Observations on interdiffusion in planar copper/tin-nickel/gold tricouples
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PINNEL
AND
JAMES
E. BENNETT
Many of the b a s i c p r o p e r t i e s of the e l e c t r o d e p o s i t e d t i n - n i c k e l (Sn-Ni) a l l o y have b e e n i n v e s t i g a t e d , but two p r e v i o u s l y u n r e p o r t e d a s p e c t s a r e i n t e r d f f f u s i o n b e h a v i o r in the C u / S n - N i / A u s y s t e m and the t h e r m a l s t a b i l i t y of S n - N i when a s s o c i a t e d with gold. P l a n a r couples w e r e aged at t e m p e r a t u r e s b e t w e e n 100 and 500~C f o r t i m e s to one y e a r and e v a l u a t e d by s c a n n i n g e l e c t r o n m i c r o s c o p y , e l e c t r o n probe m i c r o a n a l y s i s , and A u g e r e l e c t r o n s p e c t r o s c o p y . R e s u l t s show that above 250~ the l a y e r d i s i n t e g r a t e s into fine p a r t i c l e s and the gold and copper i n t e r d i f f u s e as if the S n - N i l a y e r were n e v e r p r e s e n t . At lower t e m p e r a t u r e s , the d i s i n t e g r a t i o n of the l a y e r is not d e t e c t a b l e b y the t e c h n i q u e s used or within the t i m e p e r i o d s t u d i e d . However, t i n is s t i l l r e l e a s e d f r o m the l a y e r and diffuses v e r y r a p i d l y , e v e n t h r o u g h thick gold, to the s u r f a c e w h e r e it is oxidized. The t h i c k e r the t i n - n i c k e l l a y e r , the g r e a t e r the a m o u n t of s u r f a c e oxide which is produced for a fixed aging condition. T h u s s o m e q u a n t i t i e s of t i n - o x i d e can be e x pected to f o r m on the s u r f a c e of gold i n t i m e s of j u s t a few y e a r s for a p p l i c a t i o n t e m p e r a t u r e s b e t w e e n 100~C and r o o m t e m p e r a t u r e .
I. INTRODUCTION
AN
electrodeposited equiatomic tin-nickel (Sn-Ni) alloy has found application in the electronics industry as an etch resist I and as an underplate for thin porous golds in electrical contacts, e as a result of its high resistance to tarnishing. ~ It has also been reported to be harder and more durable than either tin or nickel alone, 4 and the latter is frequently used as an underplate or "diffusion barrier" between copper and gold in separable electrical connectors. For these reasons, the practicality of using tin-nickel has been fully explored and previously reported by others. These included rather extensive studies on the chemistry of the passivating film ~-7 and the thermal stability, s-l~ corrosion resistance, n and structure '~ of the electrodeposit. Although the results of the above evaluations yielded no reservations toward the use of t i n - n i c k e l , the e v a l u a t i o n of the m e c h a n i c a l p r o p e r t i e s and, in p a r t i c u l a r , the lack of d u c t i l i t y and the i m p r o b a b i l i t y for i m p r o v e m e n t of this p r o p e r t y ~3 g r e a t l y d i m i n i s h e d its a t t r a c t i v e n e s s . One a s p e c t on t i n - n i c k e l which is p e r t i n e n t to its use a s an u n d e r p l a t e is the i n t e r d i f f u s i o n b
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