An Assessment of Thermoset Injection Molding for Thin-Walled Conformal Encapsulation of Board-Level Electronic Packages

Journal of Manufacturing and Materials Processing

Article An Assessment of Thermoset Injection Molding for Thin-Walled Conformal Encapsulation of Board-Level Electronic Packages

Romit Kulkarni 1,* , Peter Wappler 1, Mahdi Soltani 2 , Mehmet Haybat 1, Thomas Guenther 2, Tobias Groezinger 1 and André Zimmermann 1,2 1 Hahn-Schickard, 70569 Stuttgart, Germany; [email protected] (P.W.); [email protected] (M.H.); [email protected] (T.G.); [email protected] (A.Z.) 2 University of Stuttgart, Institute for Micro Integration (IFM), 70569 Stuttgart, Germany; [email protected] (M.S.); [email protected] (T.G.) * Correspondence: [email protected]; Tel.: +49-711-685-84780

Received: 21 December 2018; Accepted: 25 January 2019; Published: 1 February 2019

Abstract: An ever-growing market demand for board (second) level packages (e.g., embedded systems, system-on-a-chip, etc.) poses newer challenges for its manufacturing industry in terms of competitive pricing, higher reliability, and overall dimensions. Such packages are encapsulated for various reasons including thermal management, protection from environmental conditions and dust particles, and enhancing the mechanical stability. In the due course of reducing overall sizes and material saving, an encapsulation as thin as possible imposes its own signiﬁcance. Such a thin-walled conformal encapsulation serves as an added advantage by reducing the thermo-mechanical stresses occurring due to thermal-cyclic loading, compared to block-sized or thicker encapsulations. This paper assesses the encapsulation process of a board-level package by means of thermoset injection molding. Various aspects reviewed in this paper include the conception of a demonstrator, investigation of the ﬂow simulation of the injection molding process, execution of molding trials with different encapsulation thicknesses, and characterization of the packages. The process shows a high dependence on the substrate properties, injection molding process parameters, device mounting tolerances, and device geometry tolerances. Nevertheless, the thermoset injection molding process is suitable for the encapsulation of board-level packages limiting itself only with respect to the thickness of the encapsulation material, which depends on other external aforementioned factors.

Keywords: encapsulation; electronics; manufacturing; thermoset; epoxy molding compound; EMC; injection molding; resistors; QFN; board level package; surface mount technology; SMT; SMD; transfer molding; injection molding simulation; ﬂow simulation; reliability; PCB

1. Introduction The production of packages at different levels plays a major role in the microassembly technology [1]. The wide range of different requirements related to packaging solutions in the ﬁelds of automotive, medical, and automation industries is also increasing by the day [2]. The requirement of integrating more and more functions in an electronic assembly stimulates more challenges regarding miniaturization, robustness, reliability, and cost reduction in manufacturing [3]. Encapsulations are used to protect the electronic components from adverse environmental conditions and mechanical shock, and to undertake the functions of structural support and electrical insulation [4]. Additionally, encapsulations assume the thermal management of the assembly. The use of epoxy molding

J. Manuf. Mater. Process. 2019, 3, 18; doi:10.3390/jmmp3010018 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2019, 3, 18 2 of 16 compounds (EMC) for the encapsulation of packages offers a large set of advantages such as low coefﬁcients of thermal expansion (CTE), decent thermal conductivity, good chemical stability, higher Young’s modulus, etc. [5]. Due to their reduced shrinkage and low viscosity values compared to thermoplastic materials, even highly sensitive electronic assemblies can be packaged owing to the possibility of processing at lower injection pressures. Packaging is carried out at various stages to enable system integration at different levels of complexity, which usually begins at the chip or wafer level [1] and has no upper limit. Table1 gives examples of packages of different levels (up to the fourth level). This article deals with the encapsulation of the second level (or board-level) packages.

Table 1. Deﬁnition of different levels of packages according to Ref. [1]. (IC: integrated circuit, RAM: random-access memory).

Level Stage 0 IC chip 1 Encapsulated microelectronic package 2 Printed circuit board with various mounted devices 3 Multiple PCBs (e.g., RAM) integrated on a mother board 4 Housed electronics system (e.g., laptop computer)

Transfer molding is the most commonly used technology for the encapsulation of microsystems [1]. For constant development and newer ideas, the application of an easily automatable, flexible, and cost-effective production method is needed. Furthermore, an injection molding machine can be flexibly used for thermoplastic and thermosetting plastics with suitable necessary attachments. Thermoset injection molding offers these required characteristics. A major portion of injection molding service providers already owns machines that can be easily upgraded to deploy the thermoset injection molding process. A small and medium-sized enterprise (SME) can profit from this availability to easily order for non-standard packages and in lower order volumes. The biggest hurdle in the use of thermoset injection molding, however, lies in constraining the material within the processing window with a temperature high enough to achieve minimum viscosity but not as high as to facilitate rapid curing of the material as explained in Refs. [6,7]. The feasibility of using thermoset injection molding for encapsulating board-level packages was checked and presented in a previous article [8]. As a conclusion from Ref. [8], and according to other pretests conducted in-house regarding filling behavior and mechanical properties, thermoset injection molding can be implemented for thin-walled encapsulations of board-level packages. A further challenge is posed by limiting the wall thickness of the encapsulation to a minimum. This enables material saving, reduces the end size of an assembly and should lower the thermo-mechanical loading, e.g., at the solder joints. Thermoset injection molding has shown to be a cost-effective alternative for encapsulation of electronic components and assemblies [7]. However, the process needs to be verified to investigate the minimum possible encapsulation thickness that conforms to the profile of the printed circuit board (PCB) and the mounted devices. A well-suited process simulation is also required to reduce the risk of defects during the molding trials. Numerical simulation methods for thermoset injection molding have been available since the 80s, e.g., Ref. [9]. While numerical simulation methods for transfer molding of encapsulations are also already presented in the literature since the 90s [10], not many references are available in the field of simulation of thermoset injection molding for the encapsulation of microelectronic devices. Similarly, hardly any literature is found that lays the foundation for the implementation of thermoset injection molding specifically for the manufacturing of thin-walled conformal encapsulation using an EMC supplied in granular form. An effort to bridge this gap in the literature is made with the help of this article. In the forthcoming sections, a concept for a demonstrator is introduced (Section 2.1), the process simulation is explained (Section 2.2), the lessons learned through the molding trials are described J. Manuf. Mater. Process. 2019, 3, 18 3 of 16

