Silicone Gels for Semiconductor Applications - American Chemical

Chapter 20

Silicone Gels for Semiconductor Applications Chemistry and Properties Gust J . Kookootsedes

Downloaded by COLUMBIA UNIV on September 19, 2017 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch020

Dow

Corning Corporation, Midland, MI 48686-0995

The use of soft silicone gels to encapsulate and protect semiconductor devices is becoming increasingly important as device complexity increases, greater rel i a b i l i t y and cost are emphasized and packaging trends change. In addition, stresses encountered during the packaging of larger semiconductor chips, and concern for alpha particle induced errors, in many cases necessitates the use of a protective coating. This paper reviews the synthesis, chemistry, cure and general properties of silicone gels. The information provided is intended for those who may be unfamiliar with this technology and who are using or contemplating their use. The escalating complexity and density of c i r c u i t elements has i n creased the lead count and the size of semiconductor chips. At the same time, there i s a growing emphasis on greater r e l i a b i l i t y and lower cost. These simultaneous events are prompting the semiconductor industry to investigate moving from costly ceramic packages to non-hermetic types such as pre-molded p l a s t i c chip c a r r i e r s , both leaded and leadless, and pin grid arrays. Where post molding can s t i l l be used, stresses induced by the molding material, either during the molding operation or during thermal stress, and the threat of alpha p a r t i c l e s emitted by f i l l e r s i n the compound are of concern. In many instances the solution to these problems i s the use of s i l i c o n e gels as protective coatings. In recent years we have read and heard much about the benefits of s i l i c o n e gels. For example, Kanji Otsuka of Hitachi (1) and Rachel M i l l e r of Burroughs (2) described results achieved using s i l i c o n e gels as protective coatings for non-hermetic pin grid arrays. R.E. Thomas of Motorola (3) observed that the deformation of aluminum m e t a l l i z a t i o n of p l a s t i c encapsulated semiconductor chips could be eliminated by coating the chip surface with a soft gel. Jonathan King of Singer Company (4) reported the use of gels as p a r t i c l e getters inside hermetic micro-microcircuit packages. 0097-6156/89/0407-0230S06.00/0 o 1989 American Chemical Society

Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


20. KOOKOOTSEDES

Silicone Gels for Semiconductor Applications

231

A l l have stimulated interest i n the properties and performance of s i l i c o n e gels. In addition, i n 1986 the IEEE Computer Packaging Committee set up a special task force chaired by Jack W. Balde to investigate the p o s s i b i l i t y of gaining m i l i t a r y approval to use gels for protecting non-hermetic packages. Recent interest not withstanding, the use of s i l i c o n e gels to protect m i c r o - c i r c u i t r y i s not new. S i l i c o n e gels have been used to protect various automotive electronic modules for a number of years. E. S a i l e r and A. Kennedy of IBM reported the use of a s i l i c o n e gel to protect the c i r c u i t r y of a non-hermetic module used i n t h e i r IBM System/360 computers as early as 1966 (5). The object i v e of this paper i s to give those unfamiliar with s i l i c o n e materi a l s a better understanding of the synthesis, chemistry and propert i e s of s i l i c o n e gels. Silicone Gels!

What Are They?

S i l i c o n e gels may be viewed as s l i g h t l y cross-linked f l u i d s or under cured elastomers. They are unique i n their c h a r a c t e r i s t i c s and t h e i r a b i l i t y to protect both i n d i v i d u a l integrated c i r c u i t s and hybrid c i r c u i t s . S i l i c o n e gels may vary from very soft and tacky to moderately soft and only s l i g h t l y sticky to the touch. In fact, the hardness of a gel as defined can not be measured by the normal methods employed for elastomers and p l a s t i c materials. The t e n s i l e strength and modulus of s i l i c o n e gels are very low, which allows their use without unduly stressing small interconnect wires during thermal stress conditions. H i s t o r i c a l l y , firmer high purity s i l i c o n e coatings, and more recently the gels, have demonstrated t h e i r extreme compatibility with most semiconductor devices. In many cases these devices provide improved performance, greater s t a b i l i t y and higher y i e l d s than their uncoated counterparts. Several mechanisms for curing gels are possible; however, most have l i m i t a t i o n s involving either processing or the f i n a l gel prop e r t i e s . Condensation type cures form water or alcohol by-products which cause outgassing and voids. Free r a d i c a l peroxide-activated addition cures make i t d i f f i c u l t to control gel consistency from batch to batch. These problems are not evident i n the most prevalent cure mechanism used today, the addition of silicon-bonded hydrogen atoms to silicon-bonded o l e f i n i c r a d i c a l s , usually v i n y l , i n the presence of a few parts per m i l l i o n of a platinum catalyst ( I ) . This system creates no by-products and i s e a s i l y controlled. Pt ~ w SiCH=CH + H S i ^ 2

