Grain Formation and Intergrain Stresses in a Sn-Ag-Cu Solder Ball

The thermo-mechanical behavior of near eutectic lead-free SnAgCu (SAC) solder joints under Deep Thermal Cycling (DTC) and in-situ thermal loading was examined. Crossed polarizer, optical microscopy revealed that in ball grid array (BGA) solder joints, these Sn rich, Pb-free solders exhibit large grained Sn structures. After imaging, these SnAgCu solder joints were subjected to repeated thermal stresses under an inert atmosphere. Subsequent to this thermal loading, the samples were again examined with optical microscopy. Using both data sets, the intergrain strains and deformations were quantified by Digital image correlation, a full field optical measurement technique. The relations between the positions of grains as well as intermetallics compounds, their boundaries and Sn deformation fields were examined.