Reliability of Sn-Pb solder joints with Cu and Co-P surface finishes under thermal cycling

In this work, ball grid array (BGA) Sn-Pb solder joints on Co-P (4±0.5 at.%P and 8±0.8 at.%P) and Cu substrates were investigated to clarify the effects of interfacial reaction and morphology on the shear strength of the joints during thermal cycling. The interfacial intermetallic compounds (IMCs) in the solder joints were observed by back-scattered electron scanning electron microscopy. The composition of Co-P finishes and IMCs was determined by energy dispersive spectroscopy. The shear strength of Sn-Pb/Cu and Sn-Pb/Co-P solder joints was obtained by ball shear testing. The results showed that Co-P surface finishes retarded the formation of IMCs at the interface as compared with Cu finish. The shear strength of Sn-Pb/Co-4%P solder joints was slightly lower than that of Sn-Pb/Cu joints after thermal cycling, whereas the Sn-Pb/Co-8%P solder joints exhibited the highest shear strength. The fracture mode of all three kinds of joints was ductile mode. The experimental data showed that Co-8%P could be a good surface finish used in BGA packages.