Microstructure evolution in Sn–Bi and Sn–Bi–Cu solder joints under thermal aging

Binary eutectic Sn–Bi and ternary Sn–Bi–Cu alloys were fabricated and soldered to conductor metallized substrates to study the interfacial morphology and microstructural evolution of the joints under thermal aging. Different types of intermetallic compound (IMC), such as Cu6Sn5, (Cu, Ni)6Sn5 and Ag3Sn, are observed among various metallized substrates including Cu/FR4, Ni/Cu/FR4, Cu/Al2O3 and Pt–Ag/Al2O3. Three major effects of aging on the microstructure of the solder joints are observed. First, the coarsening rate of the Bi-rich phase in the solder joint under thermal aging at 120°C is reduced by adding 1 wt.% Cu to the 42Sn–58Bi solder. Second, the Ag3Sn IMC forms planar layer at the solder/PtAg interface, while the Cu6Sn5 intermetallic grows as scallop-like grains into the solder at the Cu/solder interface. Third, the growth of Ag3Sn intermetallic in the SnBi–1Cu/PtAg solder joint is slower than that in the 42Sn–58Bi/PtAg joint after thermal aging at 120°C.