J.(SectionJ. Manuf.Manuf. Mater.Mater. 3.1), Process.Process. and the 2019 outcome,, 3,, 1818 of the characterization is discussed (Section 3.2). The interpretation3 of 16 from and conclusions of the results are laid down in Section4. 2. Materials and Methods 2. Materials and Methods 2.1. Conception of the Demonstrator 2.1. Conception of the Demonstrator To investigate the above-mentioned importantt aspectsaspects regardingregarding thethe implementationimplementation ofof thermosetthermosetTo investigate injectioninjection the moldingmolding above-mentioned forfor thethe encapsulationencapsulation important aspects of board-level regarding packages, the implementation an exemplary of thermoset PCB (55 mminjection × 55 molding mm × for1.55 the mm) encapsulation with mounted of board-level devices packages,was considered an exemplary as shown PCB in (55 Figure mm × 1.55 mmThe demonstrator× 1.55 mm) with is based mounted on a devices PCB with was a considered relatively ashigh shown glass in transition Figure1. Thetemperature demonstrator (Tg = is 170 based °C) ◦ withon a devices PCB with mounted a relatively on it. high Various glass electronic transition devic temperaturees of different (Tg =sizes 170 andC) withgeometries devices were mounted taken intointoon it. account.account. Various TwoTwo electronic ceramicceramic devices resistorresistor ofs differentof standard sizes sizes and CR1206 geometries (4.7 werekΩ) and taken CR0603 into account. (1 kΩ) were Two chosenceramic to resistors be mounted of standard in a parallel sizes CR1206 configuration (4.7 kΩ to) and eacheach CR0603 other.other. TheseThese (1 kΩ ) areare were positionedpositioned chosen to (two(two be mounted ofof each)each) inin a linear parallel repeating conﬁguration pattern to to each facilitate other. the These examinatio are positionedn of the (two effects of each) of being in a placed linear repeatingeither near pattern or far fromto facilitate the gating the examinationposition. One of of the the effects possible of being four ga placedtingting positionspositions either near isis shownshown or far from inin FigureFigure the gating 2.2. InIn aa position. similarsimilar fashion,One of the a micro-leadframe possible four gating (MLF28), positions representing is shownin broad Figure devices,2. In a similarand an fashion,induction a micro-leadframe coil (DO1608C), representing(MLF28), representing tall devices, broad were devices, mounted and on the an inductionother normal coil axis. (DO1608C), The layout representing enables electric tall devices, tests of eachwere mountedmounted ondevice the otherindividually. normal axis.This Thelayout layout can enables also be electric effectively tests ofput each into mounted use for device later implementationimplementationindividually. This ofof layout inlineinline canfailurefailure also detection be effectively during put environmental into use for later tests implementation in accordance ofwith inline Refs. failure [11– 14].detection Figure during 2 shows environmental a CAD model tests of inthe accordance demonstrator with with Refs. a [ 11thin-walled–14]. Figure encapsulation2 shows a CAD following model thetheof the contourcontour demonstrator ofof thethe mountedmounted with a thin-walled devices.devices. encapsulation following the contour of the mounted devices.

Figure 1. PCBPCB withwith mountedmounted devicesdevices (C(C (CR1206,R1206, CR0603, MLF28, DO1608C).

Figure 2.2. CAD CADCAD model modelmodel of theofof thethe demonstrator demonstratordemonstrator forinvestigating forfor investinvestigatingigating the implementation thethe implementationimplementation of thermoset ofof thermosetthermoset injection injectioninjectionmolding molding formolding thin-walled forfor thin-walledthin-walled encapsulations encapsulationsencapsulations of board-level ofof board-levelboard-level packages. packages.packages.

Molding trials trials with with different different reducing reducing encapsulation encapsulation thicknesses thicknesses were were planned, planned, starting starting from from 1 mm.1 mm. The The further further stages stages corresponded corresponded to to thickne thicknessessses of of 0.5 0.5 mm mm and and 0.25 0.25 mm. mm. Respectively, Respectively, three different molding molding tool tool inserts inserts (Figure (Figure 33 shows shows one one example) example) fitted ﬁtted within within the the standard standard molding molding tool availabletool available in-house in-house at Hahn-Schickard at Hahn-Schickard were were ma machinedchined using using the the milling milling process. process. The The design supported moldingmolding trials trials in in four four directions, directions, which which allowed allowed a closer a closer contemplation contemplation of the ﬂow of the behavior flow behaviorin each case. in each case.

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Figure 3. Figure Molding3. Molding tool tool insert insert forfor a 0.25 mm mm thick thick encapsulation. encapsulation.

The materialThe material for the for encapsulation the encapsulation of the of board-level the board-level packages packages was chosenwas chosen from commonfrom common suppliers in thesuppliers European in market the European for the EMCsmarket deemed for the ﬁt EMCs for a granulardeemed feedfit for to enablea granular reproducibility feed to enable owing to easy availabilityreproducibility for owing SMEs to in easy the availability region. The for chosenSMEs in EMCthe region. is available The chosen in the EMC market is available with in the the name NU6110Vmarket [15 with] from the the name manufacturer NU6110V Duresco [15] from GmbH the (Witterswil,manufacturer Switzerland). Duresco GmbH The key(Witterswil, properties of this materialSwitzerland). are shown The key in properties Table2. of this material are shown in Table 2.

