of shear test machine. Materials 2021, 14, x FOR PEER REVIEW 11 of 15 Materials 2021, 14, 3850 11 of 15

FigureFigure 8. 8. ShearShear force force measurement measurement results results of of soldering soldering joints. joints. Al Alll composite composite surfaces surfaces were were mechanically mechanically cleaned cleaned with with sandpaper and RF800 flux was applied before soldering. sandpaper and RF800 ﬂux was applied before soldering.

ItIt was was also noticednoticed thatthat soldering soldering joints joints prepared prepared with with the the hot hot iron iron method method significantly signifi- cantlydiffered differed from eachfrom other, each other, with a with noticeable a noticeable spread spread of measured of measured shear forceshear values.force values. These Theseresults results confirm confirm the previous the previous observation observation that the that quality the ofquality soldering of soldering joints fabricated joints fabri- with catedthis method with this is method variable. is The variable. significant The sign differencesificant differences in the shear in force the shear measurement force measure- results mentobtained results are obtained also related are toalso the related impossibility to the impossibility of fabricating of surface-repeatablefabricating surface-repeatable connections connectionssoldered with soldered a soldering with a iron.soldering It should iron. It also should be mentioned also be mentioned that a solder that a jointsolder is joint only isformed only formed with the with surface the surface of the of copper, the copper, not the not polymer the polymer matrix, matrix, in the in composite. the composite. The Thespread spread of the of the obtained obtained results results is also is also due du toe the to the variable variable number number of copper of copper grains grains on theon thecomposite composite surface surface where where the the soldering soldering process process was was performed. performed. TheThe results results of of shear shear force force measurements measurements al alsoso correspond correspond with with the the thermal thermal stability stability propertiesproperties of of the the polymer polymer matrix. matrix. It It can can be be ob observedserved that that for for composites composites based based on on polymer polymer matrixesmatrixes with with lower lower glass glass transition transition temperatures, temperatures, the the maximum maximum value value of the of force the force at sol- at dersolder connection connection failure failure is lower. is lower. Materials Materials wi withth lower lower glass glass transition transition temperatures temperatures show show lowerlower thermal thermal stability, stability, and and thermal thermal degradatio degradationn is is quicker, quicker, resulting resulting in in failure failure at at lower lower shearshear forces forces (Table (Table 55).).

Table 5.TableGlass 5. transition Glass transition temperature temperature for different for differ polymersent polymers according according to literature. to literature.

Glass Transition Temperature Average Shear Force (Reﬂow Average Shear Force Polymer Glass Transition (T )[◦C] Soldering)Average Shear [N] Force (Re- (HotAverage Iron Soldering)Shear Force [N] (Hot Polymerg Temperature (Tg) flow Soldering) [N] Iron Soldering) [N] ABS 105[°C] 48.1 45.5 PSABS 100 105 46.7 48.1 35.4 45.5 PLA 60 36.1 28.7 PS 100 46.7 35.4 PLA 60 36.1 28.7 Comparing the results obtained with other examples of soldering tests on 3D-printed compositeComparing substrates the results is not obtained possible, with because other thereexamples are noof soldering such reports tests in on the 3D-printed literature. compositeTherefore, substrates the obtained is not results possible, were because compared there with are the no shear such forcereports values in the for literature. standard Therefore,joints soldered the obtained on metal results substrates. were Thecompar sheared force with reported the shear in force the literature, values for depending standard on the soldering paste used, for the 1206 SMD composite size soldered to a copper surface Materials 2021, 14, x FOR PEER REVIEW 12 of 15

