Influence of Surface Finishes and Solder Alloys on Solder Ball Joint Reliability

ABSTRACT The influence of surface finishes and solder alloys on the solder ball joint reliability after multiple reflow cycles and thermal aging were investigated by conducting a high-speed solder ball shear test. In the case of SAC305, It was found that the electroless Ni/Pd/Au provided excellent reliability comparable to the conventional electrolytic Ni/Au after multiple reflow cycles and thermal aging. In the case of the combination involving Sn-37Pb with electroless Ni/Pd/Au, electrolytic Ni/Au, and OSP, the excellent joint reliability was observed after multiple reflow cycles. However, the brittle fracture increased after thermal aging, regardless of the type of the surface finish. The causes of poor joint reliability of Sn-37Pb after thermal aging, and that of the excellent joint reliability of SAC305 were also investigated. In the case of Sn-37Pb, there was no change in microstructure after multiple reflow cycles, but the coarsening and separation of Sn and Pb particles by the recrystallization were observed after thermal aging. The brittle fracture mode of samples observed after 1000 h thermal aging was changed to ductile mode in subsequent reflow. In response to this, the microstructure became fine as before thermal aging. The shear strength and elastic modulus increased by thermal aging. On the contrary, in the case of SAC305, there was no change in the microstructure and the shear strength decreased after thermal aging. After thermal aging, a smooth phase of the intermetallic compounds (IMCs) was formed at the solder joint interface. The IMC layers in Sn-37Pb were thicker than those in SAC305. It was concluded that the thick IMC layers and hardening of the solder might be the root causes of poor solder joint reliability of Sn-37Pb, and the thin IMC layers and softening of the solder might be the root causes of excellent solder joint reliability of SAC305 after thermal aging.