TableTable 2. Key 2. Key properties properties of of material material NU6110V according according to toRef. Ref. [15]. [15 ]. Material Property Value Material Property Value Density 2.0 g/cm3 GlassDensity transition temperature 1602.0 °C g/cm3 ◦ Glass transitionThermal temperature conductivity 0.85 W/mK 160 C CoefficientThermal conductivityof thermal expansion (20–105 °C) 18 ppm/K 0.85 W/mK ◦ Coefficient of thermalYoung’s expansion modulus (flexural (20–105 test)C) 18 18GPa ppm/K Young’s modulus (flexural test) 18 GPa 2.2. Process Simulation 2.2. ProcessNumerous Simulation iterations during molding trials can be effectively reduced by virtue of optimizing the encapsulation design in terms of sprue length, gate position, tempering channels, and injection Numerous iterations during molding trials can be effectively reduced by virtue of optimizing parameters. The injection molding simulations were carried out using SIGMASOFT Virtual Molding the encapsulation design in terms of sprue length, gate position, tempering channels, and injection (v5.2) provided by SIGMA Engineering GmbH (Aachen, Germany). Relevant results for the parameters.optimized The gate injection positioning molding were simulationsalso taken from were ancarried earlier in-house out using project SIGMASOFT [16] and verified Virtual with Molding (v5.2)experiments provided by during SIGMA the Engineering same project GmbH and as (Aachen, presented Germany). in Ref. [17] Relevant involving results the (film for theassisted) optimized gate positioningtransfer molding were process. also taken These from results an recommended earlier in-house an offset project positioning [16] and of verified the gate with to the experiments axis on duringwhich the samethe devices project were and mounted as presented in the inflow Ref. direct [17]ion involving to reduce the the (film possibility assisted) of occurrence transfer molding of process.weld-lines These resultsand voids recommended in the flow shadow an offset regions positioning of the mounted of the gatedevices. to the axis on which the devices were mountedThe material in the flowmodel direction used for the to reducesimulation the was possibility based on of an occurrence earlier version of weld-lines of the material and [18], voids in the flowreplenished shadow with regions the help of the of mounted data measured devices. via external service providers, and confirmed by the material manufacturer and simulation software provider. The material model used for the simulation was based on an earlier version of the material [18], A computer aided design (CAD) model for the planned demonstrator was developed in Creo ® replenished with the help of data measured via external service providers, and confirmed by the Parametric 2.0 developed by PTC Inc. (Needham, MA, USA) and implemented in SIGMASOFT as materialillustrated manufacturer in Figure and 2. The simulation PCB and softwarethe mounted provider. devices (the board-level package) acted as insert ® Acomponents computer in aided the process design simulation. (CAD) model A finer for mesh theplanned (Figure 4) demonstrator was generated was in the developed areas near in the Creo Parametricmounted 2.0 devices developed to enhance by PTC the Inc.accuracy (Needham, of the simulation. MA, USA) An andexample implemented of the fineness in SIGMASOFT of the mesh as illustratedis shown in Figure in Figure2. The 5, PCBwhich and ensured the mounted a minimum devices of (thethree board-level element layers package) throughout acted asthe insert componentsencapsulation in the thickness process (except simulation. in the thin A finer gaps mesh under (Figure the devices).4) was generated in the areas near the mounted devices to enhance the accuracy of the simulation. An example of the fineness of the mesh is shown in Figure5, which ensured a minimum of three element layers throughout the encapsulation thickness (except in the thin gaps under the devices). J. Manuf. Mater. Process. 2019, 3, 18 5 of 16 J.J. Manuf.Manuf. Mater.Mater. Process.Process. 2019,, 3,, 1818 5 of 16 J. Manuf. Mater. Process. 2019, 3, 18 5 of 16

Figure 4. FineFine meshmesh inin thethe areasareas especiallyespecially aroundaround thethe devices.devices. Figure 4. Fine mesh in the areas especially around the devices.

Figure 5. ExemplaryExemplary fine finefine mesh mesh around around the MLF with atat least threethree elementselements overover thethe wallwall thickness.thickness. Figure 5. Exemplary fine mesh around the MLF with at least three elements over the wall thickness. Numerous simulations were carried out involvinginvolving thethe fourfour possiblepossible injectioninjection directionsdirections (each(each ◦ Numerous simulations were carried out involving the four possible injection directions (each 9090° apart). Figure Figure 6 shows the the fill fill behavior behavior for for one one such such direction. direction. The The filling filling behavior behavior was was seen seen to tobe 90° apart). Figure 6 shows the fill behavior for one such direction. The filling behavior was seen to be beacceptably acceptably uniform uniform with with a slightly a slightly slower slower filling filling above above the the induction induction coil coil (tall (tall cylindrical cylindrical part). part). acceptably uniform with a slightly slower filling above the induction◦ coil (tall cylindrical part). Besides, thethe melt temperature whilewhile enteringentering thethe gategate regionregion rose to 100100 °C,C, which is a suitable filling filling Besides, the melt temperature while entering the gate region rose to 100 °C, which is a suitable filling temperaturetemperature recommendedrecommendedrecommended by byby the thethe material materialmaterial manufacturer. manumanufacturer.facturer. Attaining AttainingAttaining the right thethe temperature rightright temperaturetemperature is necessary isis temperature recommended by the material manufacturer. Attaining the right temperature is tonecessary reach a compromiseto reach a compromise between the between least possible the least viscosity possible of viscosity the melt of during the melt filling during and maximumfilling and necessary to reach a compromise between the least possible viscosity of the melt during filling and possiblemaximum curing possible during curing the holdingduring the time holding (30–60 time s) [19 (30–60] after s) filling [19] isafter complete filling is as complete explained as in explained Section1. maximum possible curing during the holding time (30–60 s) [19] after filling is complete as explained Theinin SectionSection mounted 1.1. TheThe side mountedmounted of the PCB sideside was ofof seen thetheto PCBPCB befilled waswas fasterseseen to than be thefilled unmounted faster than side, the which unmounted reduced side, the in Section 1. The mounted side of the PCB was seen to be filled faster than the unmounted side, riskwhich of reduced formation the of risk a large of formation weld line of on a the large former. weld line on the former. which reduced the risk of formation of a large weld line on the former.

Figure 6.6. SimulatedSimulatedSimulatedfilling fillingfilling behavior behaviorbehavior (temperature (temperature(temperature scale) scale)scale) at differentatat differentdifferent filling fillingfilling stages stagesstages on the onon mounted thethe mountedmounted (first Figure 6. Simulated filling behavior (temperature scale) at different filling stages on the mounted row)(first(first androw)row) unmounted andand unmountedunmounted (second (second(second row) sidesrow)row) forsidessides an forfor encapsulation anan encapsulationencapsulation thickness thicknessthickness of 1 mm. ofof 11 mm.mm. (first row) and unmounted (second row) sides for an encapsulation thickness of 1 mm.

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A comparison of the simulations for the filling patterns for the different levels of encapsulation A comparison of the simulations for the filling patterns for the different levels of encapsulation thickness showed a similar behavior, though the filling speed was different owing to different filling thicknessvolumes. showed The comparison a similar behavior,can be seen though in Figure the 7. filling speed was different owing to different filling volumes. The comparison can be seen in Figure7.