SiCH CH Si~"~+ no by-product 2

2

(I)

Synthesis Silicones are unique synthetic polymers that are p a r t i a l l y organic and p a r t i a l l y inorganic i n nature. They have a quartz-like backbone made up of alternating s i l i c o n and oxygen atoms rather than the carbon-carbon chain c h a r a c t e r i s t i c of organic polymers. Fluid polymers, t y p i c a l l y , have two organic groups attached to each s i l i con atom. They are usually methyl; however, groups such as phenyl and v i n y l provide special properties.


232

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

The preparation of s i l i c o n e s begins with quartzite which i s reduced to s i l i c o n metal i n an e l e c t r i c arc furnace ( I I ) . Si0

+ 2 C

2

Si + 2 COf

(II)


The next step involves the conversion of s i l i c o n to chloro silanes, the basic building blocks for s i l i c o n e s . The most impor tant series of chlorosilanes are those having methyl attached to the s i l i c o n atoms. They are produced by reacting the s i l i c o n metal i n the presence of catalyst and heat with methyl chloride and hydro gen chloride, y i e l d i n g a family of silanes ( I I I ) . Si + CH C1 +

HC1

3

R H SiCl, , χ y 4-(x+y) A

(III)

N

For the purposes of this discussion the most important silane i s dimethyldichlorosilane, ( C H ^ S i C ^ . The controlled hydrolysis of dimethyldichlorosilane y i e l d s octamethylcyclotetrasiloxane, [(CH^) SiO]^, which i s the basic ingredient used to generate d i methyl polymers. Other siloxanes needed to produce the necessary polymers are obtained from the hydrolysis of respective c h l o r o s i lanes. Chlorosilanes and siloxanes having substituents on s i l i c o n other than methyl and hydrogen are usually prepared by other means, and t h e i r syntheses are outside the scope of this discussion. How ever, t h e i r importance w i l l become evident l a t e r as we discuss poly mer make-up and some low temperature considerations of the f i n a l cured gel. Most polymers used for s i l i c o n e gels are of two types. In one case the polymer chain i s made up of predominantly dimethylsiloxy units with dimethylvinylsiloxy end groups (IV) or a mixture of d i methyl and methylvinylsiloxy units with trimethylsiloxy end groups (V). In some cases methylvinylsiloxy units may be used in the chain and dimethylvinylsiloxy units used as end groups (VI). In a l l cases the number of v i n y l groups present are very small. 2

CH CH =CHSiC3 - S i 3 — àiite CH CH„ CHJ CH 3* χ 3

CH. CH CH.JiO—rSid 3 · £Η^ CH

0

n

0

u

(IV) ICHJ

fc:

â|—S±ÔH=

=CHSi0CH

0

(V)

ÇH CH.

C H . CH„ •SiC SiO-SiC: CH CH IftL

3

LCH^J

x

kï

CH.

(VI) Polymers of this type are produced by either a c i d - or basecatalyzed polymerizations of c y c l i c siloxanes where t r i s u b s t i t u t e d siloxanes are used to end-cap the polymer chains and control the v i s c o s i t y (VII).


20. KOOKOOTSEDES

Silicone Gehfor Semiconductor Applications

233

Acid or x [(CH ) SiO] 3

2

+ y [CH RSiO]

4

3

+ [(CH ) RSi] 0

4

3

2

£

Base (VII) CH

CH„

0

CH.

CH

0

Rài0îo4îoMiR


CH

L(5H

3

CH - or CH =CH3

2

CH„ 3JX

If R i s other than methyl on the disubstituted siloxane, they w i l l be distributed randomly i n the polymer chain. The other important type of polymer contains a hydrogen atom attached d i r e c t l y to s i l i c o n . These polymers would be very s i m i l a r i n structure to those shown above, (IV), (V), and (VI), i f hydrogen groups were substituted f o r v i n y l . These polymers are usually lower i n molecular weight and v i s c o s i t y . Oligomers such as (VIIIa-c) have also been used. The combinations and ramifications are nearlv l i m i t l e s s i n siloxane chemistry. R R (VIIIc) (VHIb) Si lOSiH R S i |0SiH (Villa) i-O-SiH CH„ OH, CH R = CH - or C H -