joints soldered on metal substrates. The shear force reported in the literature, depending Materials 2021, 14, 3850 12 of 15 on the soldering paste used, for the 1206 SMD composite size soldered to a copper surface varies from 86 to 111 N [38,39]. The shear forces obtained for polymer composites as substrates are around 50% lower. The mechanical properties of the prepared soldered joints arevaries still fromgood 86enough to 111 to N prep [38,39are]. Thehigh-quality shear forces electrical obtained connections for polymer that composites easily pass as the “rulesubstrates of thumb”. are around Developing 50% lower. a method The mechanical for soldering properties standard of the preparedelectrical soldered components joints to 3D-printedare still good composites enough to allows prepare us high-qualityto merge traditional electrical electronics connections with that the easily rapidly pass thegrow- “rule of thumb”. Developing a method for soldering standard electrical components to ing field of structural electronics. It is also worth mentioning that structural electronics 3D-printed composites allows us to merge traditional electronics with the rapidly growing are expected to embed electronic elements in the housing of structural materials with ex- field of structural electronics. It is also worth mentioning that structural electronics are cellentexpected mechanical to embed properties electronic elements [40]. Therefore, in the housing the mechanical of structural properties materials of with electrical excellent con- nectionsmechanical are propertiesof secondary [40 ].importance, Therefore, the becaus mechanicale components properties in ofstructural electrical electronics connections sys- temsare ofwill secondary be sealed importance, inside the becausestructure components of the whole in structuraldevice and electronics thus protected systems from will forces be thatsealed could inside damage the structure the electrical of the connections whole device between and thus the protected components. from forces that could damage the electrical connections between the components. 3.3. Demonstrator Fabrication 3.3. Demonstrator Fabrication To demonstrate the functionality of the developed conductive composites together with theTo soldering demonstrate method, the functionality the electrical of structure the developed was fabricated. conductive Conductive composites togetherpaths were 3Dwith printed the soldering on an unmodified method, the FDM electrical printer, structure and an was SMD fabricated. LED component Conductive was paths soldered were to its3D surface printed (Figure on an unmodified 9). Conductive FDM paths printer, were and printed an SMD with LED componentPS/Cu composite was soldered filled towith its surface (Figure9). Conductive paths were printed with PS/Cu composite filled with 85.5 wt.% copper powder. The LED was soldered to the surface using the hot iron method 85.5 wt.% copper powder. The LED was soldered to the surface using the hot iron method with Sn63Pb37 solder alloy. Previous tests showed that reflow soldering gave far better with Sn63Pb37 solder alloy. Previous tests showed that reflow soldering gave far better results,results, but but using using the the hot hot iron methodmethod inin demonstratordemonstrator fabrication fabrication proved proved that that it couldit could alsoalso be be used used in in the the fast fast prototyping ofof structuralstructural electronics electronics systems. systems. A A 5 V5 USBV USB charger charger poweredpowered the the electronic electronic circuit. circuit. The The electrical electrical resistivity resistivity of of the the printed printed composites composites and and soldersolder connections connections was was low low enough enough to topower power the the LED LED with with high high efficiency. efficiency. The The example of thisof thisdemonstrator demonstrator shows shows that that it is it possible is possible to tomanufacture manufacture complex complex electronic electronic systems systems usingusing additive additive techniques techniques despite despite their their apparent apparent limitations.limitations. TheThe limitation limitation of of obtaining obtaining miniatureminiature integrated integrated circuits circuits with FDMFDM technologytechnology can can be be solved solved by by combining combining printed printed elementselements with with standard standard electronic components.components. Soldering Soldering the the components components to to composite composite substratessubstrates turns turns out out to to be be a method thatthat cancan bebe used used to to obtain obtain fully fully functional functional 3D-printed 3D-printed electronicelectronic systems. systems.

FigureFigure 9. 3D-printed 9. 3D-printed demonstrator demonstrator with with soldered soldered LED LED on on PS/Cu PS/Cu conductive conductive substrate substrate using using a a hot hot iron iron and and SnPb soldering solder- alloy. ing alloy.

