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Humidity Testing of Silicone Polymers for Corrosion Control of Implanted Medical Electronic Prostheses Philip R. Troyk, Michael J. Watson, and James J. Poyezdala Pritzer Institute of Medical Engineering, Illinois Institute of Technology, IIT Center, Chicago, IL 60616

Adhesion tests are insufficient to qualify polysiloxanes for the corrosion control of electronic assemblies, and parameters such as material hardness may be of equal importance as the bond strength of the polymer to the protected assembly. Silane coupling agents in the form of commercially available surface primers do not seem to enhance the corrosion control of silicone encapsulated electronic devices despite higher measured bond strengths for primed samples as opposed to unprimed ones. This is probably due to water soluable contaminants in the p r i mers. Accelerated temperature-humidity tests upon 72 interdigitated test substrates encapsulated with various silicone elastomers treated with and without surface p r i mers show a strong dependence upon the material hardness in controlling corrosion. Electrical leakage current measurments combined with visual examination were used as performance criteria for the encapsulants. S i l i c o n e e l a s t o m e r s have been used f o r t h e p r o t e c t i o n o f i n t e g r a t e d c i r c u i t s , epoxy g l a s s p r i n t e d w i r i n g b o a r d s , h i g h v o l t a g e a s s e m b l i e s and o t h e r e l e c t r o n i c a s s e m b l i e s exposed t o h a r s h e n v i r o n mental c o n d i t i o n s . Among t h e most s e v e r e c o n d i t i o n s i s exposure t o high humidity with r e s u l t a n t c o r r o s i o n o f the e l e c t r i c a l conductors and components. The c o r r o s i o n t a k e s p l a c e not o n l y as t h e r e s u l t o f e l e c t r o c h e m i c a l c o u p l e s , due t o t h e v a r i e t y o f m e t a l s used i n a t y p i c a l e l e c t r o n i c a s s e m b l y , b u t a l s o due t o f o r c e d v o l t a g e p o t e n t i a l s p r e s e n t w i t h i n t h e a s s e m b l y d u r i n g normal o p e r a t i o n . The l a t t e r may p r e s e n t a h i g h e r s t r e s s t h a n t h e former s i n c e t h e p o t e n t i a l d i f f e r e n c e s d u r i n g o p e r a t i o n may exceed t y p i c a l e l e c t r o c h e m i c a l c o u p l e s by o r d e r s o f magnitude. T h i s i s p a r t i c u l a r l y the case i n h i g h d e n s i t y a s s e m b l i e s where t h e s p a c i n g between p r i n t e d - w i r i n g b o a r d (PWB)conductors may be a s s m a l l as 0.003" t o 0.0010 . With t h e a d v e n t o f t e c h n o l o g y such a s s m a l l - o u t l i n e p a c k a g i n g ( w i t h p i n s p a c i n g s o f 0.03 ) t h e c o r r o s i o n p r o t e c t i o n o f PWBs has become a major i n d u s t r y p r o b l e m . W

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0097-6156/86/0322-0299S06.00/0 © 1986 American Chemical Society

Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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cones, have been used f o r corrosion control i s that of medical implanted e l e c t r o n i c devices. In the past decade, hermetic cans have emerged as the preferred method of corrosion control f o r implanted medical devices. Although hermetic cans with glass-onmetal e l e c t r i c a l feedthroughs have been s u i t a b l e f o r devices such as pacemakers and simple monopolar neurostimulators, they are not e a s i l y miniaturized. M i n i a t u r i z a t i o n i s important because the effectiveness of sensors and stimulators i s often s i g n i f i c a n t l y enhanced by reducing t h e i r i n v a s i v e side effects. The miniaturizat i o n of e l e c t r o n i c c i r c u i t r y i s possible using modern t h i n - f i l m techniques, and such technology has already resulted i n the design and f a b r i c a t i o n of numerous recording and stimulation micro-electrode arrays, biosensors, and integrated c i r c u i t s a p p l i c a b l e to the development of subminiature medical e l e c t r o n i c implants ( 1 2 3 4 5 6). However, the successful design and implementation of complete and functional implants depends not only upon the f a b r i c a t i o n of these t h i n - f i l m devices, but a l s o upon the a b i l i t y to protect them during use from the h o s t i l e p h y s i o l o g i c a l environment. Unless s i g n i f i c a n t advances i n hermetic packaging are achieved, the r e l a t i v e l y large size of hermetic cans w i l l continue to prelude t h e i r use when miniaturization i s important and presently no univers a l l y accepted method of encapsulation e x i s t s f o r moisture protect i o n of non-hermetic implants. Even when size might a l l o w hermetic packaging such as sealed welded cans, the need f o r m u l t i p l e feed-throughs i n devices such as neuromuscular stimulators presents new packaging problems which have not formerly been present i n other medical implants such as pacemakers. These feed-throughs must be protected from corrosion and e l e c t r i c a l leakage, and t h i s i s u s u a l l y accomplished by overcoating the hermetic package with an encapsulant (7 8). The r e s u l t i n g insulated feed-through and wire connector can occupy as much as onet h i r d the volume of a modern pacemaker. E l e c t r i c a l leakage, which s i g n i f i c a n t l y a f f e c t s the impedance at these s i t e s , i s permissible only f o r low impedance c i r c u i t s . The increased use of sensors with high impedance outputs i n implanted systems places greater demands upon the encapsulants i n s u l a t i n g c a p a b i l i t i e s , and the use of highly corrosion-resistant metals as compensation f o r encapsulant inadequacies i n preventing implant degradation i s no longer a v i a b l e design philosophy. Encapsulants are an a l t e r n a t i v e to the welded can. A properly chosen encapsulation system can provide adequate corrosion protect i o n f o r e l e c t r o n i c assemblies and a number of organic b a r r i e r coatings and encapsulants have been used to protect e l e c t r o n i c systems from deleterious e f f e c t s of water vapor i n c l u d i n g products based upon epoxy, imide, and s i l i c o n e polymers. The l a t t e r , espec i a l l y those polymerized by addition mechanisms have gained wide acceptance f o r m i l i t a r y and space assemblies which must be protected from humidity and a l t i t u d e effects, since polysiloxanes are e a s i l y processed, t h e i r performance has been found satisfactory, and they provide v e r s a t i l i t y i n package designs. However, i n medical e l e c tronic implants the f a i l u r e incidence of encapsulants ( e s p e c i a l l y s i l i c o n e polymers) f o r e l e c t r o n i c devices such as neuromuscular stimulators i s much greater than i n nonmedical devices such as plastic-pak integrated c i r c u i t s , even when the l a t t e r are subjected to accelerated stress tests It i s not c l e a r whether t h i s poor f