(a) (b) (c)

Figure 7. Comparative fill behavior from the simulation for (a) 1 mm, (b) 0.5 mm, and (c) 0.25 mm Figure 7. Comparative fill behavior from the simulation for (a) 1 mm, (b) 0.5 mm, and (c) 0.25 mm encapsulation thickness. encapsulation thickness. The images in Figure7 show the filled melt on a temperature scale at different time points The images in Figure 7 show the filled melt on a temperature scale at different time points measured after the injection start (1.5–2.5 s). The cavity with a 0.25 mm wall thickness was filled measured after the injection start (1.5–2.5 s). The cavity with a 0.25 mm wall thickness was filled already at 2 s, 0.5 mm at 2.5 s, and 1 mm (filled cavity not shown) filled at 4 s after injection. Due to already at 2 s, 0.5 mm at 2.5 s, and 1 mm (filled cavity not shown) filled at 4 s after injection. Due to different wall thicknesses, the melts attained different temperature levels in each case (higher in the different wall thicknesses, the melts attained different temperature levels in each case (higher in the casecase of of lesser lesser wall wall thickness). thickness). Though Though thethe attainedattained melt temperature was was high high (highest (highest in in the the case case of of 0.250.25 mm mm thickness), thickness), the the curing curing degree degree achieved achieved at the at endthe end of filling of filling was was under under 4% with 4% with an initial an initial degree of 3%degree included of 3% inincluded the feedstock. in the feedstock. At the end At of the the end packing of the andpacking holding and time,holding this time, variant this attainedvariant a curingattained degree a curing of 72%, degree which of necessitated 72%, which thenecessitate requirementd the ofrequirement an additional of an accelerated additional curing accelerated process aftercuring the injectionprocess after molding the injection process. molding The optimized process. simulation The optimized parameters simulation are shownparameters in Table are shown3. Every thicknessin Table variant 3. Every showed thickness a pressure variant requirementshowed a pressu of are maximum requirement of 250 of bar,a maximum which is of comfortably 250 bar, which under theis limits comfortably (≈2000 under bar) of the a commonlimits (≈2000 injection bar) of molding a common machine. injection molding machine.

TableTable 3. 3.Inputs Inputs usedused forfor the simulation.

MaterialMaterial NU 6110 NU 6110V V Melt injection temperature 70 °C Melt injection temperature 70 ◦C Mold tempering 180 °C Mold tempering 180 ◦C 3 Flow control (ﬂux) 2 cm /s J. Manuf. Mater. Process. 2019, 3, 18 7 of 16

Flow control (flux) 2 cm3/s J. Manuf. Mater. Process. 2019, 3, 18 7 of 16 The simulation phase was, thus, concluded assuming that an acceptably good filling behavior would be achieved during the experimental molding trials. The resulting molding trials and the characterizationThe simulation that phase followed was, are thus, presented concluded in the assuming forthcoming that an Section acceptably 3. good ﬁlling behavior would be achieved during the experimental molding trials. The resulting molding trials and the characterization3. Results that followed are presented in the forthcoming Section3.

3.3.1. Results Experimental Molding Trials 3.1. ExperimentalAll molding Molding trials Trials were carried out using a setup for thermoset injection molding at Hahn-Schickard as depicted in Figure 8 on a horizontal Arburg Allrounder 375 V (a), with a All molding trials were carried out using a setup for thermoset injection molding at mounted standard molding tool (b), and an attachment (Ø 20 mm) suitable for processing of Hahn-Schickard as depicted in Figure8 on a horizontal Arburg Allrounder 375 V (a), with a mounted thermosetting plastics (c). The upper half of the molding tool was free to move only in the vertical standard molding tool (b), and an attachment (Ø 20 mm) suitable for processing of thermosetting direction. The lower half, which also held the board-level package to be encapsulated, was mounted plastics (c). The upper half of the molding tool was free to move only in the vertical direction. The lower on a turntable rotating around the vertical axis. The injection feeding system and the molding tool half, which also held the board-level package to be encapsulated, was mounted on a turntable rotating inserts were cleaned before every trial with the use of a melamine resin Meloplas MF 152 (white around the vertical axis. The injection feeding system and the molding tool inserts were cleaned before color) [20] provided by Raschig GmbH as recommended by the machine and EMC manufacturers. every trial with the use of a melamine resin Meloplas MF 152 (white color) [20] provided by Raschig Having similar flow properties as the encapsulation material, Meloplas was also used for the filling GmbH as recommended by the machine and EMC manufacturers. Having similar ﬂow properties as study of the mold cavity. A minimum of 20 warming up cycles were taken up before every new the encapsulation material, Meloplas was also used for the ﬁlling study of the mold cavity. A minimum molding trial. All packages were subjected to a drying step at 130 °C for 1 h before encapsulation. of 20 warming up cycles were taken up before every new molding trial. All packages were subjected to After encapsulation (injection molding procedure), the packages were placed into the oven for the a drying step at 130 ◦C for 1 h before encapsulation. After encapsulation (injection molding procedure), accelerated completion of the cross-linking (curing) of the EMC. This involved holding the packages the packages were placed into the oven for the accelerated completion of the cross-linking (curing) of at the curing temperature (close to the mold temperature) of 150–180 °C for at least three hours as the EMC. This involved holding the packages at the curing temperature (close to the mold temperature) advised by the material manufacturer. As a comparison with market standards (and as a low-cost of 150–180 ◦C for at least three hours as advised by the material manufacturer. As a comparison with variant), PCBs with Tg = 125 °C were also investigated. market standards (and as a low-cost variant), PCBs with Tg = 125 ◦C were also investigated.

(a) (b) (c)

FigureFigure 8. Setup8. Setup for for thermoset thermoset injection injection molding: molding: (a) Arburg (a) Arburg 375 V injection375 V injection molding molding machine, machine, (b) closed (b) standardclosed standard molding molding tool, and tool, (c) attachment and (c) attachment for processing for processing of thermosetting of thermosetting plastics. plastics.

