Silicone Gels and Coatings for Integrated-Circuit Packaging - ACS

Chapter 23

Silicone Gels and Coatings for Integrated-Circuit Packaging

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 17, 2016 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch023

Justin C. Bolger Emerson & Cuming/W. R. Grace and Company, 25 Hartwell Avenue, Lexington, MA 02173

This chapter describes the use of very soft (Shore 00 scale) s i l i c o n e gels and coatings for use i n hybrid and monolithic IC's f o r : 1. Stress r e l i e f on large dies i n post-molded p l a s t i c packages. 2. Passivation/corrosion protection i n pre-molded p l a s t i c packages. 3. Prevention of s i l v e r migration i n hybrid circuits. The mechanism for each of these e f f e c t s i s explained. Rules are provided for selecting s i l i c o n e s with optimum hardness, rheology, cure temperature and other properties needed i n production.

D i e l e c t r i c grade s i l i c o n e g e l s a r e now b e i n g used i n i n c r e a s i n g quan tities f o r p a c k a g i n g h y b r i d and m o n o l i t h i c i n t e g r a t e d circuits. These g e l s a r e now used e i t h e r i n the form o f t h i n c o a t i n g s on d i e s o r on h y b r i d c i r c u i t s , o r i n t h i c k e r s e c t i o n s as e n c a p s u l a n t s f o r h y b r i d c i r c u i t s used i n a u t o m o t i v e e l e c t r o n i c s . S i l i c o n e g e l s are a l s o used t o e n c a p s u l a t e d i e s packaged i n pre-molded p l a s t i c pack ages and have been s u g g e s t e d f o r use as " b l o b t o p " c o a t i n g s f o r low c o s t , l o c a l i z e d p r o t e c t i o n o f d i e s on PC b o a r d s . Compared t o e p o x i e s and o t h e r r e s i n - t y p e s used i n the IC i n d u s t r y , s i l i c o n e s have s e v e r a l unique advantages: 1. Flexibility. S i l i c o n e g e l s have h a r d n e s s measured i n t h e low end o f t h e Shore 00 s c a l e . They r e t a i n u s e f u l e l o n g a t i o n a t t e m p e r a t u r e s f a r below 0°C. 2. E x c e l l e n t thermal s t a b i l i t y . No o u t - g a s s i n g o r de c o m p o s i t i o n up t o 200°C. 3. Cleanliness. No c h l o r i d e s o r o t h e r p o t e n t i a l l y c o r r o s i v e contaminants. T h i s p r o v i d e s good s e m i c o n d u c t o r c o m p a t i b i l i t y . S i l i c o n e s have, however, s e v e r a l unique d i s a d v a n t a g e s v s . o t h e r e n c a p s u l a n t s such as e p o x i e s , which must be t a k e n i n t o a c c o u n t f o r proper performance:

0097-6156/89/0407-0268$06.00/0 ο 1989 American Chemical Society

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


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1. They have a v e r y h i g h t h e r m a l e x p a n s i o n c o e f f i c i e n t a t ambient temperatures. This creates a l a r g e p o t e n t i a l thermal mis-match i n c o n t a c t w i t h s i l i c o n , a l u m i n a , and other surfaces. T h i s e x p a n s i o n mis-match can cause w i r e breakage and o t h e r damage d u r i n g temperature c y c l i n g . 2. Low m e c h a n i c a l s t r e n g t h . S i l i c o n e g e l s must be p r o t e c t e d by some s o r t o f c o v e r or o u t e r s h e l l a g a i n s t m e c h a n i c a l damage. They a l s o have v e r y low r e s i s t a n c e t o a t t a c k o r s w e l l i n g i n c l e a n i n g solvents. 3. U n t i l r e c e n t l y , most s i l i c o n e g e l s p r o v i d e d inadequate adhesion to coated or encapsulated parts. Modern silicone encapsulants and coatings contain, however, silane "coupling a g e n t s " , which are a b l e t o form a permanent water p r o o f cova'Lent bond t o most s u b s t r a t e s and t h e r e f o r e a v o i d f a i l u r e a s s o c i a t e d w i t h l o s s o f a d h e s i o n t o the s u b s t r a t e . Other c h a p t e r s i n t h e s e p r o c e e d i n g s d i s c u s s the e l e c t r i c a l and mechanical properties of silicone elastomers. This chapter describes the use of silicone gels to provide stress relief, p a s s i v a t i o n and c o r r o s i o n p r o t e c t i o n , and f o r the p r e v e n t i o n of s i l v e r migration. S t r e s s R e l i e f on L a r g e D i e s