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s u c c e s s r a t e i s due t o i m p r o p e r a p p l i c a t i o n and p r o c e s s i n g o f the encapsulants or to m a t e r i a l inadequacies. S i l i c o n e p o l y m e r s are o f t e n m i s u s e d i n the sense t h a t they r e g a r d e d as v a p o r b a r r i e r s . However f o r any system, the h i g h f a i l u r e r a t e , u n d o u b t e d l y , i s a r e s u l t o f the s e v e r e exposure c o n d i t i o n s t h a t e x i s t i n a p h y s i o l o g i c a l environment. Not o n l y i s the i m p l a n t exposed t o an aqueous system, but the e l e c t r o l y t i c e n v i r o n m e n t and i o n i c m o b i l i t y p r e s e n t a m i l i e u conducive to current leakage. I n t h i s r e g a r d , i t s h o u l d be noted t h a t c h l o r i d e s , as w e l l as sodium i o n s , have been i m p l i c a t e d i n the d e g r a d a t i o n o f s i l i c o n e n i t r i d e and s i l i c o n e r u b b e r e n c a p s u lated circuits MO). Mechanisms o f I m p l a n t F a i l u r e Body f l u i d s , c o n t a i n a h i g h c o n c e n t r a t i o n o f i m p l a n t f a i l u r e i n the f o l l o w i n g two ways: 1)

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M i g r a t i o n o f i o n s ( e s p e c i a l l y sodium) i n t o u n p r o t e c t e d s e m i c o n d u c t o r s r e s u l t s i n the d e g r a d a t i o n o f j u n c t i o n c h a r a c t e r i s t i c s such as r e v e r s e b i a s l e a k a g e c u r r e n t s so t h a t the c h i p no l o n g e r performs t o s p e c i f i c a t i o n s and eventually fails. Leakage p a t h s form between n o r m a l l y i n s u l a t e d c o n d u c t o r s due t o f l u i d i n v a s i o n , so t h a t a r e a s o f the c i r c u i t which must be i n s u l a t e d from each o t h e r do not r e m a i n s o , degrading c i r c u i t operation. These l e a k a g e c u r r e n t s may cause redox r e a c t i o n s r e s u l t i n g i n the c o r r o s i o n o f c o n d u c t o r s such as c h i p m e t a l l i z a t i o n s . Similarly, electro c h e m i c a l c o u p l e s i n an aqueous e n v i r o n m e n t can produce corrosion. The e n t r y o f f l u i d may be d i r e c t , t h r o u g h m e c h a n i c a l d e f e c t s i n the package, o r by d i f f u s i o n o f water t h r o u g h the p o l y m e r e n c a p s u l a n t .