AA test test plan plan was was made made to to correctly correctly investigate investigate all all the the possible possible combinations combinations while while involving involving the the minimumminimum consumption consumption of of PCBs. PCBs. This This test test plan plan is is shown shown in in Figure Figure9. 9. The The tests tests were were planned planned to to start start ◦ fromfrom the the top-left top-left corner, corner, namely, namely, the the PCB PCB with with Tg Tg = 125= 125C °C and and an an encapsulation encapsulation thickness thickness of of 1 1 mm. mm. AA successful successful molding molding trial trial was was to to be be followed followed by by a patha path led led by by a greena green arrow arrow and and an an unsuccessful unsuccessful moldingmolding trial trial by by a reda red arrow. arrow. In In this this way, way, it wasit was ensured ensured that that the the molding molding trials trials proceeded, proceeded, as as far far as as possible,possible, using using the the PCBs PCBs with with the the lower lower Tg. Tg. When When these these PCBs PCBs were were rendered rendered useless useless in in a particulara particular moldingmolding trial, trial, the the same same encapsulation encapsulation thickness thickness could could be be tested tested using using the the PCBs PCBs with with the the higher higher Tg. Tg. TheThe subsequent subsequent molding molding trials trials wouldwould thenthen be carried out out only only with with the the PCBs PCBs with with the the higher higher Tg Tg(170 ◦ (170°C). C).A Acomparison comparison of ofthe the values values of of thermal thermal expa expansionnsion coefficients coefﬁcients of thesethese twotwo PCB PCB variants variants is is shownshown in in Table Table4. 4.

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FigureFigure 9. Test 9. Test plan plan for thefor moldingthe molding trials trials starting starting from from top-left. top-left. A A successful successful molding molding trial trial was was toto be followedfollowedfollowed by by the by green the green arrow arrow and and an unsuccessfulan unsuccessful moldingmo moldinglding trial trial was was to to be be followed followed byby by thethe the redredred arrow.arrow.arrow. Table 4. Comparison of the CTE values of the two PCB variants (ppm/K). TableTable 4. Comparison 4. Comparison of the of the CTE CTE values values of ofthe the two two PCB PCB variants variants (ppm/K). (ppm/K). ◦ ◦ DirectionDirectionDirection Tg Tg =Tg= 125125 = 125 °CC °C Tg Tg = 170= 170 Tg °C °C = 170 C XX X 12.812.812.8 13.513.5 13.5 YY Y 13.213.213.2 15.615.6 15.6 Z (belowZ Tg)(belowZ (below Tg) Tg) 43 43 43 55 55 55 Z (aboveZ Tg)(aboveZ (above Tg) Tg) 242 242 242 234 234 234

A preliminary filling study (Figure 10) for the first molding trial (MT01) showed similar filling A preliminary filling filling study (Figure 10)10) for the first first molding trial (MT01) showed similar filling filling behavior to the simulation (Figure 6) on both sides of the PCB. behavior to the simulation (Figur (Figuree 66)) onon bothboth sidessides ofof thethe PCB.PCB.

(a) (a)

(b) FigureFigure 10. Filling 10. Filling behavior behavior during during MT01 MT01 on on (a) ( mounting/fronta) mounting/front side side but but on on an an unmountedunmounted PCB, and (b) (b) back(b) back side side of the of PCB.the PCB. Figure 10. Filling behavior during MT01 on (a) mounting/front side but on an unmounted PCB, and (b) back side of the PCB.

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J. Manuf. Mater. Process. 2019, 3, 18 9 of 16 The molding trials (MT01) with an encapsulation thickness of 1 mm for the board-level packages The molding trials (MT01) with an encapsulation thickness of 1 mm for the board-level based on the PCBs with Tg = 125 ◦C were successfully carried out. An encapsulated package from packages based on the PCBs with Tg = 125 °C were successfully carried out. An encapsulated these molding trials is shown in Figure 11. No signiﬁcant problems were noted during these trials. package from these molding trials is shown in Figure 11. No significant problems were noted during When sufﬁcient packages were encapsulated with considerable repeatability, these molding trials for these trials. When sufficient packages were encapsulated with considerable repeatability, these anmolding encapsulation trials for thickness an encapsulation of 1 mm werethickness concluded. of 1 mm The were injection concluded. molding The injection setup was molding then retooledsetup forwas the then encapsulations retooled for with the encapsul a 0.5 mmations thickness. with a 0.5 mm thickness.

Figure 11. Encapsulated (1 mm thick) board-level package. Figure 11. Encapsulated (1 mm thick) board-level package. The molding trials (MT02) based on the PCB with Tg = 125 ◦C and an encapsulation thickness of The molding trials (MT02) based on the PCB with Tg = 125 °C and an encapsulation thickness of 0.5 mm were, however, not successful. Figure 12 shows an example of a large uncovered areas on the 0.5 mm were, however, not successful. Figure 12 shows an example of a large uncovered areas on the package.package. Various Various combinations combinations of of process process parameters parameters were were tried tried out out to to remove remove this this behavior. behavior.When When all theall combinations the combinations were were rendered rendered useless, useless, a few a few samples samples were were generated generated with with holes holes drilled drilled on on the the PCB at randomPCB at random positions positions to act asto pressureact as pressure equalizers equali betweenzers between both both sides sides of the of PCB.the PCB. These These samples samples were thenwere included then included in the molding in the molding trials. Nevertheless,trials. Nevertheless, these these measures measures could co notuld avoid not avoid the bendingthe bending of the PCBof asthe seen PCB inas Figureseen in 13Figure. As a13. further As a further measure, measure, a supporting a supporting pin waspin was provided provided in thein the molding molding tool inserttool on insert the loweron the (mounted) lower (mounted) side of the side PCB of to the prevent PCB to it fromprevent bending. it from Thereafter, bending. theThereafter, molding the trials weremolding repeated, trials however, were repeated, in vain. however, A stark in bending vain. A of stark the PCBbending was of still the noticed PCB was in still spite noticed of the in presence spite of theof the supporting presence pinof the and supporting pressure equalizing pin and pressure in the form equalizing of drilled in holes. the form The thermalof drilled expansion holes. The had, therefore,thermal a expansion very strong had, effect therefore, on the resultsa very ofstrong these ef moldingfect on the trials. results The moldingof these molding trials were trials. concluded The asmolding being unsuccessful trials were concluded and the path as being led byunsuccessful the red arrow and the asguided path led by by Figure the red9 arrowwas followed as guided in by the transverseFigure 9 direction.was followed in the transverse direction.