i n Post-Molded P l a s t i c

Packages

An i n c r e a s i n g l y s e r i o u s r e l i a b i l i t y problem, f o r l a r g e d i e s packaged i n p o s t - m o l d e d p l a s t i c packages, i n v o l v e s the p o s s i b i l i t y o f d i e breakage or o t h e r damage, due t o c o m p r e s s i v e s u r f a c e s t r e s s e s on the die surface ( F i g . 1). These c o m p r e s s i v e s t r e s s e s were a minor c o n s i d e r a t i o n when IC d i e s were r e l a t i v e l y s m a l l , but are now becoming a v e r y s e r i o u s cause o f f a i l u r e as d i e s a r e a p p r o a c h i n g 400 m i l s i n l e n g t h and the l e a d count i s i n c r e a s i n g t o 100 l e a d s p e r d i e or more. F i g u r e 1 d e p i c t s use o f a t h i n c o a t i n g o f s i l i c o n e e l a s t o m e r o r g e l , which can be a v e r y e f f e c t i v e way t o a v o i d l o c a l i z e d s u r f a c e s t r e s s e s on the d i e i n post-molded p l a s t i c p a c k a g e s . The t h i c k n e s s o f the s i l i c o n e c o a t i n g need o n l y be t e n m i c r o n s or so t o be effective. Thin s i l i c o n e coatings apparently can d e l o c a l i z e the s t r e s s e s and p r e v e n t d i e damage d u r i n g m o l d i n g o r t h e r m a l c y c l i n g . A p p l y i n g a u n i f o r m c o a t i n g on the s u r f a c e o f the d i e , w h i l e k e e p i n g the coating away from the gold wires, is a difficult p r o d u c t i o n problem. I t would be e a s i e r t o a p p l y a t h i c k e r c o a t i n g . The q u e s t i o n i s , t h e r e f o r e , i f a t e n m i c r o n c o a t i n g i s e f f e c t i v e i n r e d u c i n g s t r e s s , why not use a t h i c k e r , more e a s i l y a p p l i e d c o a t i n g ? The answer i n v o l v e s the h i g h e x p a n s i o n c o e f f i c i e n t o f s i l i c o n e elastomers. The epoxy m o l d i n g compounds, used t o e n c a p s u l a t e the c h i p i n the p o s t - m o l d e d package o f F i g u r e 1 , do not b r e a k the f r a g i l e (1 m i l d i a m e t e r ) g o l d w i r e s because o f t h e i r low e x p a n s i o n c o e f f i c i e n t below Tg and because t h e i r Tg v a l u e i s h i g h enough t o p r e v e n t t r a n s i t i o n i n t o the r u b b e r y s t a t e a t a l l a n t i c i p a t e d s e r v i c e temperatures. T y p i c a l v a l u e s f o r t h e s e epoxy e n c a p s u l a n t s , and f o r the s i l i c o n e g e l s , as measured by TMA(thermo-mechanical-analysis) are :

