Mechanisms for Sn whisker growth in rare earth-containing Pb-free solders

It has recently been documented that Pb-free solder alloys doped with trace amounts of rare earth (RE) elements show a very strong propensity to grow Sn whiskers. In this work, we have investigated the effect of the addition of 2 wt.% Ce, La or Y on the whiskering behavior of Sn-3.9Ag-0.7Cu. Hillock-type whiskers around particle peripheries were observed in water-quenched alloys with smaller RESn₃ particles, while furnace-cooled alloys with larger RESn₃ particles formed needle-like whiskers from within the particle. Phase separation between Sn and RE oxides occurred during oxidation of the RESn₃ intermetallics. A focused ion beam serial sectioning approach was used to visualize the Sn whiskers and the oxide structure. We show that the driving force for whisker growth is related to the compressive stresses that develop in these particles during the oxidation of the RE intermetallic phases.