M i g r a t i o n o f Ions. I n r e c e n t y e a r s , the f i r s t type o f f a i l u r e has been f a i r l y easy t o a v o i d . C h i p s are p r o t e c t e d from c o n t a m i n a t i o n d u r i n g normal h a n d l i n g by a p r o c e s s known as " p a s s i v a t i o n . " In t h i s procedure, s e v e r a l p r o t e c t i v e l a y e r s o f ceramic m a t e r i a l are d e p o s i t e d o r t h e r m a l l y grown o v e r the c h i p . The p a s s i v a t i n g l a y e r s e r v e s as a b a r r i e r t o m i g r a t i n g i o n s . S i l i c o n dioxide offers p a r t i a l p r o t e c t i o n a g a i n s t some c o n t a m i n a n t s . U n f o r t u n a t e l y , i t i s permeable t o sodium i o n s and does not o f f e r c o m p l e t e p r o t e c t i o n (11 12). A l t h o u g h used f o r i n t e r n a l p a s s i v a t i o n l a y e r s , a l m o s t no c o m m e r c i a l l y a v a i l a b l e c h i p s have f i n a l p a s s i v a t i o n l a y e r s o f s i l i c o n dioxide. I n t h e p a s t few y e a r s , s i l i c o n n i t r i d e has emerged as a n o t h e r p a s s i v a t i o n m a t e r i a l . Sodium p e n e t r a t i o n o f a p r o p e r l y a p p l i e d n i t r i d e l a y e r i s v e r y l o w — e v e n u n d e t e c t a b l e ( 1 2 1 3 14). Many s e m i c o n d u c t o r m a n u f a c t u r e r s now o f f e r n i t r i d e p a s s i v a t e d c h i p s . However, the a p p l i c a t i o n o f s i l i c o n n i t r i d e c a n h a v e v a r i a b l e r e s u l t s and the p r e s e n c e o f i m p u r i t i e s w i l l r e d u c e the effectiveness o f t h e l a y e r as an i o n i c b a r r i e r . T r a c e amounts o f oxygen w i t h i n the d e p o s i t i o n equipment c a n r e s u l t i n an o x y - n i t r i d e l a y e r w h i c h i s l e s s e f f e c t i v e against i o n i c penetration. I n a d d i t i o n , many n i t r i d e - p a s s i v a t e d d e v i c e s have exposed u n d e r l y i n g t h e r m a l o x i d e a t e a c h edge o f the d i e . L a t e r a l m i g r a t i o n o f i o n s i s p o s s i b l e and has been shown t o a d v e r s e l y a f f e c t d e v i c e r e l i a b i l i t y (15). At the f

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p r e s e n t time the most commonly used f i n a l p a s s i v a t i o n l a y e r s a r e the p h o s p h o s i l i c a t e g l a s s e s (PSG). PSG i s p a r t i c u l a r l y e f f e c t i v e a s a f i n a l p a s s i v a t i o n c o a t i n g , and i s c h a r a c t e r i z e d by s i m p l i c i t y o f a p p l i c a t i o n . L a y e r s o f PSG have been w i d e l y used because they are i n l e s s t e n s i l e s t r e s s than l a y e r s of chemically-vapor-deposited s i l i c o n d i o x i d e ( 1 6 1 7 1 8 ) , and have the a b i l i t y t o t r a p i o n s (16). The l o w e r s t r e s s p r e v e n t s c r a c k i n g o f the c o a t i n g . f