Epoxy Encap. S i l i c o n e Gel

60

Wt % F i l l e r t o 70% SiO unfilled

Tg ° C 140 t o 160 -30 t o -40

20 40

°* 1 t o 30 t o 60

150 180

°*2 t o 200 t o 300


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Here ^ i s the average l i n e a r c o e f f i c i e n t o f t h e r m a l e x p a n s i o n i n the g l a s s y s t a t e , i n p a r t s p e r m i l l i o n p e r ° C . Οζ^ i s the v a l u e i n the r u b b e r y s t a t e . A t o r near room temperature t h e r e f o r e , the s i l i c o n e g e l i s i n i t s r u b b e r y s t a t e , and the h i g h 0 ^ v a l u e may cause w i r e breakage i f the s i l i c o n e d i e c o a t i n g i s t h i c k enough t o e x t e n d onto the w i r e s o f F i g u r e 1. I f the s i l i c o n e g e l i s s o f t enough, however, such t h a t e x p a n s i o n does n o t a p p l y any s i g n i f i c a n t f o r c e t o the g o l d w i r e , t h e n t h i s w i r e breakage can be a v o i d e d . The r u l e , therefore, i s t h a t s i l i c o n e c o a t i n g s are a very e f f e c t i v e way t o a v o i d damage t o d i e s u r f a c e s e n c a p s u l a t e d i n epoxy t r a n s f e r m o l d i n g compounds p r o v i d e d t h a t : a) The s i l i c o n e c o a t i n g i s t h i c k enough ( t e n m i c r o n s o r more) to d e l o c a l i z e the s t r e s s , but n o t t h i c k enough t o e x t e n d above t h e t o p s o f the g o l d b a l l bonds, o r , b) i f a t h i c k e r s i l i c o n e c o a t i n g i s used, t h e s i l i c o n e must be a v e r y s o f t g e l which can expand w i t h o u t b r e a k i n g t h e w i r e s .


9

P a s s i v a t i o n i n Automotive

H y b r i d s and PCC

Packages

One way t o a v o i d the thermal stresses on d i e s shown i n t h e post-molded package o f F i g . 1 i s t o use a pre-molded p l a s t i c package for IC's o r h y b r i d s . T h i s i n v o l v e s u s i n g a p l a s t i c box which c o n t a i n s the d i e o r h y b r i d i n a c a v i t y , and t h e n s e a l i n g the c a v i t y by b o n d i n g a p l a s t i c l i d as shown i n F i g . 2. Pre-molded p l a s t i c packages, f r e q u e n t l y c a l l e d p l a s t i c c h i p c a r r i e r s (PCC's), have l o n g been p r o p o s e d f o r m o n o l i t h i c I C ' s ( 1 ) , b u t because o f h i g h e r c o s t s r e l a t i v e t o post-molded p l a s t i c packages, d i d n o t s e l l i n volume u n t i l the r e c e n t i n c r e a s e s i n d i e s i z e and i n l e a d count made pre-molded packages more a t t r a c t i v e f o r s t r e s s and tooling-cost reasons. One long established use of silicones has been in the pre-molded p l a s t i c packages used i n automotive h y b r i d s o f F i g . 2 . D e l c o f i r s t p i o n e e r e d the use o f h y b r i d c i r c u i t s ( 2 ) i n a p l a s t i c c a v i t y package over 20 y e a r s ago, and v i r t u a l l y a l l a u t o m o t i v e c i r c u i t s now use t h i s p a c k a g i n g method. The key t o r e l i a b i l i t y has been the use o f a s i l i c o n e g e l w i t h i n t h e package, which c o m p l e t e l y e n c a p s u l a t e s a l l components. For reasons noted elsewhere, this s i l i c o n e i s v e r y e f f e c t i v e i n p r o v i d i n g p a s s i v a t i o n and c o r r o s i o n p r o t e c t i o n , and i n a v o i d i n g s i l v e r m i g r a t i o n problems. C l e a r l y , the s i l i c o n e does n o t f u n c t i o n as a b a r r i e r c o a t i n g and does n o t e x c l u d e m o i s t u r e , oxygen o r o t h e r i o n s . Rather, the s i l i c o n e p r o v i d e s t h i s l o n g - t e r m p r o t e c t i o n i n the v e r y h o s t i l e under-the-hood automotive environment, by r e t a i n i n g l o n g - t e r m f l e x i b i l i t y , even a f t e r extended exposures t o h i g h t e m p e r a t u r e s , and by r e t a i n i n g a d h e s i o n t o t h e h y b r i d s u b s t r a t e and a l l e n c a p s u l a t e d s u r f a c e s . As used above, " a d h e s i o n r e t e n t i o n " means t h a t the s i l i c o n e g e l must r e t a i n a s u f f i c i e n t degree o f i n t i m a t e i n t e r f a c i a l c o n t a c t such t h a t the t r a n s p o r t o f e l e c t r o n s , as w e l l as l i q u i d water, i o n s and o t h e r r e a g e n t s , must t a k e p l a c e t h r o u g h the b u l k polymer, rather than through gaps o r a l o n g an adsorbed aqueous phase a t the substrate surface. Moreover, t h i s i n t e r f a c i a l c o n t a c t must be m a i n t a i n e d a f t e r t h e r m a l c y c l i n g , m e c h a n i c a l shock and/or exposure to h i g h h u m i d i t i e s . A d e t a i l e d d i s c u s s i o n o f t h i s p r o t e c t i o n mechanism (3,4) is



