Abstract: In spite of many beneficial effects obtained from the doping of rare earth(RE) to lead-free solders,it is newly found that an exceeded addition of RE would cause a risk of Sn whisker growth in the alloys.The results indicate that with the addition of 0.7% Pr to Sn-9Zn-0.5Ga solder,many Sn whiskers spontaneously grow from the surface of Sn-Pr phases in the bulk solder exposed for 12 hours only.And the whiskers show a continuous growth with the exposure time being increased.The final longest length of the whisker can reach 100 μm,which can lead to a serious reliability problem for electronic assemblies.Finally,the mechanical reason for whisker growth is discussed.