an evaluation of thermoplastic materials and injection ... · pdf filedrools a lot 3 2’...

8 International Microelectronics And Packaging Society

Intl. Journal of Microcircuits and Electronic Packaging

The International Journal of Microcircuits and Electronic Packaging, Volume 21, Number 1, First Quarter 1998 (ISSN 1063-1674)

An Evaluation of Thermoplastic Materials andInjection Molding as a Discrete PowerSemiconductor Packaging AlternativeLonne L. Mays and Alexandra HubenkoMotorola Semiconductor Products Sector5005 East McDowell RoadPhoenix, Arizona 85008Phone: 602-244-6667/3399Fax: 602-244-4015/4201e-mails: [email protected]: Z312 (Mays) [email protected]: Z210 (Hubenko)

Abstract

This paper documents the evaluation of injection molding of power semiconductor packages using several new high temperature engineer-ing thermoplastics as an alternative to conventional power semiconductor packaging materials and techniques. The properties of an idealmaterial are described in this paper, as well as an example package evolution which takes full advantage of one such advanced material’sproperties.Investigation and evaluation was accomplished by prototype molding trials with candidate materials followed by performance tests suchas thermal shock, temperature cycling, and autoclave. As a result of the research and the experiments performed, a new thermoplasticmaterial has been developed which has a thermal conductivity of 1.26 W/m•K, almost an order of magnitude greater than that of aconventional thermoplastic material. This material maintains the electrically insulating characteristics required for microelectronicspackaging while dissipating heat from a high power semiconductor device.

Key words:

Injection Molding, Thermal Conductivity, Power Semiconductor,and Electronic Packaging.

1. Introduction

The current molding process for power semiconductor pack-ages is epoxy transfer molding. This process uses moderate pres-sures to transfer B-stage epoxy into a hot mold where it cures to asolid. A potentially faster, lower cost process is thermoplastic in-jection molding. This method uses heat, high pressure, and shear-ing forces to liquefy and inject material into a relatively cool moldwhere the material “chills” to a solid status. Mold cycle time istypically 5X to 10X faster compared to transfer molding, and run-ners and sprues can be recycled.

This paper documents the evaluation of injection molding usingseveral new high temperature engineering thermoplastics as an al-ternative to conventional power semiconductor packaging materi-als. An ideal material would have good thermal/mechanical andelectrical properties as well as the ability to shield the internal semi-conductor structure from moisture and contamination. The reli-ability tests used in this evaluation were selected to specificallyverify the mechanical and electrical properties. In addition, sinceconventional package geometry did not maximize the benefits of athermal-conductivity enhanced material, a new package design wasdeveloped which took advantage of thermally enhanced materialproperties from a power dissipation standpoint.

2. Background

Epoxy transfer molding has been the process of choice for mold-ing semiconductor packages for decades (the transfer molding pro-cess is one of the first technologies developed in the plastics indus-

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An Evaluation of Thermoplastic Materials and Injection Molding as a Discrete Power Semiconductor Packaging Alternative

23

try, and has been around since 19401). The moderate pressuresused to drive (transfer) the catalyzed mold compound into the heatedmold and hold it during initial polymerization are below the levelswhere mechanical damage would be induced in the semiconductorleadframes, semiconductor chip wirebonds, or discrete insert as-semblies captured in the mold. In operation, the mold remainsclamped shut on the leadframe or insert while the transferred ep-oxy chemically crosslinks to achieve a partial cure (sufficient toachieve mechanical stability). The product is then removed fromthe mold, singulated (freed from leadframe or runners), and thensubjected to post-mold curing in batch ovens to complete the cur-ing reaction and achieve full mechanical properties. It is impor-tant to note that since epoxies are thermoset materials, they cannotbe recycled; this results in a high percentage of the expensive moldcompound becoming scrap. Typical discrete large cavity-countmolds may have as much as 50% of the material wasted in therunners and the pot cull. Additionally, molding cycle times arelong (minutes), and the in-mold curing process emits EPA regu-lated fumes (fume hoods and the corresponding permits are re-quired).