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beyond the scope o f t h i s c h a p t e r , o t h e r t h a n t o note t h a t the d i s p l a c e m e n t mechanism d i f f e r s f o r a r i g i d polymer, where v e r y h i g h t e n s i l e and s h e a r f o r c e s can be g e n e r a t e d a t the p o l y m e r - s u b s t r a t e i n t e r f a c e , v e r s u s the p r e s e n t case o f v e r y s o f t s i l i c o n e g e l s where i n t e r f a c i a l s e p a r a t i o n i s caused p r i m a r i l y by c h e m i c a l ( r a t h e r t h a n mechanical) displacement e f f e c t s . Thermodynamics always strongly f a v o r s the d i s p l a c e m e n t o f a s i l i c o n e r e s i n , by water, a t an o x i d e s u r f a c e , whenever the o n l y i n t e r f a c i a l bonds a r e s e c o n d a r y v a l e n c e f o r c e s (3,4). The b e s t p r e s e n t s i l i c o n e g e l e n c a p s u l a n t s r e t a i n a d h e s i o n because t h e y c o n t a i n s m a l l (ca 1% by wt) a d d i t i o n s o f s i l a n e c o u p l i n g agents. These a r e m o b i l e , s u r f a c e - a c t i v e , m o l e c u l e s which a r e a b l e t o form c o v a l e n t bonds t o the b u l k s i l i c o n e phase, and a l s o t o form h y d r o l y s i s - r e s i s t a n t s i l o x a n e bonds t o s i l i c a and other oxide s u r f a c e s . Plueddemann (5) and o t h e r s have d e s c r i b e d t h e s e s i l a n e c o u p l i n g agents i n d e t a i l . The l e a s t e x p e n s i v e way t o package an I.e. c h i p , on a PC b o a r d , i s the "Chip-On-Board" method shown i n F i g . 2. T h i s method, u s i n g epoxy " b l o b t o p " c o a t i n g s , i s now w i d e l y used i n e l e c t r o n i c watches, games, p o c k e t c a l c u l a t o r s , and o t h e r l o w - c o s t consumer p r o d u c t s , b u t has not y e t been a c c e p t e d f o r use i n h i g h e r v a l u e , h i g h e r r e l i a b i l i t y c i r c u i t s , p r i m a r i l y because o f s t r e s s e s g e n e r a t e d when the epoxy b l o b t o p i s c u r e d o r temperature c y c l e d . Very s o f t s i l i c o n e g e l s can a v o i d t h e s e s t r e s s e s , and a l s o p r o v i d e the i o n i c p u r i t y and o t h e r p r o p e r t i e s needed f o r l o n g - t e r m p a s s i v a t i o n , b u t a r e t o o s o f t t o p r o v i d e m e c h a n i c a l p r o t e c t i o n o r t o w i t h s t a n d the s o l v e n t s used i n PC b o a r d p r o d u c t i o n . Hence, s i l i c o n e g e l s a r e used o n l y under a p l a s t i c s h e l l o r c o v e r , o r i n one o f the e n c l o s u r e s shown i n F i g . 2. Silver Migration Silver migration w i l l o c c u r whenever a s i l v e r - f i l l e d adhesive, c o a t i n g or ink i s i n c l o s e p r o x i m i t y to another conductor i n a c i r c u i t , p r o v i d e d t h a t a DC v o l t a g e p o t e n t i a l (the s i l v e r - f i l l e d polymer b e i n g the anode), and a f i l m o f l i q u i d water, e x i s t on the s u r f a c e s e p a r a t i n g the s i l v e r compound from the o t h e r c o n d u c t o r . L i c a r i , e t a l . (6) showed t h a t by p l a c i n g a d r o p o f d e i o n i z e d water a c r o s s a 20 m i l gap between a c o n d u c t o r and a s i l v e r - f i l l e d adhesive, and then a p p l y i n g a one volt DC potential, silver d e n d r i t e s began t o grow a c r o s s the gap w i t h i n 30 seconds. Bridging, c a u s i n g an e l e c t r i c a l s h o r t , o c c u r r e d w i t h i n 3-4 m i n u t e s . Within e i g h t minutes, s i l v e r p a r t i c l e s had c o m p l e t e l y f i l l e d the gap. L i c a r i t e s t e d specimens w i t h water d r o p l e t s b r i d g i n g the gap between the two c o n d u c t o r s as w e l l as w i t h o t h e r specimens exposed t o r e l a t i v e h u m i d i t i e s up t o 100% a t a c o n s t a n t 8 0 ° C . He showed t h a t even w i t h a p p l i e d v o l t a g e s o v e r 20 v o l t s , and time p e r i o d s up t o two weeks, s i l v e r m i g r i a t i o n d i d not o c c u r u n l e s s t h e r e was a f i l m o f l i q u i d water on the s u r f a c e . L i c a r i ' s c o n c l u s i o n was t h a t water i n the vapor phase i s n o t sufficient to induce silver migration. Silver migration i s t h e r e f o r e p o s s i b l e o n l y when packages a r e o p e r a t i n g below the dew p o i n t o f the atmosphere w i t h i n the package. Recent work (7) which r e p e a t e d L i c a r i ' s t e s t s used a l a r g e number o f c o m m e r c i a l l y a v a i l a b l e c o n d u c t i v e i n k s and a d h e s i v e s t o d e t e r m i n e the e f f e c t o f r e s i n - t y p e , o f Tg, o f i o n i c p u r i t y and o f