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4. Conclusions The possibility of fabricating soldering joints on 3D-printed conductive substrates was investigated in the current work. Two soldering methods (hot iron soldering and reflow soldering) and two soldering alloys (SnPb and low-temperature SnBiAg) were tested to find the optimal technique for soldering. The main conclusions can be summarized as follows: 1. The surface preparation of conductive composite plays a significant role in forming high-quality solder joints. The influence of surface preparation was evaluated by measuring the contact angle of the melted solder alloy. It can be observed that the best results were obtained with the mechanical removing of a thin polymer layer, revealing a copper powder inside a composite structure, and using flux that allows the removal of oxides and other compounds from the soldered surfaces. After the surface preparation, the contact angle value was reduced to 55◦ for reflow soldering with low-temperature SnBiAg solder paste, which fits the criteria of acceptable wetting. 2. The shear test results show that the maximum force value that could be applied before damaging soldered joints is strongly dependent on the polymer matrix used in the composite substrate. In general, maximum shear force values for reflow soldering are slightly higher than for hot iron soldering. Shear forces obtained for polymer composites as substrate are around 50% lower compared to a typical copper substrate. 3. The highest average shear strength of the joint occurs on ABS/Cu substrates, then on PS/Cu, and the lowest results were obtained for the PLA/Cu substrate. The obtained results correlate directly with the thermal stability of the polymer matrixes used. The higher the thermal stability of the substrate material, the higher the maximum shear force of the soldered joints fabricated on that substrate. 4. Hot iron soldering can be used to fabricate soldered joints on prepared composite composites. Still, it is recommended that this method only be used for rapid prototyping of individual structures because of its low repeatability. 5. The soldering joints with the best properties were obtained using low-temperature SnBiAg solder paste and reflow soldering. This allows the fabrication of repeatable, high-quality solder joints without damaging the composite substrate. This method can be used for the mass production of structural electronics elements.

Author Contributions: Conceptualization, B.P. and A.S.; methodology, B.P., A.S. and M.S.; validation, B.P.; formal analysis, B.P. and M.S.; investigation, B.P.; resources, A.S. and M.S.; writing—original draft preparation, B.P.; writing—review and editing, M.S.; visualization, B.P.; supervision, A.S. and M.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Foundation for Polish Science within the First TEAM/2016- 1/7 project. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: This work was performed as a part of the “Functional heterophase materials for structural electronics” project carried out within the First TEAM/2016-1/7 programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. It was also supported by the Institute of Metrology and Biomedical Engineering, Warsaw University of Technology. Conflicts of Interest: The authors declare no conflict of interest. Materials 2021, 14, 3850 14 of 15