f

Leakage. By f a r the m a j o r i t y o f i m p l a n t e d e l e c t r o n i c d e v i c e s f a i l due t o mechanisms o f the second t y p e ( i . e . l e a k a g e ) . I n a p o o r l y e n c a p s u l a t e d i m p l a n t , t h i s f a i l u r e mode can cause r a p i d d e t e r i o r a t i o n i n a m a t t e r o f days, hours, o r even minutes. As the s i z e o f the i m p l a n t d e s i g n s are reduced, the r i s k o f t h i s type o f f a i l u r e i s g r e a t l y i n c r e a s e d because due t o the s m a l l dimensions i n v o l v e d , the time f o r f l u i d e n t r y i n t o the package becomes v e r y short. 1. M e c h a n i c a l D e f e c t s . D i r e c t e n t r y o f h i g h l y i o n i c f l u i d s such a s s a l i n e i n t o the package r e s u l t s i n the f o r m a t i o n o f conduct i v e paths between e l e c t r o c h e m i c a l c o u p l e s o r c o n d u c t o r s o f d i f f e r i n g p o t e n t i a l s . The s i t e o f e n t r a n c e can v a r y , depending upon the package d e s i g n , but most f r e q u e n t l y i n v o l v e s the i n t e r f a c e between the e n c a p s u l a n t and a d i s s i m i l a r m a t e r i a l . F a i l u r e due t o d i r e c t s a l i n e e n t r y can be one o f two g e n e r a l t y p e s : those caused by d e f e c t s i n the b u l k m a t e r i a l used f o r e n c a p s u l a t i o n and those caused by t h e mechanism o f creep. The n a t u r e o f b u l k d e f e c t s i s s t r a i g h t f o r w a r d . P i n h o l e s , cracks, o r other surface imperfections r e s u l t i n r a p i d d i f f u s i o n o f s a l i n e i n t o t h e i m p l a n t r e s u l t i n g i n c o r r o s i o n o f e l e c t r i c a l conduct o r s . T h i s c o r r o s i o n l o w e r s the r e s i s t a n c e o f the o r i g i n a l l e a k a g e p a t h u n t i l normal o p e r a t i o n o f the e l e c t r o n i c c i r c u i t i s impared and the i m p l a n t f a i l s . Creep f a i l u r e s a r e not o f t e n a s easy t o p r e d i c t and may not show up i m m e d i a t e l y . A l t h o u g h the d e v i c e may not f a i l a s r a p i d l y a s i n t h e case o f a b u l k d e f e c t , t h e f i n a l r e s u l t i s i d e n t i c a l : c o r r o s i o n o f e l e c t r i c a l conductors. I n a c r e e p f a i l u r e , the s a l i n e d i f f u s e s between two a d j o i n i n g and u s u a l l y d i f f e r i n g m a t e r i a l s . The presence o f the f l u i d tends t o s e p a r a t e the two s u r f a c e s , thus p e r m i t t i n g more f l u i d t o l e a k i n t o t h e d e v i c e . A creep f a i l u r e i s p r i m a r i l y caused by the l a c k o f , o r poor a d h e s i o n between the two m a t e r i a l s . A common s i t e f o r such l e a k a g e i s the i n t e r f a c e between an e x i t i n g l e a d w i r e and t h e e n c a p s u l a t i o n m a t e r i a l . Often, an i n a p p r o p r i a t e c h o i c e f o r the w i r e i n s u l a t i o n may a c c e l e r a t e the f a i l u r e . M a t e r i a l s such a s T e f l o n are d i f f i c u l t t o bond t o , ( u n l e s s c h e m i c a l l y etched) however are f r e q u e n t l y used f o r i m p l a n t e d w i r e s . The o u t p u t l e a d s from i m p l a n t s such a s n e u r o s t i m u l a t o r s are u s u a l l y o f a low impedance n a t u r e and many i m p l a n t s a r e d e s i g n e d on t h e premise t h a t some e l e c t r i c a l l e a k a g e i s t o l e r a b l e a t t h i s s i t e . N o b l e m e t a l c o n n e c t i o n s can reduce the c o r r o s i o n t o an " a c c e p t a b l e " l e v e l . T h i s a s s u m p t i o n i s not t r u e f o r l e a d s w h i c h e n t e r the package from s e n s o r s such as m i c r o - e l e c t r o d e s w h i c h a r e c h a r a c t e r i z e d by r e l a t i v e l y h i g h impedances. The t r e n d f o r n e u r o p r o s t h e t i c d e v i c e s i s towards c l o s e d - l o o p c o n t r o l i n w h i c h the use o f h i g h impedance b i o e l e c t r i c s e n s o r s w i l l be common. I n a d d i t i o n , d i f f e r i n g p o t e n t i a l s w i t h i n m u l t i - c i r c u i t c a b l e s can r e s u l t i n c o r r o s i o n e v e n when the c o n d u c t o r s are f a b r i c a t e d from h i g h l y c o r r o s i o n r e s i s t a n t m a t e r i a l s such as MP35N.