FigureFigure 12. 12.Signiﬁcant Significant uncovered uncovered areas areasof of thethe packagepackage du duringring mold mold trials trials MT02 MT02 due due to to the the bent bent PCB. PCB.

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J. Manuf. Mater. Process. 2019, 3, 18 10 of 16

Figure 13. Pressure equalizing holes were also not useful in avoiding the PCB from bending during Figure 13. Pressure equalizing holes were also not useful in avoiding the PCB from bending during moldmold trials trials MT02. MT02. Figure 13. Pressure equalizing holes were also not useful in avoiding the PCB from bending during mold trials MT02. MoldingMolding trials trials (MT02a) (MT02a) for for the the unchangedunchanged encapsulation thickness thickness of of 0.5 0.5 mm mm but but for for PCBs PCBs with with ◦ TgTg = 170= 170C °CMolding were were conducted. trialsconducted. (MT02a) The The for encapsulation theencapsulation unchanged processencapsulation process was was successfullythickness successfully of 0.5 carried mmcarried but out forout evenPCBs even with without without the presencethe presenceTg of = 170 the °Cof supporting werethe supporting conducted. pin and Thepin the encapsulationand pressure the pressure equalizingprocess equalizing was holessuccessfully holes assuming carriedassuming the out earlier eventhe earlierwithout set of set process of parameters.processthe parameters.presence A few encapsulatedof the A supportingfew encapsulated samples, pin and having thesamples, pressure equalizing ha equalizingving holes,equalizing holes however, assuming holes, exhibited however, the earlier alower exhibited set of injection a process parameters. A few encapsulated samples, having equalizing holes, however, exhibited a pressurelower injection requirement. pressure requirement. Thelower succeeding injection pressure molding requirement. trials (MT03) according to the test plan in Figure 9 for thickness 0.25 The succeedingThe succeeding molding molding trials trials (MT03) (MT03) according according to to the the testtest planplan in Figure Figure 99 for for thickness thickness 0.25 0.25 mm mm and PCBs with Tg◦ = 170 °C were conducted thereafter. They were partly successful. The filling and PCBsmm with and TgPCBs = 170with CTg were = 170 conducted°C were conducted thereafter. thereafter. They wereThey were partly partly successful. successful. The The ﬁlling filling behavior behavior was acceptable, but minor uncovered areas on the mounted devices were discovered. The was acceptable,behavior was but acceptable, minor uncovered but minor uncovered areas on areas the mountedon the mounted devices devices were were discovered. discovered. The actual actual path followed on the test plan is marked and shown in Figure 14 by the blue dotted line. path followedactual path on followed the test on plan the istest marked plan is andmarked shown and shown in Figure in Figure 14 by 14 the by bluethe blue dotted dotted line. line. Further Further characterization of the encapsulations only from MT03 is discussed in detail in the next characterizationFurther characterization of the encapsulations of the encapsulations only from MT03only from is discussed MT03 is discussed in detail inin thedetail next in the Section next 3.2 due SectionSection 3.2 due3.2 dueto itsto itscriticality criticality and and relevance relevance of being thethe minimum minimum thickness thickness tested tested during during the the to its criticality and relevance of being the minimum thickness tested during the experiments. experiments.experiments.

FigureFigure 14. Original 14. Original test test plan plan superimposed superimposed by th thee actual actual path path followed followed (blue(blue dotted dotted line). line). Figure 14. Original test plan superimposed by the actual path followed (blue dotted line). 3.2. Characterization3.2. Characterization of the of Encapsulation the Encapsulation 3.2. CharacterizationThe encapsulated of the Encapsulation samples were subjected to various stages of failure analysis. The PCBs were The encapsulated samples were subjected to various stages of failure analysis. The PCBs were tested for electrical connectivity of all the mounted devices at the following stages: tested forThe electrical encapsulated connectivity samples ofwere all thesubjected mounted to variou devicess stages at the of following failure an stages:alysis. The PCBs were • tested for afterelectrical mounting connectivity of the electronic of all thedevices, mounted devices at the following stages: • after• mountingafter the drying of the procedure/before electronic devices, injection molding, • • • afterafter the aftermounting drying the injection procedure/before of the molding electronic process, devices, injection and molding, • after• afterthe dryingthe accelerated procedure/before curing at elevated injection temperatures. molding, • • after the injection molding process, and after Afterthe injection completion molding of the process, accelerated and curing and stabilization of the packages at room • after the accelerated curing at elevated temperatures. • aftertemperature the accelerated for a minimum curing of at 24 elevated h, randomly temperatures. chosen packages were tested using X-ray scanning to look for any abnormalities present in the encapsulation. The images in Figure 15 shows examples of AfterAfter completion completion of theof acceleratedthe accelerated curing curing and stabilization and stabilization of the packagesof the packages at room temperatureat room packages with well-filled encapsulations over the resistors (a), MLF (b), and the induction coil (c). A for a minimum of 24 h, randomly chosen packages were tested using X-ray scanning to look for any temperature for a minimum of 24 h, randomly chosen packages were tested using X-ray scanning to abnormalitieslook for any abnormalities present in the present encapsulation. in the encapsulat The imagesion. The in images Figure in15 Figureshows 15 examples shows examples of packages of withpackages well-ﬁlled with encapsulationswell-filled encapsulations over the over resistors the resistors (a), MLF (a), (b), MLF and (b), the and induction the induction coil coil (c). (c). A A few

J. Manuf. Mater. Process. 2019, 3, 18 11 of 16 J. Manuf. Mater. Process. 2019, 3, 18 11 of 16 J. Manuf. Mater. Process. 2019, 3, 18 11 of 16 void-likefew void-like areas areas could could be observed be observed ineach in each of of the the images, images, which which werewere also noticed noticed in in the the packages packages few void-like areas could be observed in each of the images, which were also noticed in the packages beforebefore encapsulation, encapsulation, corresponding corresponding to the to typical the typical voids invoids the solder in the joints. solder The joints. smaller The round-shaped smaller round-shapedbefore encapsulation, darker regions corresponding in the image to of the the MLtypicalF (b) voidscan be inattributed the solder to solder joints. spatter, The whereassmaller darker regions in the image of the MLF (b) can be attributed to solder spatter, whereas the larger but theround-shaped larger but less darker dark regions region in maythe image correspond of the ML withF (b) either can be an attributed unfilled togap solder under spatter, the devicewhereas or less dark region may correspond with either an unﬁlled gap under the device or remnants of solder remnantsthe larger of butsolder less paste dark erroneously region may present correspond in the with no-contact either anregion. unfilled gap under the device or pasteremnants erroneously of solder present paste in erroneously the no-contact present region. in the no-contact region.

