Effects of multiple reflows on microstructure and shear strength of (Au-20Sn)-2Ag/Au/Ni(P)/Kovar joints

摘要

The multiple reflow reliability of Au-20 wt%Sn solder has attracted great interests in optoelectronic and high power electronic industries. But the growing dendrite in the solder has a negative effect on the reliability of the solder joint. In this study, to control the microstructure and improve the reliability of the Au-20Sn electronic-packaging solders, we successfully designed and prepared a (Au-20 wt%Sn)-2wt.%Ag solder using micro-alloying method. It was found that the addition of 2 wt%Ag can change the heterogeneous nucleation conditions and refine the grains of primary Au_5Sn phase, forming globular-shaped (Au,Ag)_5Sn phase and eutectic (AuSn + (Au, Ag)_5Sn) lamellar structure. The (Au-20Sn)-2Ag solder showed a strong reflow reliability even after 200 times of reflow (the shear strength of (Au-20Sn)-2Ag/Au/Ni(P)/Kovar joint maintained 88.33% of that measured after one reflow, and the shear strength of Au-20Sn/Au/Ni(P)/Kovar joint only maintained 63.37% of that measured after one reflow). The interface of (Au-20Sn)-2Ag/substrate joint after 200 times of reflow consisted of less accumulated (Ni,Au)_3Sn_2 grains and thick (Au,Ni)_5Sn phase, so the refined microstructure can reduce the stress concentration and thus hinder the crack initiation and propagation, thereby significantly improving the reliability of the joint.