Effects of thermal cycling on rare earth (Pr)-induced Sn whisker/hillock growth

The addition of rare earth (RE) into Pb-free solders has been shown to induce spontaneous Sn whisker growth on RE-rich intermetallics (IMCs). In this research, whisker growth behaviors under ambient and thermal cycling conditions were investigated to address the RE-induced Sn whisker growth mechanism. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition. It was discussed that the CTE mismatch between RE-rich PrSn3 IMCs and surrounding solder matrix induces compressive stress within the IMCs during thermal cycling, which contributes to the driving force for hillock growth.

► It was found that RE addition can induce Sn whisker/hillock growth in bulk Sn-rich solders.
► Sn whisker/hillock growth behaviors under ambient and thermal cycling conditions were investigated.
► Effect of CTE mismatch between RE-rich IMC and matrix on the driving force of whisker/hillock growth was discussed.