Effect of initial Cu concentration on the IMC size and grain aspect ratio in Sn-xCu solders during multiple reflows

摘要

AbstractThe effect of initial Cu concentration in Sn–xCu solders, on grain size and morphology of interfacial intermetallic compound (IMC) during multiple reflow process, was investigated in this work. It was found that the initial Cu content in solders is closely related to the IMC nucleation and Cu migration behavior, which can govern the IMC crystal growth as well as grain aspect ratio evolution. In general, higher initial Cu in solder would lead to an increase in thickness of IMC, regardless of the reflow number. For Sn, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu, the grain aspect ratios show decreased values with rising initial Cu percentage in solder; but for Sn0.7Cu, it has an even smaller aspect ratio than pure Sn. Moreover, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu present a pronounced suppression of grain aspect ratio with reflow number, whereas this is not observed in context of pure Sn and Sn0.7Cu solders. Fine Cu6Sn5nanoparticles on the surface of interfacial IMC grains in Sn0.7Cu alloy were observed in both isothermal heating and cooling, which may have served as the reason for its unique behavior of grain aspect ratio evolution. The relationship between Cu addition and IMC grain growth can be utilized in the reliability design of Pb-free solder materials.