*

5

n

3

6

5

The v i s c o s i t y of the v i n y l functional polymers used to produce gels may run from approximately 200 to 7000 centipoise, which r e lates to molecular weights of approximately 7500 to 35000. The hydrogen functional oligomers or polymers are usually much lower i n v i s c o s i t y , 2 to 100 centipoise f o r molecular weights of 134 to 5000. In formulated systems the hydrogen-functional polymer i s usually blended with some of the higher v i s c o s i t y v i n y l functional polymers to make mixing of the two components easier. One-component systems have no problems i n t h i s respect. (The reader i s directed to "Organosilicon Compounds" by C. Eaborn, Butterworths, London, 1960 and "Chemistry and Technology of S i l i c o n e s " by W. N o l l , Academic Press, New York and London, 1965, f o r a comprehensive r e view of the synthesis and chemistry of s i l i c o n e materials.) Cure Mechanism As stated before, the predominant cure mechanism used today i s the addition of silicon-bonded hydrogen atoms to silicon-bonded o l e f i n i c r a d i c a l s ( I ) . For gelation or cure to take place, either the vinyl-containing polymer or the hydrogen-containing polymer, or both, must contain more than two reactive groups per polymer chain. A very s i m p l i f i e d example i s shown below (IX). As can be seen, silethylene cross-links are formed which serve to gel the structure. If the polymers contain two or fewer reactive groups per chain, one would obtain only chain extension with an increase i n v i s c o s i t y but no c r o s s - l i n k i n g .



234 Me

Me

Me

Me

Me

Me

Me

(Me

2 Meâi0îC|—éio4sioLsiMe + 3 H$iO-4sicU-èiH CH CH

Mel

OH

Me ^ OH n

Me

Me I Me *- x J

2

Me Me Me MeSiO-lsid CH Me^ n

Me SiH Me

CH

-

0

(IX)

-> To another v i n y l containing polymer chain.

CH [Me I Me [Me I Me Me [Me I Me MeSiO-isid—Si0îO--SiCH^CH SiO-4£id—èiMe 2* p e ^ tH Me Mel OH \Ae L Me 0


2

CH:

7

MeSiMe

Me CH Me Me Me MeéiO—IsicJ—Si8-4iid—£iCH' fee. Me CH n

Me [Me SiO-}—SiMe ι χ H

n

To another hydrogen containing polymer chain. To another v i n y l containing polymer chain. The ideal mixture would have an equal molar amount of s i l i c o n bonded v i n y l and silicon-bonded hydrogen so that the desired consistency i s obtained at cure without excess v i n y l or hydrogen r e maining. Unfortunately, this idealized s i t u a t i o n i s very d i f f i c u l t to achieve. Without exception, some of one or both w i l l remain unreacted. Even i n the idealized case, not a l l v i n y l and hydrogen groups would be i n a position to react with each other once crossl i n k i n g began to r e s t r i c t mobility of the polymer chains. Uncured Properties Many of the currently used products are two-part systems that r e quire mixing to activate the cure mechanism. Some are mixed oneto-one while others are mixed ten to one. Generally speaking, the mixing r a t i o may be by weight or by volume. In the past few years one-part gels which eliminate the need for mixing but make heat curing mandatory have made their way into the market. A l l currently available systems are free flowing l i q u i d s having v i s c o s i t i e s as low as 300 and as high as several thousand centipoise. In some cases high v i s c o s i t y may cause interconnect wire deformation, poor coverage under closely spaced f l i p chips, and entrapment of a i r bubbles. On the other hand, the use of very low v i s c o s i t y products may lead to rapid creep of the f l u i d onto land areas which w i l l be used for l i d sealing.


