Synchrotron radiation imaging study on the rapid IMC growth of Sn–xAg solders with Cu and Ni substrates during the heat preservation stage

Abstract

Synchrotron radiation X-ray real-time imaging technology was used to study the interfacial reactions of Sn and Sn3.5Ag solders with Cu and Ni substrates during the heat preservation stage. This experiment allows us to in-situ observe the interfacial intermetallic compounds (IMCs) growth, provides a stage to distinctly separate their growth at isothermal reflow and cooling durations, and thus avoids the measurement errors for size and morphology of interfacial IMCs. The results show that the Cu6Sn5 morphology is scalloped at Sn–xAg/Cu interfaces, whereas (Cu,Ni)6Sn5 is wide-stripe shaped at Sn–xAg/Ni interface during the heat preservation stage. It is interesting to note that the grain boundary molten liquid gaps is observed at both of the Sn3.5Ag/Cu and Sn3.5Ag/Ni interfaces. Compared with pure Sn solder, Sn3.5Ag solders are observed with interfacial IMC of greater thickness, smaller base width, lower aspect ratio and more substrate consumption. In addition, it is confirmed that Ag3Sn grains appear in Sn3.5Ag solder, but not on the surface of existing interfacial Cu6Sn5 grains at Sn3.5Ag/Cu interface during the heat preservation stage. All of the growth kinetic index (n) for Sn–xAg/Cu is close to 1/3 indicating that grain boundary diffusion determines the interfacial reaction at the heat preservation stage. The mechanism by which Ag addition affects interfacial reaction and changes the mirostructure of solder matrix through creation of grain boundary gaps and decrease of the growth activation energy of IMC is also clarified. The knowledge for morphology and thickness control of interfacial IMCs by alloy elements addition as outlined by this study can be utilized to improve the reliability of solder joints in electronic packaging and solar photovoltaic (PV) cells array manufacturing sectors.