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Effects of Cu Addition on Growth of Au-Sn Intermetallics at Sn-xCu/Au Interface during Aging Process

The growth of Au-Sn intermetallic compounds (IMCs) is a major concern to the reliability of solder joints in microelectronic, optoelectronic and micro-electro-mechanical system (MEMS) which has a layer of Au metallization on the surface of components or leads. This paper presents the growth behavior of Au-Sn IMCs at interfaces of Au metallization and Sn-based solder joints with the addition Cu alloying element during aging process, and growth coefficients of the Au-Sn IMCs were calculated. Results on the interfacial reaction between Sn-xCu solders and Au metallization during aging process show that three thick layers of Au-Sn IMCs including AuSn, AuSn2 and AuSn4 formed at the interface region. The thickness of each Au-Sn IMC layer vs square root of aging time followed linear relationship. Calculation of the IMC growth coefficients show that the diffusion coefficients decrease with the addition Cu elements, which indicates that Cu addition suppresses the growth of Au-Sn IMCs layer.