Layer growth in Au-Pb/In solder joints
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ALLOYS
of P b - S n , P b - I n , and P b - S n - I n a r e often used in the e l e c t r o n i c s i n d u s t r y to s o l d e r v a r i o u s Au p a r t s . S e v e r a l i n t e r m e d i a t e p h a s e s a r e known to exist b e t w e e n Au and the s o l d e r alloy components.l-a The a p p e a r a n c e of these p h a s e s d u r i n g aging affect the m e c h a n i c a l and p h y s i c a l i n t e g r i t y of t h e s e s o l d e r j o i n t s . S e v e r a l a u t h o r s have studied aging k i n e t i c s in related solder alloys. P o w e l l and B r a u n 4 s t u d i e d p u r e A u - I n diffusion coup l e s aged at 142 and 151~ They found a p r e d o m i n a n c e of AuIn2 and thin l a y e r s of AuIn and AugIn4. The o r i g i n a l i n t e r f a c e was m a r k e d with q u a r t z m i c r o s p h e r e s which r e m a i n e d at the In-AuIn2 i n t e r f a c e a f t e r 1056 h at 142~ T h i s i n d i c a t e d that In diffused m u c h m o r e r a p i d l y through the r e a c t i o n zone than did Au. U s i n g only t h r e e m e a s u r e m e n t s at each aging t e m p e r a t u r e , t h e s e a u t h o r s concluded that AuIn2 t h i c k n e s s v a r i e s l i n e a r l y with the s q u a r e root of diffusion t i m e . With apologies for t h e i r s c a r c e data, t h e s e a u t h o r s quoted an a c t i v a t i o n e n e r g y for diffusion equal to 0.97 eV. B r a u n and R h i n e h a m m e r 5 i n v e s t i g a t e d the r e a c t i o n b e t w e e n 0.0015 in. d i a m Au w i r e and a P b - S n - I n alloy. The c y l i n d r i c a l r e a c t i o n l a y e r was found to c o n s i s t of AuIn2; no other compounds w e r e found. The r e a c t i o n l a y e r contained no Sn and v e r y little P b in solid s o l u t i o n . T h i s suggested that I n was a g a i n the f a s t e r diff u s i n g c o m p o n e n t . The a u t h o r s stated that for aging t e m p e r a t u r e s of 100 and 120~ the s q u a r e d r a d i u s of the c o m b i n e d Au w i r e and r e a c t i o n zone r e g i o n v a r i e d l i n e a r l y with aging t i m e . R e c e n t l y , u s i n g the s a m e P b - S n - I n alloy, J a n c o and B r a u n 6 extended t h e i r data to lower aging t e m p e r a t u r e s . However, they r e p o r t e d this data i n t e r m s of f r a c t i o n of Au c o n s u m e d and p r o p o s e d an e x p o n e n t i a l r e l a t i o n s h i p b e t w e e n this f r a c t i o n and aging t i m e . T h e s e a u t h o r s concluded that t h e i r r e s u l t s c o n f i r m e d the e a r l i e r work of B r a u n and R h i n e h a m m e r . The 50 wt pct P b - I n a l l o y has r e c e i v e d r e c e n t i n t e r e s t 7 s i n c e it does not d i s s o l v e Au r a p i d l y d u r i n g the s o l d e r i n g o p e r a t i o n . T h i s alloy lies in the f a c e c e n t e r e d cubic solid s o l u t i o n r e g i o n on the P b r i c h FREDERICK G. YOST, FLOYD P. GANYARD,and MAURICE M. KARNOWSKYare Staff Member, Engineeringand ScienceAssistant and Staff Member, respectively, Sandia
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