Thermoplastic injection molding, on the other hand, representsa potentially faster, lower cost process applicable to power discretesemiconductors (which are mechanically more robust than inte-grated circuit semiconductors). As a category, the thermoplasticinjection molding process represents the majority of plastic com-ponents manufactured today. As a consequence, its process pa-rameters are well documented and understood. The process usesheat, high pressure, and shearing forces to liquefy granules of ther-moplastic and rapidly inject the molten material into a relativelycool mold where it “chills” to a mechanical solid. The mold cycletime can be an order of magnitude faster (seconds) compared to thetransfer molding process, since no chemical reactions are takingplace. Additionally, thermoplastics are nearly 100% recyclable:the runners and sprues can be ground up and mixed in with virginmold compound at high percentages without degrading the proper-ties of the resulting plastic.

Of interest to semiconductor and other electronic componentmanufacturers are several recently developed high temperature en-gineering thermoplastics, some of which are currently widely usedin the automotive industry for under-the-hood applications2, whereoperating conditions range from -40 to +175 degrees C, and com-ponents are exposed to moisture, dirt, grease, oil, and solvents. Onthe other hand, thermoplastics do have temperature limitations, andbehave differently over temperature than cross-linked thermosetcompounds like epoxies. For components which must sustain me-chanical loads, thermoplastic designs must take into account thereduction in flexural modulus which occurs with increases in tem-perature. A thermoplastic mechanical component must be designedto sustain its required loads utilizing the mechanical properties whichwill be present at the peak temperatures expected to be encoun-tered in the operating environment. However, for non-load bear-ing components such as semiconductor packages, the selected ma-terial needs only to retain its electrical characteristics and the abil-ity to shield the internal semiconductor structure from moistureand contamination. Additionally, it should be considered a benefitif at the higher temperatures, the protective plastic body exhibits a

lower stress to the semiconductor chip and the internal solder bonds,such as would be the case with a reduction in the flexural modulus.

Investigation and evaluation were accomplished by prototypemolding trials with candidate materials (selected based onmanufacturer’s recommendations and materials data sheets) fol-lowed by performance tests such as thermal shock, temperaturecycling, and autoclave. As a result of the research and experiments,a new thermoplastic material has been developed which has a ther-mal conductivity of 1.26 W/m•K, almost an order of magnitudegreater than that of a conventional thermoplastic material3. Thismaterial maintains the electrically insulating characteristics requiredfor a semiconductor package, while dissipating heat from a highpower semiconductor device.

Molding trials were conducted via two different 8-cavity moldsprocured and set up in an Arburg 221 injection molding press at theMotorola Space Systems Technology Group’s Plastics Laboratory.

3. Button Rectifier Injection MoldingExperiment

The first experiments were conducted using a mold designed toduplicate the Motorola button rectifier package. This product con-sists of a semiconductor die (chip) soldered between two cylindri-cal heatsinks; the sandwich assembly is then overmolded such thatthe opposing heatsinks are allowed to protrude from the mold com-pound. This package represents a worst case condition for injec-tion molding, as the button sandwich floats inside the mold cavity(for example it is captured, but is not subjected to any clampingpressure). High injection or holding pressures could theoreticallyforce the upper and lower halves of the button sandwich apart, frac-turing the solder joint or die.

The first experiment was divided into 8 runs, each using a dif-ferent combination of high temperature thermoplastics on buttonsandwich inserts both with and without die filming. (Note: diefilming is a silicone room temperature vulcanized (RTV) rubbercoating applied as a thin film to protect the semiconductor junctionfrom contamination.) After molding, parts were subject to the fol-lowing reliability tests,· H

2O Immersion— This test consisted of immersing the parts

in room temperature water for four hours, followed by ambi-ent air drying.

· TSHOCK — This thermal shock test consisting of heating theparts to +175°C in an oven, then plunging them into liquidNitrogen (5 cycles.)

· TCYCL — An air-to-air thermal cycle test from -65°C to+175°C (100 cycles.)

· PTH — 96 hours of autoclave during which the devices weresubjected to steam (100% RH) at 14.7 psig.

All devices came from the same wafer lot (4mm square planarSiO

2 passivated epitaxial die) and same manufacturing lot. The

1000 device lot was split at the die filming process, with 500 de-vices receiving die filming, and 500 skipping the die filming op-




eration.The lot was further split so that 100 of the die filmed and 100 of

the not-die filmed devices went through the normal button transfermolding process and post mold curing only. They did not receivedeflashing or plating processes. These units became the two trans-fer molded epoxy control groups (Hysol MG40F). The thermo-plastics evaluated were Valox (polybutylene terphthalate - PBT),Santoprene (thermoplastic elastomer), and Amodel(polyphthalamide -PPA); the PPA used was specifically formulatedby the manufacturer to withstand wave soldering.