20. KOOKOOTSEDES

Silicone Gelsfor Semiconductor Applications

235

Cure Time and Temperatures


Room temperature curing two-part systems require from as l i t t l e as twenty-four hours to as long as seven days to reach ultimate properties. In most cases heat i s used to accelerate cure and to speed up the manufacturing process. Cure times from a few minutes to hours are used at temperatures of 100° to 175°C. Times and temperatures are predicated on coating thickness as well as device type. Work reported by K. Otsuka et a l . indicates that high temperature cures may improve device performance (6). In some instances, however, temperatures are limited by device construction; e.g., use of low temperature solders. Purity And Cleanliness Silicone gels intended f o r direct application to active semiconductor devices, especially VLSI's, are controlled f o r sodium, potassium and chloride content. Values range from one to two parts per m i l l i o n each, to as l i t t l e as 0.1 part per m i l l i o n . Test methods can have an impact on the value quoted. Needless to say, purity can affect device performance. I f the coating i s pure then i t only stands to reason that the surface to which i t w i l l be applied must also be pure and clean. Anything coming between the coating and the surface w i l l lead to poor performance. This i s e s p e c i a l l y true when adhesion, whether physical or chemical, determines performance. It i s well known that most structures w i l l have at least a monomolecular layer of moisture on their surfaces and i n many cases, i . e . , s i l i c o n , s i l i c o n dioxide, and aluminum, w i l l also form surface hydroxides. K. Otsuka and co-workers observed that devices which were coated with g e l and then cured at temperatures above the norma l l y used 150°C had less corrosion of the chip pads and aluminum bonding wires when subjected to a pressure cooker cycling test. (6) An explanation for the observed finding i s that at the elevated temperatures the s i l i c o n hydride groups had s u f f i c i e n t energy to react with the hydroxyl groups, thus dehydrating the surface and possibly forming some covalent bonds with the coated surface (X). 4 *

-w^MOSi^- + H *~>~> s i o s i * — + n

2

(X)

For this to take place, the surface presented to the g e l must be clean and free of foreign matter so that the g e l can attach i t s e l f to the primary surface. Recently reported work by P.R. Troyk, J. E. Anderson and V. Markovac further emphasizes the need f o r clean surfaces i f ultimate performance i s to be expected. (7) Physical Properties Cured Consistency. The hardness of these gels can not be measured by the normal methods used for elastomeric materials. Test methods for characterizing their physical properties are i n various stages of development. The most commonly used technique to describe the gel's softness i s the penetration test, which i s emerging as a



236


standard. The method involves the use of a grease penetrometer, which i s described i n ASTM D217, modified by replacing the normal plunger mechanism with one weighing 19.5 grams. The plunger end i s 3/16 inch long and 1/4 inch i n diameter. To perform the test, the instrument i s c a r e f u l l y lowered u n t i l the plunger t i p just touches the top surface of the cured sample. The plunger i s released and allowed to f a l l into the sample for f i v e seconds. A reading i s then made to tenths of a millimeter. The higher the reading, the softer the g e l . Most gels have values of 3.0 to 8.0 millimeters, although some as soft as 20.0 millimeters are possible. Gels having values above 20.0 are edging close to becoming f l u i d at elevated temperatures. Tests for other physical properties are more d i f f i c u l t to de v i s e because of the sticky nature of gels. With great resolve we were successful i n determining the t e n s i l e strength and elongation of a few gels having penetration values of less than 7.2 mm. Our objective was to determine i f gels of approximately equal penetra tions would y i e l d similar t e n s i l e strength and elongation values. The determinations where made at 25°C using a Model 1122 Instron tester with a 500 gram load c e l l . P u l l rate was two inches per minute. Apparent t e n s i l e strengths averaging 144 to 420 grams per square inch were found with elongations of 85 to 408 percent. (Table I) As can be seen by the results there i s no clear r e l a t i o n ship between penetration values and t e n s i l e strength or elongation. The differences i n t e n s i l e strength and elongation noted between some samples may be due to the d i f f i c u l t y in preparing and handling the specimens; however, the high and low values must be considered r e a l , and the difference due to formulation. Continuing the search for other methods of characterizing gels using simple and e a s i l y available tools, we examined the u t i l i t y of Τ pins used by upholsterers. The pins are Τ shaped and made of 44 mil diameter wire. They are available i n various lengths; however the one used was 1.75 inches long. The Τ pin was centered and suspended i n a 10 ml beaker which was then f i l l e d with a given v o l ume of gel and cured. By means of a Model 4202 Instron tester, operating at a withdrawal rate of one inch per minute, the maximum force required to p u l l the pin through the gel was determined. In this case two other softer gels were also tested. The r e s u l t s , (Table I ) , ranged from 88 to 645 grams of force required to extract the pin. As can be seen, the gel having a penetration of 10.6 mm actually had more tenacity than two of the firmer gels. Again, this can be attributed to formulation. The values seem to follow reason ably well the determined t e n s i l e strengths. While these tests contribute to our understanding of the physi c a l properties of s i l i c o n e gels, t h e i r significance i s unknown. The use of the Τ pin test as a means of characterizing gels i s uncer t a i n . Its p r e c i s i o n and r e p r o d u c i b i l i t y as well as the e f f e c t of p u l l rate have yet to be determined. The ultimate method of char a c t e r i z i n g gels may be by using today's sophisticated p r e c i s i o n rheometers. This work i s i n the planning stages. Low Temperature Formulations. In some applications temperatures below -40°C may be encountered. Here i t i s important that the gel