References 1. Conner, B.P.; Manogharan, G.P.; Martof, A.N.; Rodomsky, L.M.; Rodomsky, C.M.; Jordan, D.C.; Limperos, J.W. Making sense of 3-D printing: Creating a map of additive manufacturing products and services. Addit. Manuf. 2014, 1, 64–76. [CrossRef] 2. Huang, Y.; Leu, M.C.; Mazumder, J.; Donmez, A. Additive Manufacturing: Current State, Future Potential, Gaps and Needs, and Recommendations. J. Manuf. Sci. Eng. 2015, 137, 014001. [CrossRef] 3. Uriondo, A.; Esperon-Miguez, M.; Perinpanayagam, S. The present and future of additive manufacturing in the aerospace sector: A review of important aspects. Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng. 2015, 229, 2132–2147. [CrossRef] 4. Yap, Y.L.; Yeong, W. Additive manufacture of fashion and jewellery products: A mini review. Virtual Phys. Prototyp. 2014, 9, 195–201. [CrossRef] 5. Yoshida, H.; Igarashi, T.; Obuchi, Y.; Takami, Y.; Sato, J.; Araki, M.; Miki, M.; Nagata, K.; Sakai, K.; Igarashi, S. Architecture-Scale Human-Assisted Additive Manufacturing. ACM Trans. Graph. 2015, 34, 88. [CrossRef] 6. Ngo, T.D.; Kashani, A.; Imbalzano, G.; Nguyen, K.T.Q.; Hui, D. Additive Manufacturing (3D Printing): A Review of Materials, Methods, Applications and Challenges. Compos. Part B Eng. 2018, 143, 172–196. [CrossRef] 7. Farahani, R.D.; Chizari, K.; Therriault, D. Three-dimensional printing of freeform helical microstructures: A review. Nanoscale 2014, 6, 10470–10485. [CrossRef][PubMed] 8. Ahn, B.Y.; Duoss, E.B.; Motala, M.J.; Guo, X.; Park, S.-I.; Xiong, Y.; Yoon, J.; Nuzzo, R.G.; Rogers, J.A.; Lewis, J.A. Omnidi-rectional Printing of Flexible, Stretchable, and Spanning Silver Microelectrodes. Science 2009, 323, 1590–1593. [CrossRef][PubMed] 9. Lewis, J.A.; Ahn, B.Y. Three-dimensional printed electronics. Nature 2015, 518, 42–43. [CrossRef][PubMed] 10. Maalderink, H.H.H.; Bruning, F.B.J.; De Schipper, M.M.R.; Van Der Werff, J.J.J.; Germs, W.W.C.; Remmers, J.J.C.; Meinders, E.R. 3D Printed structural electronics: Embedding and connecting electronic components into freeform electronic devices. Plast. Rubber Compos. 2017, 47, 35–41. [CrossRef] 11. Lopes, A.J.; MacDonald, E.; Wicker, R.B. Integrating stereolithography and direct print technologies for 3D structural electronics fabrication. Rapid Prototyp. J. 2012, 18, 129–143. [CrossRef] 12. MacDonald, E.; Salas, R.; Espalin, D.; Perez, M.; Aguilera, E.; Muse, D.; Wicker, R.B. 3D Printing for the Rapid Prototyping of Structural Electronics. IEEE Access 2014, 2, 234–242. [CrossRef] 13. Espera, A.H.; Dizon, J.R.C.; Chen, Q.; Advincula, R.C. 3D-printing and advanced manufacturing for electronics. Prog. Addit. Manuf. 2019, 4, 245–267. [CrossRef] 14. Dul, S.; Fambri, L.; Pegoretti, A. Fused deposition modelling with ABS–graphene nanocomposites. Compos. Part A Appl. Sci. Manuf. 2016, 85, 181–191. [CrossRef] 15. Hwang, S.; Reyes, E.I.; Moon, K.-S.; Rumpf, R.C.; Kim, N.S. Thermo-mechanical Characterization of Metal/Polymer Composite Filaments and Printing Parameter Study for Fused Deposition Modeling in the 3D Printing Process. J. Electron. Mater. 2015, 44, 771–777. [CrossRef] 16. Kuester, S.; Merlini, C.; Barra, G.M.; Ferreira, J.C.; Lucas, A.; de Souza, A.C.; Soares, B.G. Processing and characterization of conductive composites based on poly(styrene-b-ethylene-ran-butylene-b-styrene) (SEBS) and carbon additives: A comparative study of expanded graphite and carbon black. Compos. Part B Eng. 2016, 84, 236–247. [CrossRef] 17. Ning, F.; Cong, W.; Qiu, J.; Wei, J.; Wang, S. Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling. Compos. Part B Eng. 2015, 80, 369–378. [CrossRef] 18. Ryder, M.A.; Lados, D.A.; Iannacchione, G.S.; Peterson, A.M. Fabrication and properties of novel polymer-metal composites using fused deposition modeling. Compos. Sci. Technol. 2018, 158, 43–50. [CrossRef] 19. Hirman, M.; Steiner, F. A comparison of the shear strength of conductive adhesives and soldering alloys. In Proceedings of the 2015 38th International Spring Seminar on Electronics Technology (ISSE), Eger, Hungary, 6–10 May 2015; pp. 167–172. 20. Jojibabu, P.; Jagannatham, M.; Haridoss, P.; Ram, G.J.; Deshpande, A.; Bakshi, S.R. Effect of different carbon nano-fillers on rheological properties and lap shear strength of epoxy adhesive joints. Compos. Part A Appl. Sci. Manuf. 2016, 82, 53–64. [CrossRef] 21. Jeevi, G.; Nayak, S.K.; Kader, M.A. Review on adhesive joints and their application in hybrid composite structures. J. Adhes. Sci. Technol. 2019, 33, 1497–1520. [CrossRef] 22. Ji, X.; Xiao, G.; Jin, T.; Shu, X. Shear properties of isotropic conductive adhesive joints under different loading rates. J. Adhes. 2018, 95, 204–217. [CrossRef] 23. Sánchez-Romate, X.X.F.; Jiménez-Suárez, A.; Molinero, J.; Sánchez, M.; Güemes, A.; Ureña, A. Development of bonded joints using novel CNT doped adhesive films: Mechanical and electrical properties. Int. J. Adhes. Adhes. 2018, 86, 98–104. [CrossRef] 24. Podsiadły, B.; Skalski, A.; Wałpuski, B.; Słoma, M. Heterophase Materials for Fused Filament Fabrication of Structural Elec-tronics. J. Mater Sci. Mater Electron. 2019, 30, 1236–1245. [CrossRef] 25. Noor, E.E.M.; Singh, A.; Chuan, Y.T. A review: Influence of nano particles reinforced on solder alloy. Solder. Surf. Mt. Technol. 2013, 25, 229–241. [CrossRef] 26. Jothi Prakash, C.G.; Prasanth, R. Approaches to Design a Surface with Tunable Wettability: A Review on Surface Properties. J. Mater. Sci. 2021, 56, 108–135. [CrossRef] 27. Zhevnenko, S.N. Surface Free Energy of Copper-Based Solid Solutions. J. Phys. Chem. C 2015, 119, 2566–2571. [CrossRef] 28. Vitos, L.; Ruban, A.; Skriver, H.; Kollár, J. The surface energy of metals. Surf. Sci. 1998, 411, 186–202. [CrossRef] 29. Shimizu, R.N.; Demarquette, N.R. Evaluation of Surface Energy of Solid Polymers Using Different Models. J. Appl. Polym. Sci. 2000, 76, 1831–1845. [CrossRef] Materials 2021, 14, 3850 15 of 15