4. Thermally Conductive FillerExperiment

An experimental matrix was set up to evaluate two differentthermoplastics (PPS and PPA) and three different fillers (fiberglass,silica coated aluminum nitride, and carbon graphite fibers). Theexperimental objective was to determine if a thermoplastic can befilled with thermal conductivity-enhancing particles and retain ac-ceptable moldability characteristics. The Full Factorial experiment,one replication matrix was used since two-factor interactions wereanticipated to occur. The response variables were identified as thejunction-to-air thermal resistance (R2JA) and the junction-to-leadthermal resistance (R2JL). The response parameters of interestwere moldability (ease of processing during molding) and R2JAof the molded device. The matrix is summarized in Table 1.

Table 1. Thermally conductive filler DOE matrix.

EXP MATRIX FILLER RESINTYPE

RESULTS VARS COMPOUNT #

COLOR MOLDABILITY

LIKERT

Run#

Levels Cfiber

Glass A1N Thermo-plastic

1/ReJA 1/ReJA

1 --- 0% `40% 0% PPS 1.09 4.85 RYTONR9-02

BLK Good, butdrools a lot

3

2 --+ 0% `40% 0% PPA 0.925 3.6 AMODEL 1115

TAN Excellent 5

3 -+- 0% 0% >50% PPS 0.928 4.92 13-68712B

CHGRY

excellent,but drools

4

4 -++ 0% 0% >50% PPA 1.2 4.31 40-68713B

LT GRY excellent 5

5 +-- 5% 0% 0% PPS 1.21 4.63 13-68712A

BL GRY poor,drools,flashes, softon eject

1

6 +-+ 5% 0% 0% PPA 0.925 3.94 40-68713A

BLK poor, verysoft oneject

2

7 ++- 5% 0% >50% PPS 1.27 4.7 13-68712C

BLK fair, soft oneject

3

8 +++ 0% 0% >50% PPA 1.2 4.91 40-68713C

BLK excellent 5

1’ --- 0% `40% 0% PPS 1.17 5.1 RYTONR9-02

BLK good, butdrools a lot

3

2’ --+ 0% `40% 0% PPA 1.1 4.26 AMODEL 1115

TAN excellent 5

3’ -+- 0% 0% >50% PPS 1.32 5.05 13-68712B

CHGRY

excellent,but drools

4

4’ -++ 0% 0% >50% PPA 1.26 4.82 40-68713B

LT GRY excellent 5

5’ +-- 5% 0% 0% PPS 1.24 4.17 13-68712A

BL GRY poor,drools,flahses, softon eject

1

6’ +-+ 5% 0% 0% PPA 1.1 4.09 40-68713A

BLK poor, verysoft oneject

2

7’ ++- 5% 0% >50%+ PPS 1.3 4.31 13-68712C

BLK fair, soft oneject

3

8’ +++ 5% 0% >50%+ PPA 1.2 4.91 40-68713C

BLK excellent 5

5. Results

Figure 1 shows the button rectifier injection molding experi-ment results as a total percentage of units that survived each of theabove reliability tests. It can be seen in each case except the con-trol groups, that die filming improved the reliability. The controlgroup performed slightly less well with die filming, since the film-ing process has the known side effect of degrading the adhesion ofthe epoxy mold compound to the nickel plated heat sinks. The ther-moplastics, on the other hand, benefited from the use of die film.

The results of a second button molding experiment are docu-mented in Figure 2. In this experiment, an additional high tem-perature thermoplastic material, polyphenylene sulfide (PPS), wasevaluated. PPS is a semicrystalline polymer which has a relativelyhigh melting point (280°C). It is resistant to solvents and radiation,and is well-suited for metal replacement applications2, such as theelimation of metal heat sinks for this rectifier application. Two dif-ferent formulations of PPS were added to the experiment, Ryton R-9-02 and Ryton R-4-02. (Note: The R-9-02 has been formulatedby the manufacturer to withstand infrared and vapor phase solder-ing.) The parameter of interest in these runs was electrical leakage(Ir), induced by penetration of ionic moisture into the package. Allruns were subjected to immersion in hot salt water which was sub-sequently allowed to cool to ambient temperature while the de-vices soaked for 4 hours minimum. Devices were then rinsed andair dried.