20. KOOKOOTSEDES

Silicone Gels for Semiconductor Applications

TABLE I:

PROPERTIES OF SOME SILICONE GELS


PENETRATION TENSILE STRENGTH GEL NUMBER mm GRAMS/SQ INCH 5.2 144 1 2 219 5.9 6.0 420 3 7.2 4 283 10.6 5 6 16.3

-

ELONGATION % 85 117 408 150

-

Τ PIN PULL NUMBER GRAMS 220 202 645 265 237 88

not c r y s t a l l i z e and exert undue stress on small wire bonds. Gels are available with c r y s t a l l i z a t i o n temperatures below -65°C. This i s accomplished by interrupting the symmetry of the polydimethylsiloxane chain by preparing random copolymers of dimethyl and methylphenyl or diphenyl siloxanes. Temperature transitions can be readily measured by d i f f e r e n t i a l scanning calorimetry (DSC). F i g ures 1 and 2 show the observed differences between an all-dimethyl polymer and one which i s a dimethyl copolymer. As can be seen i n Figure 2 the a l l dimethyl polymer undergoes c r y s t a l l i z a t i o n around -43°C while the copolymer of Figure 1 only exhibits a glass t r a n s i s t i o n at approximately -120°C. Moisture Absorption. The water vapor permeability of s i l i c o n e gels, l i k e s i l i c o n e elastomers, i s rather high. However, their moisture absorption i s quite low compared to many other polymeric materials. We have measured water absorptions of 0.01 to 0.05% after 100 hours at 65°C and 100% r e l a t i v e humidity. After 16 hours at 15 p s i steam while immersed i n water, the up-take was found to be only 0.3 to 0.5%; again, low compared to other materials. E l e c t r i c a l Properties. The e l e c t r i c a l properties of s i l i c o n e gels are e s s e n t i a l l y the same as those of most clean, u n f i l l e d s i l i c o n e elastomers and f l u i d s . Typical values for d i e l e c t r i c constant and d i s s i p a t i o n factor when tested at 25°C and 100 Hz are 2.7 to 2.9 and 0.001 to 0.002 respectively. Volume r e s i s t i v i t y values, usu a l l y , f a l l i n the 10 ohm- centimeter range. Conclusions The use of soft s i l i c o n e gels to protect non-hermetic as well as some hermetic semiconductor devices i s a c t i v e l y being pursued by many investigators. The chemistry of s i l i c o n e gels, which allows alterations of physical c h a r a c t e r i s t i c s to be made e a s i l y , along with their e l e c t r i c a l properties and purity make them prime candi dates for these investigations. Results to date would indicate that gels may o f f e r a reasonable way of achieving the desired re l i a b i l i t y while holding down packaging costs.


237


238


Τ — I

Γ

-160 -120 -80 -40

40

80

120 160 200

Temperature (° C)

FIGURE 1:

DIMETHYL COPOLYMER.

-160-120 -80 -40

1 — I — I — I — Γ 0 40 80 120 160 200

Temperature (°C) FIGURE 2:

ALL DIMETHYL POLYMER.


20. KOOKOOTSEDES

Silicone Gels for Semiconductor Applications239

Acknowledgments I wish to acknowledge Donavon Bryant for h i s persistence i n prepar ing the gel samples for t e n s i l e and elongation testing and testing of same, as well as carrying out the Τ pin tests and producing the DSC curves. I also wish to thank Amy Johnson for her e d i t o r i a l assistance and Mary Anne Walker for her preparation of the f i n a l document.


L i t e r a t u r e Cited (1) Otsuka, K.; S h i r a i , Y.; Okutani, K. IEEE Transactions on Components, Hybrids and Manufacturing Technology, 1984, CHMT-7, No. 3, 249. (2) M i l l e r , R. VLSI Packaging Workshop, 1985. (3) Thomas, R. E. Proceedings of the 35th Electronic Components Conference, 1985, pp. 37 - 45. (4) King, J . Proceedings 1985 International Symposium on Microelectronics; ISHM: Montgomery, AL, pp. 322 - 325. (5) S a i l e r , E.; Kennedy, Α., Electronic Packaging and Production, November 1966. (6) Otsuka, K.; Takeo, Y.; Tachi, H.; Ishida, H.; Yamada, T.; Kuroda, S. IEPS Proceedings, 1986, pp. 720 - 726. (7) Troyk, P. R.; Anderson, J . E.; Markovac, V. 1st International SAMPE Electronics Conference, 1987, pp. 590 - 601. RECEIVED February 2, 1989


Silicone Gels for Semiconductor Applications - American Chemical

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