30. Wassink, R.J.K. Weichlöten in der Elektronik: Mit 303 Abb. u. 57 Tab. im Text; Leuze: Saulgau/Württ, Germany, 1986; ISBN 978-0-901150-14-1. 31. Wedi, A.; Baither, D.; Schmitz, G. Contact angle and reactive wetting in the SnPb/Cu system. Scr. Mater. 2011, 64, 689–692. [CrossRef] 32. Lin, C.-T.; Lin, K.-L. Contact angle of 63Sn–37Pb and Pb-free solder on Cu plating. Appl. Surf. Sci. 2003, 214, 243–258. [CrossRef] 33. Garcia, L.R.; Osório, W.R.; Peixoto, L.C.; Garcia, A. Wetting Behavior and Mechanical Properties of Sn-Zn and Sn-Pb Solder Alloys. J. Electron. Mater. 2009, 38, 2405–2414. [CrossRef] 34. Plevachuk, Y.; Hoyer, W.; Kaban, I.; Kohler, M.; Novakovic, R.; Plevachuk, Y. Experimental study of density, surface tension, and contact angle of Sn–Sb-based alloys for high temperature soldering. J. Mater. Sci. 2009, 45, 2051–2056. [CrossRef] 35. Li, Y.; Wang, Z.; Li, X.; Lei, M. Effect of temperature and substrate surface roughness on wetting behavior and interfacial structure between Sn–35Bi–1Ag solder and Cu substrate. J. Mater. Sci. Mater. Electron. 2020, 31, 4224–4236. [CrossRef] 36. Vianco, P.T.; Rejent, J.A.; Vianco, P.T.; Rejent, J.A. Properties of ternary Sn-Ag-Bi solder alloys: Part II—Wettability and mechanical properties analyses. J. Electron. Mater. 1999, 28, 1138–1143. [CrossRef] 37. Fima, P.; G ˛asior, W.; Sypien, A.; Moser, Z. Wetting of Cu by Bi–Ag based alloys with Sn and Zn additions. J. Mater. Sci. 2010, 45, 4339–4344. [CrossRef] 38. Bernasko, P.K.; Mallik, S.; Takyi, G. Effect of intermetallic compound layer thickness on the shear strength of 1206 chip resistor solder joint. Solder. Surf. Mt. Technol. 2015, 27, 52–58. [CrossRef] 39. Pan, J.; Toleno, B.J.; Chou, T.; Dee, W.J. The effect of reﬂow proﬁle on SnPb and SnAgCu solder joint shear strength. Solder. Surf. Mt. Technol. 2006, 18, 48–56. [CrossRef] 40. Espalin, D.; Muse, D.W.; MacDonald, E.; Wicker, R.B. 3D Printing Multifunctionality: Structures with Electronics. Int. J. Adv. Manuf. Technol. 2014, 72, 963–978. [CrossRef]