The Effect of Solid-State Reactions Upon Solder Lap Shear Strength

The effect of solid-state reactions between gold plating and the eutectic solders 88Au:12Ge, 80Au:20Sn, 90SN:10Au, and 85Pb: 15Au and the off eutectic solder 60SN:36Pb:4Ag upon the solder lap shear strength has been investigated at four aging temperatures ranging from 75 to 150°C. Failure analysis of the lap shear samples has shown that Au-Ge and Au-Sn solders failed cohesively whereas Sn-Au, Pb-Au, and Sn-Pb-Ag solders failed adhesively. Scanning electron micrographs and microprobe analysis of gold-solder metallographic sampies aged at 125°C revealed the presence of AuSn, AuSn2, AuSn4, and voids at the plating-solder interface of Sn-Au solder. AuSn2and AuSn4intermetallic compounds were detected at the interface of SnPb-Ag solder, and AuPb2was detected at the interface of Pb-Au solder. The zeta phase of the Au-Sn binary system was detected at the plating-solder interface of the Au-Sn solder. The plating-solder interface of the Au-Ge solder was devoid of any intermediate phases or intermetallic compounds. The results of the lap shear measurements have shown that all five solders can be expected to exhibit adequate strength for microelectronic purposes for >10⁵h at a sustained temperature of <100°C when the mating materials have similar coefficients of thermal expansion. However, with dissimilar thermal expansion coefficients, the SnAu and Sn-Pb-Ag solders both failed catastrophically within 10³h at 100°C