(a()a ) ((bb)) ((cc))

FigureFigureFigure 15. 15. X-ray15. X-ray X-ray images images images of ofencapsulated of encapsulated encapsulated devices devicesdevices (MT03): (MT03):(MT03): (a )(a resistors:) resistors: voids voidsvoids in inin solder soldersolder joint joint joint visible visible visible with nowith defectwith no no indefect defect the encapsulation;in in the the encapsulation; encapsulation; (b) MLF28: ( b(b)) MLF28: MLF28: voids voids invoids solder inin soldersolder joint, joint, solder soldersolder spatter spatterspatter and and remnantsand remnants remnants of solderof of pastesoldersolder visible, paste paste and visible, visible, void inand and encapsulation void void inin encapsulationencapsulation in the gap inin under thethe gapgap the deviceunder isthethe possible; devicedevice is andis possible;possible; (c) induction andand ( c()c coil:) voidsinductioninduction in solder coil: coil: joint voids voids visible in in solder solder with joint joint no defectvisi visibleble inwith with encapsulation. nono defectdefect in encapsulation.

TheTheThe encapsulation encapsulation encapsulation packages packages packages werewere were thenthen then subjectedsubjected toto scanning scanning acousticacoustic acoustic microscopymicroscopy microscopy (SAM) (SAM) (SAM) using using using anan Echoan Echo Echo STD STD STD by by by Sonix Sonix Sonix (Springﬁeld, (Springfield, (Springfield, VA,VA, VA, USA)USA) USA) availableavaiavailablelable in-housein-house at at Hahn-Schickard.Hahn-Schickard. Hahn-Schickard. Working Working Working on on theon the the principleprincipleprinciple of of nondestructive of nondestructive nondestructive ultrasonic ultrasonic ultrasonic ﬂaw flaw flaw detection, detectiondetection, this, thisthis analysis analysis helped helpedhelped toto investigateinvestigate the the quality quality of theof adhesionof the the adhesion adhesion between between between the encapsulation the the encapsulation encapsulation and the andand PCB ththe material.e PCBPCB material. From theFromFrom images thethe imagesimages in Figure inin Figure 16Figure, generated 16, 16, bygenerated usinggenerated a 50 by MHzby using using transducer, a a50 50 MHz MHz it can transducer,transducer, be noted thatitit cancan the bebe encapsulation noted that the held encapsulationencapsulation a good adhesion heldheld foraa goodgood a major partadhesionadhesion of the interfacefor for a amajor major area part part with of of the the interface interface PCB. Slight area area delamination withwith thethe PCB. Slight occurred delamination only in the occurred occurred areas near only only thein in the edgesthe ofareas theareas encapsulation. near near the the edges edges This of of the beginningthe encapsulation. encapsulation. of delamination ThisThis beginningbeginning occurred of duedelamination to large aspect occurredoccurred ratios duedue betweento to large large the aspect ratios between the length and the thickness of the encapsulation and a lack of mechanical lengthaspect and ratios the thicknessbetween the of thelength encapsulation and the thickne andss a lackof the of mechanicalencapsulation anchorage, and a lack which of mechanical was highly anchorage,anchorage, which which was was highly highly improved improved in in enclosing enclosing packagingpackaging designs. improved in enclosing packaging designs.

FigureFigure 16. 16.Images Images from from the the scanning scanning acousticacoustic micro microscopyscopy at at the the interface interface of ofthe the PCB PCB and and the the Figure 16. Images from the scanning acoustic microscopy at the interface of the PCB and the encapsulation.encapsulation. Except Except for for a a few few delaminated delaminated areasareas (marked in in red) red) at at the the edges, edges, the the encapsulation encapsulation encapsulation. Except for a few delaminated areas (marked in red) at the edges, the encapsulation showedshowed good good adhesion adhesion with with the the PCB PCB material. material. showed good adhesion with the PCB material. Later,Later, randomly randomly chosen chosen samples samples underwent underwent an advanced an advanced analysis process.analysis Relevantprocess. cross-sectionRelevant Later, randomly chosen samples underwent an advanced analysis process. Relevant planescross-section were chosen planes and were were chosen probed and using were optical probed microscopy. using optical As microscopy. seen in Figure As seen 17, thein Figure melt flowed17, cross-section planes were chosen and were probed using optical microscopy. As seen in Figure 17, through all regions for an encapsulation thickness of 0.25 mm, but developed a few air voids in the flow shadow areas of MLF28 (a). Apart from the similar problem of unfilled areas, a significant uncovered J. Manuf. Mater. Process. 2019, 3, 18 12 of 16 J. Manuf. Mater. Process. 2019, 3, 18 12 of 16 the melt flowed through all regions for an encapsulation thickness of 0.25 mm, but developed a few air voids in the flow shadow areas of MLF28 (a). Apart from the similar problem of unfilled areas, a area cansignificant be noticed uncovered in the area case can of the be noticed induction in the coils case (b) of owing the induction to the irregular coils (b) owing shape to and the amount irregular of the glue usedshape inand the amount manufacturing of the glue of the used device. in the These manu devicesfacturing have, of the compared device. These to the devices accurate have, molding tool insertcompared with to 0.25 the mm accurate wall thickness,molding tool looser insert component with 0.25 geometry mm wall tolerances, thickness, whichlooser component sometimes also led togeometry delamination tolerances, of the which encapsulation sometimes also from led the to de toplamination of the device of the during encapsulation demolding from the (c). top Likewise, of the loosethe device mounting during tolerances demolding also (c). presentLikewise, a challengethe loose mounting to ensure tolerances a constant also encapsulation present a challenge thickness to in all areasensure (d) a especially constant encapsulation in the lateral thickness directions. in all areas (d) especially in the lateral directions.