Figure 1. Reliability test total survival percentage.

In Figure 2, points closest to the center represent the lowestleakage. Each major step outward from the origin (center of thechart) represents a ten fold increase in leakage; W/DC means withdie coating (die filming); NO DC means without die coating. Again,it is apparent that die coating assists in sealing out the environmentfrom the die. Ryton R-9-02, however, performs at acceptable lev-els even without the die coating. As an additional control, a groupof finished goods units (FGCNTL) were run in the same salt waterimmersion test. Interestingly, these control units performed ratherpoorly.

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25

Figure 2. Moisture induced electrical leakage.

Perhaps the most interesting results were from the thermallyconductive filler experiment. Recall that almost all current moldcompounds, including thermoplastic mold compounds, are ther-mal insulators. Typical thermal conductivities for Novalac epoxysilica filled mold compounds2 are in the range of 0.5 W/m•K. As acomparison, aluminum, a common heatsink material3, is ~220 W/m•K. To achieve the higher power dissipation customers desire, itwould be advantageous to be able to add fillers which enhance thethermal conductivity of the mold compound. Unfortunately, in thecase of epoxies, altering the thermal conductivity of the mold com-pound also changes the rate of the curing reaction, which is boththermally catalyzed and exothermic.

Prior attempts at developing thermally conductive mold com-pounds have centered around adding ceramic fillers (such as alu-minum oxide) to thermoset epoxies (such as Novalacs)2. Althoughimprovements of an order of magnitude in thermal conductivitywere achieved, these materials proved very difficult to mold. Pro-cessing problems encountered included short shots and incompletefills (due to the accelerating effect of the thermally conductive filleron the in-mold catalyzation), very narrow process control win-dows, and to batch-to-batch moldability variations.

Thermoplastics such as PPS do not go through a chemical reac-tion during the molding process. They simply go through a physi-cal change of state (from a hot molten state during injection into themold to a solid at temperatures below 200°C.) All process vari-ables, therefore, are under “knob” control in the injection moldingpress (for example, melt temperature, shearing velocity, injectionpressure, holding pressure, mold die temperature)4. Enhancing ther-mal conductivity of thermoplastics further reduces the possible moldcycle time by enabling the plastic to chill to a solid more quickly.Again, this chilling is totally under “knob” control in the moldingpress operation, as the mold and injection melt temperature controlcan be easily implemented with closed loop feedback.

As noted previously, thermoplastics do not require post moldbaking, runners and sprues can be recycled, and mold cycle timesrange between 5X and 10X faster than epoxy mold compounds.Engineering thermoplastics, therefore, are an ideal matrix materialfor experimenting with adding thermally conductive fillers.

Another eight cavity mold was constructed to accommodate ex-perimentation with an axial lead package. The package is the SMJ(Surface Mount Jumbo), a new high power, high voltage rectifier

package. Since customers are demanding ever higher power dissi-pation from power discretes, this new package utilizes a molded in“heatsink fins” type construction to take advantage of convectionto dissipate power (see Figure 3). The efficiency of the heatsinkfins, of course, is dependent on the thermal conductivity of the moldcompound.

Figure 3. Surface mount jumbo package.

The experiment matrix addressed two different types of ther-moplastic (PPS and PPA), and three different types of fillers (glassfiber, carbon fiber, Aluminum Nitride particles). The experimentmatrix was constructed as a Full Factorial with one replication (16runs total). A Full Factorial was utilized as beneficial two-factorinteractions were anticipated. The response variable of the experi-ment matrix was thermal resistance junction-to-air.

As can be seen from Figure 4, the statistical analysis is some-what noisy. The results tend to indicate, however, that the ther-mally conductive fillers (AlN and carbon fiber) were primary fac-tors. Surprisingly, the type of thermoplastic (PPS versus PPA) isprobable as a significant factor as well. Significant two-factor in-teractions are also evident in the experiment matrix. Figure 5 showsthe interaction of carbon fiber filler with the type of thermoplastic.There was a similar two-factor interaction with the type of thermo-plastic and the AlN filler.

