Electromigration in reduced-height solder joints with Cu pillars

This paper investigates the current-enhanced surface diffusion of solder along the side walls of Cu pillars. Two solder joints, Cu pillar/10-µm-solder/Ni and Cu pillar/Ni/10-µm-solder/Ni, were subjected to a 1.2 × 104 A/cm2 current density at 125 °C for various amounts of time. The Sn atoms may diffuse to the Cu pillars side walls and form Cu6Sn5 intermetallic compounds (IMCs). Because the Sn atoms are drawn from the solder joints, serious voids are formed in these joints. Side diffusion is more serious for the Cu pillar/Ni/10-µm-solder/Ni joints than the Cu pillar/10-µm-solder/Ni joints because, instead of forming Ni3Sn4 IMCs, the Sn atoms prefer diffusing into the Cu side walls to form Cu6Sn5 IMCs. This Sn side diffusion accelerates the depletion of solder in the joints and thus may be a serious reliability issue for solder joints with a reduced solder height.