Superior reliability of SAC305-SnBi solder joints formed in a 150 °C reflow

Studies abound aiming to optimize hybrid joints formed by soldering eutectic SnBi to SnAgCu bumped components, but there is a general trade-off between the peak reflow temperature and the fatigue resistance. While soldering can be done at as low as 150 °C, the mixing of more Bi into the solid SnAgCu at higher temperatures improves the properties of the mixed region. Still, peak temperatures near 200 °C are required before the fatigue resistance approaches that of SAC305. Instead, we have shown that the fatigue life of assemblies with hybrid joints formed at 150 °C can be improved, to the extent that it exceeds that of conventionally reflowed SAC305 joints, by subsequent annealing to distribute the Bi throughout the joint. ‘All’ that is required is for the anneal to be long and hot enough so that a few percent of Bi from the mixed region reaches the component pad. The annealing coarsens the Ag₃Sn precipitates, allowing for an enhanced rate of damage at a given stress, but that is counteracted by the addition of as little as 1.5 wt.-% Bi. A previous publication documented a fatigue life superior to that of conventional SAC305 joints under any condition of vibration or cyclic bending of assemblies at room temperature. The present work shows that life in thermal cycling also to be superior. This is done by comparing the thermal fatigue resistances in a particular accelerated thermal cycling test and then generalizing the results based on a characterization of deformation and damage properties vs temperature.