Study on Electromigration Mechanism of Lead-Free Sn3.5Ag0.5Bi8.0In Solder Joints

The electromigration mechanism of linear Cu/Sn3.5Ag0.5Bi8.0In/Cu and Cu/Sn3.0Ag0.5Cu/Cu solder joints has been systematically studied. The crystal orientation and hardness of the solder joints, the composition analysis of them, and the microstructure evolution of the interfacial intermetallic compound (IMC) during the electromigration process have been studied. The “crossing orientation” in reflowed Cu/Sn3.5Ag0.5Bi8.0In/Cu solder joints was found and defined. A large amount of the In component reduced the undercooling of the solder joints. The latent heat released by the growth of Sn grains cannot provide enough energy to melt these grains grown at other nucleation sites. The grain orientations of Sn were dominated by twins, and there were also some small grain orientations formed by nucleation sites that were not melted by the latent heat. This was expressed as “crossing orientation”. In addition, the hardness of the Sn3.5Ag0.5Bi8.0In solder joints was isotropic and about 22 HV, which was higher than that of Cu/Sn3.0Ag0.5Cu/Cu solder joints from 10 HV to 16 HV. After electromigration, the intermetallic compound (IMC) layer growth rate of the Cu/Sn3.5Ag0.5Bi8.0In/Cu linear solder interface is slower than that of Cu/Sn3.0Ag0.5Cu/Cu linear solder. The growth formulas of the IMCs have been derived by Fick's second law at the Cu/Sn3.5Ag0.5Bi8.0In/Cu solder joints. Under current stress, the preferential dissolution of Cu6Sn5 at the grain boundary and the consumption rate of the Cu plate increased rapidly, leading to the cathode dissolution phenomenon.