Trace of Ag Atoms Migration Induced by Temperature Gradient in Cu/Sn-5.0Ag/Cu Solder Joints

The growth of interfacial IMC in Cu/Sn/Cu and Cu/Sn-5.0Ag/Cu micro solder joints and the trace of Ag atoms migration in Cu/Sn-5.0Ag/Cu micro solder joint during reflow with and without temperature gradient (TG) were investigated. The growth of interfacial IMC during isothermal reflow was symmetrical and controlled by grain boundary diffusion; whereas that during reflow under TG was asymmetrical and controlled by reaction rate. In addition, the Cu and Ag atoms could migrate from the hot end to the cold end driven by TG and react with Sn atoms to form Cu6Sn5 and plate-type Ag3Sn at the cold end in the Cu/Sn-5.0Ag/Cu solder joint. As the reflow time prolonged, the plate-type Ag3Sn would embed in the Cu6Sn5 phase. Due to the larger interfacial energy of Cu6Sn5/Ag3Sn and the smaller absolute value of the enthalpy of formation for Ag3Sn, the Ag3Sn would dissolve and release Sn atoms to support Cu6Sn5 growth by consuming the surplus Cu atoms generated by the additional TG, leaving Ag atoms in the full IMC joint. Specifically, the Ag element remained in the Cu6Sn5 phase with Ag content of 0.55 wt%.