Interfacial microstructure and shear behavior of Sn–Ag–Cu solder balls joined with Sn–Zn–Bi paste

Abstract

The Sn–3.2Ag–0.5Cu solder balls were joined with Sn–8Zn–3Bi paste on Cu/Ni/Au metallized BGA substrates at 240 °C. The shear behavior of thus produced BGA package was investigated after multiple reflow and thermal aging. The two solders have mixed together and have no visible interfacial boundary. Moreover the void and the indent can be clarified occasionally close to the interface after reflow. The shear strength falls from 18.0 ± 1.6 N (one cycle reflow) to 14.8 ± 1.5 N (five cycle reflow) and remains roughly unchanged after 5 cycles up to 10 cycles (14.9 ± 2.1 N). Four types of fractures are observed in single cycle cases. The solder in which the voids exist may be sheared to descend the joint strength. The thickness of Ni–Sn–Cu–Zn compounds appears to be irrelevant to the shear behavior from 5 to 10 cycles. The shear strength drops from single to five cycles may be due to the spallation of excessive brittle Ni–Sn–Cu–Zn compounds into the solder which degrades the joint strength. The shear strength of the Sn–Ag–Cu/Sn–Zn–Bi package changes little after aging at 150 °C. The strengths are 17.5 ± 2.9, 16.9 ± 2.7, 17.6 ± 2.6 and 16.8 ± 2.4 N after aging for 100, 200, 500 and 1000 h, respectively. As aging time increases, fractures shift to occur in the solder.