As a result of the experiment, a thermoplastic mold compoundhas been developed by Motorola which is both thermally conduc-tive and electrically insulating. It consists of a PPA matrix filledwith aluminum nitride (AlN) and Carbon graphite fiber rods (Fig-ure 6). This new material molds with a wider process window thanany thermally conductive epoxy currently available, and being athermoplastic, it is 100% recyclable.




26

Figure 4. Statistical analysis.

Figure 6. Schematic representation of Motorola developed ther-moplastic mold compound.

A SEM photograph of the Motorola developed mold compound5

is shown in Figure 7. Note that the carbon graphite fibers appear asellipses since the plane of the cross section has cut through thefibers at random angles.

Figure 7. 1000X SEM image of Motorola developed thermoplas-tic mold compound.

Additional testing was performed on the molded rectifier de-vices under operating conditions with forced air cooling from atypical power supply “muffin” fan. Infrared images were capturedof the devices after thermal equilibrium was achieved. Figure 8shows the results with a cubic molded body using glass filled PPS.This material performs comparably to a standard silica filled epoxymolding compound. Note that the area of the device radiating (dis-sipating) the most heat is the lead, and not the body (the lead isseveral degrees hotter than the plastic body).

Figure 5. Carbon fiber versus plastic two factor experiment ma-trix interaction.




Figure 8. Cubic body with glass filled PPS.

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Figure 9 shows the results with AlN filled PPS and a finnedbody. Note that the body of the package is now radiating heatequal to the leads (that is it is at the same temperature). This is dueto the enhanced thermal conductivity attained via the thermally con-ductive filler. In addition, to achieve the same peak temperature asthe device in Figure 8, the power input to the device had to beincreased 20%. This is equivalent to a 20% gain in power dissipa-tion.

Figure 9. Finned body with AlN filled PPS.

6. Conclusions

Thermoplastic injection molding has several advantages overthe conventional epoxy transfer molding technique, including: voidfree, cosmetically perfect parts; complete “knob” control of keyprocess variables; mold cycle rates from 5X to 10X faster (30s forinjection molding compared to 150s for transfer molding); elimi-nation of pellet pre-heating, deflashing, and post mold curing op-erations; recycling and re-use of scrap material; smaller less ex-pensive mold die (fewer cavities required for same production ca-pacity); more cosmetically perfect parts (<0.5 PPM mold voids forinjection molding compared to >100PPM mold voids for transfermolding); and more flexible manufacturing (quicker change out of

mold die). Although thermoplastic injection molding has its draw-backs, new capital equipment would be required, the process isrelatively new to the semiconductor industry, and the molding tech-nique is not compatible with small gauge wirebonds, it shows prom-ise as an alternative molding material and process for power dis-crete devices.

References

1. J.H. DuBois and W.I. Pribble, “Plastics Mold Engineering,”Reinhold Publishing Corp., New York, 1965.

2. G.L. Collins and J.D.Menczel, Polymer Engineering and Sci-ence, Vol. 32, No. 17, 1992.

3. F.P. Incropera and D.P. DeWitt, “Fundamentals of Heat Trans-fer,” John Wiley & Sons, New York, 1981.

4. J.S. Walker and E.R. Martin, “Injection Molding of Plastics,”The Plastics Institute, London, 1966.

5. SEM photograph courtesy of M. Schade, Chemical and Sur-face Analysis Laboratory, Motorola Inc., Phoenix, Arizona,1995.

About the authors

Lonne Mays is a Principal Staff Engineer/Scientist assigned toMotorola Semiconductor Products Sector’s discrete devices. Heis a 21 year Motorolan, has applied for five patents, and has beenpublished both internally and externally. He received his Master ofTechnology Degree in Manufacturing Engineering Technology fromArizona State University in 1990, and his B.S. Degree in IndustrialTechnology in 1986. He received an Associates in Electronics in1970, and has been involved in circuit design and electronics manu-facturing for over 28 years.

Alexandra Hubenko received her B.S. Degree in Materials Sci-ence & Engineering from Cornell University, and has been withMotorola for three and a half years. She has worked on develop-ment and characterization of chemical sensor materials, and is cur-rently a product engineer in the pressure sensors Group.

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