(a) (b)

FigureFigure 17. Results 17. Results from from microscopic microscopic analysis analysis I. I. (a ()a Voids) Voids were were noticed noticed inin thethe ﬂowflow shadow region region of of the MLF.the (b) MLF. An uncovered (b) An uncovered area (marked area (marked red) was red) noticed was noti betweenced between two devices two devices and theand irregular the irregular shape of the glueshape around of the the glue induction around coil the caused induction irregularities. coil caused ( c)irregularities. Delamination (c) ofDelamination the encapsulation of the from the holder of the induction coil was noticed due to a lower thickness available in the lateral direction. (d) An offset is seen in the positioning of the induction coil and the resistor that enforced the presence of non-uniform encapsulation thicknesses on both sides of the device. J. Manuf. Mater. Process. 2019, 3, 18 13 of 16

encapsulation from the holder of the induction coil was noticed due to a lower thickness available in J. Manuf.the Mater. lateral Process. direction.2019, 3, ( 18d) An offset is seen in the positioning of the induction coil and the resistor that 13 of 16 enforced the presence of non-uniform encapsulation thicknesses on both sides of the device.

InIn areas areas where where the the tolerances tolerances of the the devices devices and and their their mounting mounting deviations deviations were were tighter tighter than the than theencapsulation encapsulation thickness, thickness, the the problems problems mentioned mentioned do not do notoccur. occur. As seen As seenin Figure in Figure 18, there 18, therewere wereno noair air voids voids in in the the flow flow shadow shadow areas areas of of the the resistor resistor (a) (a) and and MLF MLF (b), (b), and and no no delamination delamination of ofthe the encapsulationencapsulation occurs occurs during during the the demolding demolding over over thethe inductioninduction coil (c). (c). Besides, Besides, the the loose loose wires wires of ofthe the inductioninduction coil coil could could also also be successfullybe successfully encapsulated encapsulated (c). (c). The The thin thin gap gap under under the the MLF MLF was was also also filled forfilled a major for parta major (d). part This (d). phenomenon This phenomenon can be possibly can be possibly considered considered to provide to provide an alternative an alternative to theneed to of usingthe need an of underfill. using an underfill.

(a) (b)

Figure 18. Results from microscopic analysis II. (a) Resistors and (b) MLF had a well-ﬁlled

encapsulation when no irregularities in the device geometry or mounting were present. (c) The loose wires of the induction coil also had a well-packed encapsulation. (d) The thin gap under the MLF was also ﬁlled, though only partly. J. Manuf. Mater. Process. 2019, 3, 18 14 of 16

The characterization and the molding trials showed that the encapsulations with a layer thickness of 0.5 mm and 1 mm can be manufactured without hassles over board-level packages. A further study, involving the detailed characterization, reliability, and failure analysis is in progress. The results will be included in a future publication.

4. Discussions and Conclusions The comparison of the filling simulation and the filling experimental study from the molding trials showed that the simulation with the available material model was sufficient to predict the filling behavior for a similar encapsulation application. However, the material model needed to advance to a higher level of accuracy in terms of better prediction of shrinkage and warpage (stress) calculations. This phenomenon is governed by the effects of temperature dependent pressure–volume curves for the melt and the cured material. The co-existence of the two phases makes it difficult for an accurate material model to be developed. The material manufacturers and the software providers are currently working together with research institutes that specialize in this field to improve this situation. Owing to the outcome from the molding trials detailed in Section 3.1, it can be concluded that though thermoset injection molding demands use of PCBs with a higher Tg, encapsulations of 1 mm thickness of low-cost (standard) PCBs with lower Tg can also be manufactured. The results of the characterization prove that board-level packages (PCB based) can be encapsulated with EMCs using thermoset injection molding with a minimal thickness of 0.25 mm. However, this encapsulation process is strongly dependent on device geometry and device mounting tolerances. Defects were observed during the characterization, which were not acceptable due to their high influence on the performance of the encapsulation. A medium-sized thickness of 0.5 mm or 1 mm is, therefore, recommended if the tolerances are either not constant or not tight enough within the expected limits. Besides, in case only the device mounting tolerances are relevant, the encapsulation wall thickness can be slightly increased only in the lateral directions. Throughout the molding trials, a number of insights were gained. The following helpful measures for similar applications are recommended:

• Areas around tall devices, e.g., induction coils or electrolytic capacitors, should be provided with, ﬁrst, ample draft angles for easier demolding and to avoid delamination of the encapsulation, and second, with ejector pins that provide an additional function of venting the trapped air. • Ejector pins should also be placed in the sprue and melt overﬂow areas to avoid the material from staying behind in the cavity due to strong adhesion with metallic surfaces through curing and thereby blocking the way for further cycles and disrupting the repetitive process. • Pressure equalizing holes on the PCB are highly recommended. They serve an additional purpose of increasing the mechanical stability of the encapsulation.

With careful implementation, thermoset injection molding can be effectively deployed for the encapsulation of board-level packages with EMCs easily available in granular forms. SMEs can proﬁt from this implementation by being able to procure non-standard encapsulated packages in medium (or even small) order sizes. A well-known and important hindrance in the development of microtechnology with respect to functional integration can, thus, be eliminated. Further work ongoing at Hahn-Schickard on this topic involves reliability analysis of such encapsulated packages/devices by applying already-existing lifetime models of solder joints based on the Cofﬁn-Manson relation [11].

Author Contributions: Conceptualization, R.K. and T.G. (Thomas Guenther); methodology, R.K.; simulation, R.K.; experiments, P.W. and R.K.; characterization, R.K., M.H., and M.S.; writing—original draft preparation, R.K.; writing—review and editing, M.S., T.G. (Thomas Guenther), T.G. (Tobias Groezinger), and A.Z.; supervision, T.G. (Thomas Guenther) and T.G. (Tobias Groezinger); project administration, R.K.; funding acquisition, T.G. (Thomas Guenther) and A.Z. Funding: This work was done in the scope of the research project AiF-Kapselung financed by the German Federation of Industrial Research Associations (AiF) as a part of the Industrial Collective Research (IGF) program J. Manuf. Mater. Process. 2019, 3, 18 15 of 16 by the German Federal Ministry for Economic Affairs and Energy (BMWi) (IGF Vorhaben Nr.: 18253 N). Authors would like to thank the different project partners and funding institutes. Acknowledgments: The authors would like to thank the members of the project advisory group comprising of experts from the relevant industry domains for their valuable support and advice through the duration of project application and execution. Last but not the least; the authors are very thankful to the technical team at Hahn-Schickard and IFM for their quick and efficient processing of the internal assignments for this project. Conflicts of Interest: The authors declare no conflict of interest.

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