Grain-boundary character and grain growth in bulk tin and bulk lead-free solder alloys

Grain-boundary deformation is the primary failure mode observed in solder joints. Understanding the effects of alloy composition variations and cooling rates on microstructural stability and deformation processes will allow development of improved joints. The effects of these variables on grain-boundary character were investigated in a pure-tin ingot and a reflowed sample; ingots of Sn-3.5wt.%Ag and Sn-3.8wt.%Ag-0.7wt.%Cu; and solder balls with 1.63-wt.% or 3-wt.% Ag. The microstructure was characterized using orientation imaging microscopy (OIM). After aging (150degreesC for 200 h), the fine-grained polycrystalline microstructure in both pure-tin specimens grew considerably, revealing preferred misorientations and ledge formation at grain boundaries. Aging of the alloy ingots showed only slight grain growth caused by precipitate pinning. The solder balls showed similar phenomena. The role of alloying elements, cooling rate, and the anisotropy of the coefficient of thermal expansion (CTE) in tin on microstructural evolution, grain-boundary character, and properties of solder joints are discussed.