Post-annealing of hybrid SAC305-SnBi solder joints formed with a 150 °C reflow to improve fatigue resistance

Abstract

A current approach to low-temperature assembly of microelectronics is to solder to SnAgCu bumped components with eutectic or hypoeutectic SnBi. However, while the resulting hybrid joints may be more reliable than pure SnBi, their fatigue resistance cannot compete with that of pure Sn3Ag0.5Cu (SAC305). Even if fatigue failure is still through the Bi-free region near the component, the presence of Bi elsewhere reduces the overall ductility of the joint, and this gets worse for peak temperatures below 175 ˚C. We show that the fatigue resistance of hybrid joints can be improved on by annealing them to distribute the Bi all the way to the component pad as long as concentrations there remain less than 6%. Annealing of conventional SnAgCu joints is known to reduce their fatigue resistance by coarsening the Ag₃Sn precipitates, and the same is found to be true for SAC305(Bi) joints. However, the fatigue resistance of the annealed SAC305(Bi) alloys is still found to remain greater than that of unannealed SAC305. Systematic characterization of deformation and damage properties shows that this must be true for any area array assembly under isothermal cycling conditions ranging from vibration to cyclic bending, and a forthcoming publication will show the same to be true in thermal cycling. Notably, the anneal eliminates effects of the reflow parameters on the microstructure, including the interdiffusion, allowing for peak temperatures as low as 150 °C. The only significant concern is that practical considerations limit the approach to relatively short joints. Assessments are offered of minimum annealing times required at 125 °C and 150 °C, respectively, for different SAC305 joint heights with corresponding optimized SnBi volumes.