Effects of the Inter-Metallic Compounds Microstructure on Electro-Migration of Sn-Bi Solder System

In recent years, Sn-Bi solders get much attention because low temperature bonding technique is possible to reduce warpage of package and stress at solder joining. However, few studies attempted to their reliability and knowledge of microstructure of the solder bump. In this study, we investigated the difference of inter-metallic compounds (IMCs) formation between Sn57wt%Bi solder (Sn-Bi eutectic solder) and Sn30wt%Bi solder (Sn30Bi solder) during electro-migration test. Electron probe micro analyzer (EPMA) and Electron backscattered diffraction (EBSD) techniques were employed to analyze of microstructure and crystal orientation of the interconnection bump. Maximum electrical resistance increase during electro-migration test using Sn-Bi eutectic solder and using Sn30Bi solder were 82 and 20 %, respectively. In case of using Sn-Bi eutectic solder, needle-shaped Cu6Sn5 and (Cu, Ni)6Sn5 were formed at solder/ Ni pad interface after bonding. However, they transformed to (Ni, Cu)3Sn4 and thick Ni(P-rich) layer at solder/Ni pad interface after current stressing. Using Sn30Bi solder, only dense scallop-type (Cu, Ni)6Sn5 was formed at solder/ Ni pad interface after bonding. And they were stable during electro-migration test. Other IMCs did not grow at solder/ Ni pad interface.