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POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

f i l l e r t y p e on s i l v e r m i g r a t i o n . The r e s u l t s a r e summarized i n T a b l e I, and l e a d t o the f o l l o w i n g i m p o r t a n t c o n c l u s i o n s : 1. I o n i c p u r i t y and c h l o r i d e c o n t e n t . One might suppose t h a t c l e a n e p o x i e s o r o t h e r r e s i n s , c o n t a i n i n g s i l v e r f i l l e r s , would perform b e t t e r i n migration t e s t s than epoxies c o n t a i n i n g large quantities of c h l o r i d e s or other extractable ionic impurities. Table I shows, however, t h a t t h e r e i s no s i g n i f i c a n t d i f f e r e n c e between a v e r y c l e a n epoxy a d h e s i v e , a c l e a n p o l y i m i d e , and an epoxy c o n t a i n i n g o v e r 300 ppm e x t r a c t a b l e c h l o r i d e . 2. Glass t r a n s i t i o n temperature. One might a l s o e x p e c t t h a t h i g h Tg i n k s and a d h e s i v e s would p e r f o r m b e t t e r , because o f s l o w e r extraction rates for s i l v e r ions. T a b l e I a l s o shows, however, t h a t c o n d u c t i v e e p o x i e s w i t h a Tg below 100 C p e r f o r m e d a l m o s t as w e l l as the high Tg silver-filled polyimide (Tg=180 C ) or the epoxy a d h e s i v e s w i t h TG=115 C o r 130°C. Conductive g l a s s adhesives, i n which the g l a s s b i n d e r was f i r e d a t t e m p e r a t u r e s o f 430 °C, a l s o showed r a p i d f a i l u r e due t o s i l v e r m i g r a t i o n . 3. F i l l e r type. The most w i d e l y used way t o a v o i d s i l v e r migration i s to avoid s i l v e r . Gold f i l l e d epoxies are s p e c i f i e d i n many military s p e c i f i c a t i o n s , but are impractical for most commercial a p p l i c a t i o n s because o f c o s t . Silver-palladium f i l l e d i n k s and a d h e s i v e s a r e t h e r e f o r e f r e q u e n t l y used t o a v o i d m i g r a t i o n . T a b l e I shows t h a t u s i n g a s i l v e r - p a l l a d i u m f i l l e r , i n p l a c e o f pure silver, can improve the time t o f a i l u r e by metal migration, a l t h o u g h by a s u r p r i s i n g l y s m a l l d e g r e e . Silver-palladium powders which a r e now used in conductive p r o d u c t s are o f two d i f f e r e n t t y p e s . One type i s c o - p r e c i p i t a t e d powders, w h e r e i n s i l v e r and p a l l a d i u m a r e b o t h p r e c i p i t a t e d from s o l u t i o n t o form a powder i n which the two m e t a l s a r e i n t i m a t e l y a s s o c i a t e d but do not form a t r u e a l l o y . A n o t h e r , more e x p e n s i v e f i l l e r , i n v o l v e s f i r s t