Misorientations and Subgrains in Sn-Ag and Sn-Ag-Cu Solder Balls After Solidification

Abstract Subgrains and recrystallisation are common microstructural features in solder joints that have been subjected to thermal fatigue or mechanical loading. Here we study similar features in Sn-Ag and Sn-Ag-Cu solder balls after solidification. It is shown that four types of misorientation features exist to different extents in solder balls examined shortly after solidification: (i) small variations in orientation created by dendrite growth and eutectic solidification, (ii) partial polygonisation into subgrains, (iii) small grains with high angle boundaries and (iv) large anomalous grains surrounded by interlacing where the grain boundaries do not correlate with the dendrite growth pattern. The subgrains often have a boundary plane and rotation axis consistent with dislocations from a facile slip system. The recovery and misorientation features were more extensive in regions that solidified at deeper melt undercooling. The findings highlight the importance of distinguishing between the solidification and solid-state components of microstructure evolution when interpreting solder microstructures after solidification.