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Another common l o c a t i o n f o r c r e e p f a i l u r e s o f e n c a p s u l a t e d a s s e m b l i e s i s a t sharp c o r n e r s or edges. Many e n c a p s u l a n t s such as p o l y i m i d e s must be a p p l i e d i n t h i n c o a t s , and c o v e r a g e o f p o i n t s , edges, o r c o r n e r s i s d i f f i c u l t o r i m p o s s i b l e . Sharp c o r n e r s , c h a r a c t e r i s t i c o f most t h i n - f i l m d e v i c e s p r o v i d e i d e a l c o n d i t i o n s f o r the i n i t i a t i o n o f c r e e p f a i l u r e s due t o the r e s u l t i n g i r r e g u l a r i t y o f the e n c a p s u l a n t coverage. 2. D i f f u s i o n o f Water. A l t h o u g h the p r o t e c t i o n a g a i n s t mois t u r e a f f o r d e d t o e n c a p s u l a t e d i m p l a n t s by some h i g h m o l e c u l a r - w e i g h t o r g a n i c m a t e r i a l s depends on the b a r r i e r e f f e c t i v e n e s s o f the encap s u l a n t , f o r s i l i c o n e s , o t h e r f a c t o r s , n o t a b l y bond c o n d i t i o n s a t the encapsulant/substrate i n t e r f a c e (adhesion), thermal s t a b i l i t y , m e c h a n i c a l i n t e g r i t y , and e l e c t r i c a l c o m p a t i b i l i t y p l a y a more i m p o r t a n t r o l e . I n f a c t , water v a p o r p e r m e a b i l i t y may not be the most c r i t i c a l p r o p e r t y t h a t w i l l determine t h e a c c e p t a b i l i t y o f a p o l y m e r as an e n c a p s u l a n t f o r e l e c t r o n i c a s s e m b l i e s . T h i s appears t o be the case f o r i n d u s t r i a l and m i l i t a r y e l e c t r o n i c a s s e m b l i e s w h i c h use the g e n e r a l l y p r e f e r r e d p o l y s i l o x a n e s , which are by no means impermeable t o w a t e r v a p o r , a consequence o f t h e i r h e l i c a l c h a i n m o l e c u l a r s t r u c t u r e . I t has been p r e v i o u s l y w e l l accepted (19J, a l t h o u g h not w i d e l y r e a l i z e d , t h a t the dominant p r o p e r t y o f s i l i c o n e p o l y m e r s f o r use a g a i n s t m o i s t u r e p e n e t r a t i o n o f e l e c t r o n i c a s s e m b l i e s i s t h e i r a b i l i t y t o adhere ( w i t h a p p r o p r i a t e p r i m e r s ) t o the p r o t e c t e d assembly. I n t e r f a c e P r o p e r t i e s o f S i l i c o n e Polymers H y d r o p h i l i c s u r f a c e s i t e s p e r m i t the a d s o r p t i o n and c o n d e n s a t i o n o f water. E f f e c t i v e b l o c k i n g o f h y d r o p h y l i c s i t e s on e n c a p s u l a t e d s u r f a c e s w o u l d r e q u i r e c h e m i c a l r e a c t i o n o f these s i t e s w i t h f u n c t i o n a l groups o f a p p r o p r i a t e c o a t i n g s , o r d e p o s i t i o n o f s t r o n g l y adherent f i l m s o f o r g a n i c compounds on the p r o t e c t e d s u r f a c e . I n a c t i v a t i o n o f h y d r o p h y l i c s u r f a c e s i t e s i n the c o r r o s i o n c o n t r o l o f e l e c t r o n i c d e v i c e s had been proposed as e a r l y as 1969 by White o f B e l l Telephone Labs (JL9J. He demonstrated the e f f e c t i v e n e s s o f many d i f f e r e n t types of c o a t i n g s i n p r o t e c t i n g i n t e g r a t e d c i r c u i t c h i p s from m o i s t u r e damage. S i l i c o n e and imide m a t e r i a l s showed e x c e l l e n t p r o t e c t i o n a g a i n s t m o i s t u r e p e n e t r a t i o n i n t h i s study. I n subse quent h i g h p r e s s u r e steam t e s t s , s u p e r i o r performance was seen f o r the s i l i c o n e p o l y m e r s , i n s p i t e o f t h e i r r e l a t i v e l y h i g h water v a p o r p e r m e a b i l i t y . B o t h c l a s s e s o f m a t e r i a l s c o n t a i n groups w h i c h can r e a c t w i t h s u r f a c e hydroxy I s p r e s e n t on a l m o s t a l l h y d r o p h i l i c s u r f a c e s . From t h i s s t u d y emerged the t h e o r y t h a t m o i s t u r e p r o t e c t i o n o f n o r m a l l y h y d r o p h i l i c s u r f a c e s by the h i g h l y semi-permeable s i l i c o n e s was a consequence o f r e a c t i o n s w i t h s u r f a c e hydroxy1 c h a r a c t e r i z e d by s t r o n g a d h e s i o n o f the polymer t o the p r o t e c t e d s u r f a c e . V a r i o u s t h e o r i e s o f a d h e s i v e b o n d i n g are w e l l documented ( 2 Û * 2 J J and w i l l not be r e p e a t e d here. S i m i l a r l y , the need f o r r i g i d c l e a n i n g p r o t o c o l s t o enhance a d h e s i o n has been p r e v i o u s l y w e l l described (22). Most c o m m e r c i a l l y a v a i l a b l e s i l i c o n e e l a s t o m e r s , and e s p e c i a l l y those w h i c h p o l y m e r i z e by a d d i t i o n - c u r e , do not, o r w e a k l y , adhere t o most s u r f a c e s . A d h e s i o n promoters known as s u r f a c e p r i m e r s are a v a i l a b l e and are o f t e n used t o adhere the s i l i c o n e t o the p r o t e c t e d


304


surface. These p r i m e r s a r e g e n e r a l l y f o r m u l a t e d from s i l a n e c o u p l i n g agents which c o n t a i n f u n c t i o n a l groups that r e a c t with the p r o t e c t e d s u r f a c e as w e l l as t h e p o l y m e r (21). There i s a t r e n d i n the c h e m i c a l i n d u s t r y towards t h e b l e n d i n g o f c o u p l i n g a g e n t s and base r e s i n s t o form " s e l f - p r i m i n g " m a t e r i a l s . Adherent f i l m s w o u l d not n e c e s s a r i l y r e q u i r e f o r m a t i o n o f c o v a l e n t bonds a t t h e i n t e r f a c e , s i n c e l o c a l i z e d i n t e r m o l e c u l a r d i s p e r s i o n forces that are o p e r a t i v e i n the adsorption o f coatings (with good w e t t i n g p r o p e r t i e s ) s h o u l d p r o v i d e s t a b l e i n t e r f a c i a l bond conditions. Among c a n d i d a t e m a t e r i a l s w h i c h c o u l d f u l f i l l t h e r e q u i r e m e n t s o f good a d h e s i o n and s u b s t r a t e p r o t e c t i o n from m o i s t u r e a r e e p o x y - m o d i f i e d p o l y u r e t h a n e s and e p o x y - s i l o x a n e p o l y m e r s .