forming a true s i l v e r - p a l l a d i u m a l l o y , and t h e n p u l v e r i z i n g t h i s t o a f i n e powder. Table I shows t h a t the s i l v e r - p a l l a d i u m a l l o y s do indeed p e r f o r m b e t t e r t h a n the c o - p r e c i p i t a t e s . S i l v e r i o n s are o b v i o u s l y e x t r a c t e d more r a p i d l y from the p h y s i c a l m i x t u r e o f s i l v e r and p a l l a d i u m i n the c o - p r e c i p i t a t e t h a n from the a l l o y , but t h a t t h i s improvement i s s t i l l not enough f o r a h i g h l e v e l o f s e c u r i t y i n c r i t i c a l hybrid c i r c u i t s . The b e s t t e c h n i q u e found (7) f o r a v o i d i n g s i l v e r m i g r a t i o n i s t o c o a t the c i r c u i t w i t h a t h i n c o a t i n g o f s i l i c o n e o r s i l i c o n e g e l . T a b l e I shows t h a t even i n the worst c a s e (the low Tg, h i g h c h l o r i d e s i l v e r - f i l l e d epoxy, which, i n the absence o f a s i l i c o n e c o a t i n g f a i l s v i a m i g r a t i o n w i t h i n 45 seconds,) the a p p l i c a t i o n o f a t h i n (4 mil t h i c k ) s i l i c o n e c o a t i n g o v e r t h i s same c i r c u i t , essentially e l i m i n a t e s s i l v e r m i g r a t i o n a t t h e s e v o l t a g e s and t e s t c o n d i t i o n s . The f a c t t h a t s i l i c o n e c o a t i n g s are a b l e t o p r e v e n t s i l v e r m i g r a t i o n has, o f c o u r s e , been known f o r over t e n y e a r s . But the mechanism f o r t h i s has not been u n d e r s t o o d . S i l i c o n e s , as a g e n e r a l polymer c l a s s , have a v e r y h i g h p e r m e a b i l i t y t o water v a p o r . Hence, s i l i c o n e s cannot be c o n s i d e r e d as " b a r r i e r c o a t i n g s " which f u n c t i o n by e x c l u d i n g water from the r e g i o n between the conductors. The e x p l a n a t i o n depends on L i c a r i ' s o b s e r v a t i o n t h a t liquid water, r a t h e r t h a n water v a p o r , i s n e c e s s a r y between the silver c o n t a i n i n g polymer and the n e a r b y c a t h o d e . As l o n g as the s i l i c o n e e

C

e



23. BOLGER

273

Silicone Gels and Coatings for IC Packaging

top

of ball

bonds.

soft

silicone

gel.

Figure 1. Silicone die coatings can be used to protect the die against damage from the compressive stresses generated during cure and thermal cycling of the transfer molding compound in post-molded plastic packages.

Figure 2. Very soft silicone gels are preferred for encapsulation in post-molded plastic chip carriers (left) and in automotive hybrid packages (center) but are too soft for use as "blob top" coatings (right) for chip-on-board encapsulation.