Silicones for Electronic Encapsulation S i n c e t h e i n i t i a l work o f White (19J, t h e B e l l System and o t h e r major s e m i c o n d u c t o r u s e r s h a v e e x t e n s i v e l y used s i l i c o n e s i n t h e p r o t e c t i o n o f numerous t h i n - f i l m and t h i c k - f i l m d e v i c e s (2JL*2u). The m a t e r i a l s p r i m a r i l y have been condensation-cure s i l i c o n e s i n xylene dispersion. The performance o f e n c a p s u l a t e d s e m i c o n d u c t o r s used w i t h i n t h e B e l l System i s w e l l documented (26) and s t u d i e s c o n t i n u a l l y i n p r o c e s s s u p p o r t t h e use o f s i l i c o n e s f o r t h e p r e v e n t i o n o f e l e c t r o n i c component d e t e r i o r a t i o n under c o n d i t i o n s s i m i l a r t o t h e i n - v i v o environment. More r e c e n t l y Wong (21)> has r e p o r t e d t h e r e s u l t s o f t e s t s d e s i g n e d t o c h a r a c t e r i z e parameters o f e l e c t r o n i c s i l i c o n e s by T h e r m o g r a v i m e t r i c , F o u r i e r T r a n s f o r m - I n f r a r e d , and Gas Chromatography/ Mass S p e c t r o m e t r y . T h i s work has c o n c e n t r a t e d upon c o n d e n s a t i o n c u r e m a t e r i a l s used w i t h i n t h e B e l l System, r a t h e r t h a n t h e e n d blocked free r a d i c a l (addition-cure) s i l i c o n e s . Based upon t h e t h e o r y t h a t t h e m o i s t u r e p r o t e c t i o n o f e l e c t r o n i c a s s e m b l i e s by e n c a p s u l a n t s i s d i r e c t l y r e l a t e d t o t h e a d h e s i v e bond between t h e e n c a p s u l a n t and t h e u n d e r l y i n g s u r f a c e , a d h e s i o n t e s t i n g has emerged has t h e p r i m a r y method o f e n c a p s u l a n t q u a l i f i c a t i o n f o r i m p l a n t u s e . Pacemaker m a n u f a c t u r e r s h a v e p e r formed l a p s h e a r and b u t t t e n s i l e t e s t i n g o f s e l e c t e d s u b s t r a t e / e n c a p s u l a n t c o m b i n a t i o n s a f t e r h i g h h u m i d i t y exposure and t h e s e t e s t s r e l a t e t h e bond s t r e n g t h s t o t h e c o r r o s i o n p r o t e c t i v e a b i l i t y o f t h e e n c a p s u l a n t (£&)· D o n a l d s o n (23) has a l s o i n v e s t i g a t e d t h e e f f e c t s p a r a m e t e r s such as e l e c t r i c f i e l d s t r e n g t h (3Û)» and t e n s i l e f o r c e s w i t h i n a c t u a l i m p l a n t s . C o r r o s i o n i s an e l e c t r o c h e m i c a l p r o c e s s whose p r e v e n t i o n depends upon t h e a b i l i t y o f t h e encapsulant to prevent the flow o f e l e c t r i c a l leakage currents. E l e c t r i c a l l e a k a g e c u r r e n t t e s t s combined w i t h bond s t r e n g t h t e s t s w o u l d p r o v i d e not o n l y a measure o f t h e p o l y m e r ' s a b i l i t y t o r e m a i n i n c o n t a c t w i t h t h e p r o t e c t e d s u r f a c e , b u t a l s o t o a c t as a n i n s u l a t o r t o e l e c t r i c a l leakage c u r r e n t s which i n e v i t a b l i t y w i l l r e s u l t i n corrosion. Other i n v e s t i g a t o r s h a v e noted t h a t a s s e m b l i e s i n w h i c h t h e s i l i c o n e e l a s t o m e r was p o o r l y bonded t o t h e p r o t e c t e d s u r f a c e h a v e s u r v i v e d i m p l a n t a t i o n l o n g e r than expected i f c o n s i d e r i n g adhesion as t h e p r i m a r y p r o t e c t i v e mechanism. In t h i s regard, Donaldson ( 3 D has a d v a n c e d t h e t h e o r y o f o s m o t i c pumping a s a n a l t e r n a t e mechanism o f c o r r o s i o n p r o t e c t i o n by s i l i c o n e s . F o r submersion i n p h y s i o l o g i c a l s a l i n e , d i f f u s i o n o f water t h r o u g h t h e s i l i c o n e i n t o t h e d e v i c e , i s opposed by a n o s m o t i c g r a d i e n t out o f t h e d e v i c e . f