Table I. Time to failure by electrical short due to silver migration. DC voltage = 5V at 25 C (data from ref. 7) β

RESIN TYPE Epoxy A Epoxy Β Epoxy C

FILLER TYPE Silver

(2)

e

95 C 115 C 130 C e

e

Ρ'imide A Glass

Tg

TIME TO FAILURE

300 10 10

NO

45 seconds 50 45

5

e

3

>200 C

Epoxy A Epoxy A

Pd-Silver(3) " " (4)

95 C 9 5* C

Epoxy A Epoxy A

Silver

95- C 95 C

(1) (2) (3) (4) (5) (6) (7)

SILICONE COATING

e

130 C ··

CI" CONTENT(l)

e

e

··

50 80

300 300 300 300

5 minutes 30 Yes Yes

(5) (6)

2 hrs(7) 2 hrs(7)

Extractable c h l o r i d e ion concent, ppm S i l v e r g l a s s mixture. F i r e d at 430*C 70% s i l v e r . 30% palladium. C o - p r e c i p i t a t e d powder 70% " " True a l l o y powder Same as Epoxy A above with 4 mil s i l i c o n e coating Awicon sc-3613 " " with 20 mil s i l i c o n e g e l coating Aeicon sc-2650 Test discontinued a f t e r 2 h r s . No f a i l u r e M


274

POLYMERS FOR ELECTRONICS PACKAGING AND

INTERCONNECTION


c o a t i n g can r e t a i n a d h e s i o n t o the s u b s t r a t e , and t o t h e

conductors,

a l i q u i d water p a t h cannot form. As n o t e d above, the b e s t modern s i l i c o n e g e l s now contain s p e c i a l o r g a n o - f u n c t i o n a l s i l a n e " c o u p l i n g a g e n t s " , which are a b l e t o form a s t r o n g , water r e s i s t a n t , c h e m i c a l bond t o the alumina and c o n d u c t i v e s u r f a c e s , t h e r e b y p r e v e n t i n g the f o r m a t i o n o f a l i q u i d f i l m on the a d h e s i v e which c o u l d p r o v i d e a p a t h f o r m i g r a t i o n . Hence, the b e s t p r e s e n t silicone coatings provide a very r e l i a b l e method t o p r e v e n t s i l v e r m i g r a t i o n i n h y b r i d c i r c u i t s by p r e v e n t i n g the f o r m a t i o n o f a l i q u i d water p a t h between c l o s e l y spaced c o n d u c t o r s . The e f f e c t i v e n e s s o f t h e s e s i l i c o n e s does not depend on h a r d n e s s . That i s , v e r y s o f t g e l s a r e as e f f e c t i v e as h a r d e r e l a s t o m e r s , as l o n g as a d h e s i o n t o the s u b s t r a t e i s r e t a i n e d . These s i l i c o n e c o a t i n g s a r e t h e r e f o r e more c o s t e f f e c t i v e than p r e v i o u s s o l u t i o n s t o the m i g r a t i o n problem which r e q u i r e the use o f g o l d - f i l l e d , or s i l v e r - p a l l a d i u m f i l l e d , i n k s or adhesives.

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Rose,A.; Fischer,M. Proc. 32nd Electronic Components Conf., 1982, pp 87-90. Bolger,J.C. Proc. 1984 International Symposium on Microelectronics; ISHM: Montgomery, AL, pp 90-94. Bolger,J.C.; Michaels,A.S. In Interface Conversion; Weiss,P.; Cheevers,S.D., Eds.; Elsevier: New York, 1969; Chapter 1. Bolger,J.C. In Adhesion Aspects of Polymeric Coatings; Mittal,K.L., Ed.; Plenum: New York, 1983 pp 3-19. Plueddemann,E.P. idem., pp 363-379. Licari,J.J.; Perkins,K.L.; Caruso,S.V. Guidelines For The Selection of Electrically Conductive Adhesives for Hybrid Microcircuits, NASA CR-161978, 1981. Bolger,J.C.; Herberg,M.J.; Mooney,C.T. SAMPE Electronic Materials Conf., 1987.

RECEIVED January 18,

1989


Silicone Gels and Coatings for Integrated-Circuit Packaging - ACS

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