27. TROY Κ ET AL.

305

Methods We t e s t e d 6 d i f f e r e n t s i l i c o n e c a s t i n g e l a s t o m e r s i n c o n j u n c t i o n with 3 surface primers. The m a t e r i a l s and t h e i r d e s i g n a t i o n s a r e as follows:

Designation Type A Type Β Type C

Surface Primerg

Name

Formulation

Manufacturer

P r i m e c o a t 1200 Chemlok 607 SS 4155

Dow C o r n i n g Lord Chemical General E l e c t r i c

S i l a n e - S i l i c a t e Blend Blended S i l a n e s Ethylorthosilicate

Silicone Elastomers Designation Type 1 Type 2 Type Type Type Type

Name RTV-615 S y l g a r d 184 MDX-4-4210 96-083 S y l g a r d 567 RTV-3140

Manufacturer General E l e c t r i c Dow C o r n i n g Dow C o r n i n g Dow C o r n i n g Dow C o r n i n g Dow C o r n i n g

Cured Hardness Durometer (Shore A) 45 35 25 63 38 22

*MDX-4-4210 was b l e n d e d w i t h 10? S y l g a r d 527, decrease v i s c o s i t y f o r c a s t i n g purposes.

by w e i g h t ,

to

The f i r s t 5 s i l i c o n e e l a s t o m e r s a r e a d d i t i o n - c u r e p r o d u c t s . The 6 t h m a t e r i a l i s a c o n d e n s a t i o n - c u r e p r o d u c t w h i c h has been p r e v i o u s l y used f o r m e d i c a l i m p l a n t p r o t e c t i o n (2). As v e r i f i e d by l a p shear t e s t s , (described below), the f i r s t 3 elastomers p r o v i d e m i n i m a l a d h e s i o n i n t h e absence o f a s u r f a c e p r i m e r . The l a s t 3 a r e self-priming materials. To p e r m i t v i s u a l e x a m i n a t i o n , we t r i e d t o select clear materials. A l l o f the elastomers are c l e a r except f o r t y p e s 5 a n d 6. Types 1, 2, and 3 were s e l e c t e d f o r t e s t w i t h each o f t h e p r i m e r s , t y p e s A, B, and C. I n a d d i t i o n , type 4 e l a s t o m e r was used as a p r i m e r f o r t y p e s 1, 2, and 3 . A l l e l a s t o m e r s t y p e s 1-6 were a l s o tested without primers. T e s t i n g was performed by e n c a p s u l a t i o n o f 4 PWB, G-10, e p o x y g l a s s i n t e r d i g i t a t e d t e s t p a t t e r n s u b s t r a t e s p e r t e s t group. The p a t t e r n s s e l e c t e d were t a k e n from t h e IPC B-25 s t a n d a r d t e s t b o a r d , p a t t e r n s A and B. These p a t t e r n s use i n t e r d i g i t a t e d l i n e w i d t h s and s p a c i n g s o f 0.006 and 0.012 r e s p e c t i v e l y . Two s i n g l e - s i d e d A p a t t e r n s (0.006 l i n e s and s p a c i n g s ) and two s i n g l e - s i d e d Β p a t t e r n s (0.012" l i n e s and s p a c i n g s ) were p r e p a r e d f o r each e l a s t o m e r - p r i m e r combination tested. F o l l o w i n g e n c a p s u l a t i o n , each o f t h e p a t t e r n s was c o n t i n u o u s l y submersed i n 8 5 ° C water and p l a c e d under a c o n t i n u a l v o l t a g e s t r e s s o f 20VDC. E l e c t r i c a l l e a k a g e c u r r e n t (under 9VDC b i a s ) was measured f o r each o f t h e t e s t s u b s t r a t e s on a d a i l y basis. w

n

n

P r e p a r a t i o n o f each t e s t s u b s t r a t e was performed as f o l l o w s : A 10" l o n g , Kynar ( p o l y v i n y l i d e n e f l u o r i d e ) i n s u l a t e d w i r e ( i n s u l a t i o n t h i c k n e s s , 0.005") was s o l d e r e d t o each e l e c t r i c a l h a l f o f t h e


306


s u b s t r a t e . The s u b s t r a t e was t h e n c o m p l e t e l y g r i t - b l a s t e d (40 PSI) u s i n g 240 mesh (50um) aluminum o x i d e g r i t . F o l l o w i n g g r i t - b l a s t i n g , the s u b s t r a t e was c l e a n e d i n a s e r i e s o f s o l v e n t s a s f o l l o w s : M e t h y l e n e C h l o r i d e - 1 min. wash, I s o p r o p y l A l c o h o l r i n s e , Acetone-1 min. wash, I s o p r o p y l A l c o h o l r i n s e , F r e o n TMS-2 min. u l t r a s o n i c c l e a n i n a Branson B-220 u l t r a s o n i c c l e a n e r , F r e o n TF r u n n i n g r i n s e . D u r i n g the c l e a n i n g p r o c e d u r e the s u b s t r a t e was h a n d l e d , w i t h g l o v e s , o n l y by the ends o f the c o n n e c t i n g w i r e s . A f t e r c l e a n i n g , a vacuum-bakeout was performed f o r 12hrs a t 120°C i n a P r e c i s i o n model 10 vacuum o v e n a t 1-2 t o r r . The vacuum system was f i l t e r e d by an a c t i v a t e d a l u m i n a f i l l e d vacuum f i l t e r . Upon r e m o v a l from the vacuum oven, d e s i g n a t e d s u b s t r a t e s were primed w i t h the a p p r o p r i a t e p r i m e r a c c o r d i n g t o t h e manufacturer's i n s t r u c t i o n s . Each o f the m a n u f a c t u r e r s was c o n t a c t e d d i r e c t l y i n o r d e r t o v e r i f y the p r o c e dure a s o u t l i n e d i n the d a t a sheet. A s u b s t r a t e ready f o r encapsu l a t i o n was p l a c e d i n a h a l f - o p e n mold w h i c h supported the s u b s t r a t e h o r i z o n t a l l y by i t s w i r e s . The a p p r o p r i a t e e n c a p s u l a t i n g e l a s t o m e r was mixed and d e - a i r e d and poured i n t o the mold under vacuum c a s t i n g . T h i s procedure produced e n c a p s u l a t e d s u b s t r a t e s w i t h 0. 100"±0.015" e n c a p s u l a n t t h i c k n e s s o v e r and under the s u b s t r a t e . The d i a m e t e r o f the mold was such t h a t a t h i c k n e s s o f a t l e a s t 0.3" o f e n c a p s u l a n t c o v e r e d the s u b s t r a t e ' s edge. As a r e s u l t o f the vacuum c a s t i n g no b u b b l e s were v i s i b l e i n the e n c a p s u l a t e d sub s t r a t e s . C u r i n g o f the a d d i t i o n - c u r e e l a s t o m e r s was by f o r c e d - a i r c o n v e c t i o n a t a p r e s c r i b e d temperature o f 120°-165°C depending upon the p a r t i c u l a r e l a s t o m e r cured. C u r i n g o f the type 6 m a t e r i a l was done i n a h u m i d i t y chamber o f >90$rh f o r 72hrs. The s p e c i f i c com b i n a t i o n s o f p r i m e r s and e l a s t o m e r s t e s t e d are l i s t e d i n t a b l e 1. Type 4 e l a s t o m e r was used a s a p r i m e r due i t s p r e v i o u s l y demon s t r a t e d h i g h bond s t r e n g t h i n e x p e r i m e n t s i n our l a b o r a t o r y . I t was a p p l i e d i n a v e r y t h i n (jcc c d c c c d c c c d c c c Î c e c c c c a c n - i c e c c c f i q C c C d qo^voNoovoNoooqôocs ôoonooooa ο ^· a ο co ο ι TSor^voodvoooooojM^'oH^vo · > · · · « · ·CN « o ^ c N C N ^ m v o i n J ^ V O CN H ιΗ ι—I I I cN I CN Ivo ^* co ml CN a c n o o m m c N ^ r r r m u in 3 1 1-H H C C C ΙΟ ΟΟ >4θ V © ( Ν ία r- co ιΗ

G G G d G Ο G d G Ο G s cC cO aG c Gc Gf i c c e f i c G G G G 3 C C G C G Ο Ο C CN Ο Ο G C0 O CC N N C00 O ί Ï^ VOD ΟOΟ O O f f OfS CN VO CN · CN CNJ VO · < V O CN · · C N CN · CΓ O 0 · ·r—I · Η i—I ι Η ·· ·· « · γ Η · « ^•tniHcsrocNvocioo CNH CO C N ιΗ CN I CO CN iH Oi VO H H < ο CN - H co C N m m C N CN 4-1 Ο Ο

c f i c c e e M C C C C C C C C C 3 C Cc c c c c c c o c c c c vOÔ^NvOOIflOOn C O C N ^ C O C O ÔOO^JOOOOO Ο 00 C N Ο O cr> · a · · · «cr · · « m η (H « C N ^ x: m r- C N C N · CN C N · co CN vo vor^r^r~>cNinrorH^rm r* m ΓΟ iH r- ι—ι Φ H H m

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ininiriinoo · · · «j vo ^· ι—ι Ο Ο < O OΟO C N1 rH VVV

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