Evolution of Microstructure and Mechanical Properties of Eutectic Sn-Pb Solder Joint Aged under Thermal Gradient

Thermal gradients exist in solder joints used in high power device packages due to high device temperature and low substrate temperature. This paper presents the effect of thermal gradient on the microstructure and mechanical properties of eutectic Sn-37Pb solder joint. The solder joint comprises a multilayered Cu/electroless Ni-P/Sn-37Pb/electroless Ni-P/Cu material system. Thermal gradients ranging from 1.3 times 10³ to 1.5 times 10³degC/cm were imposed on the solder joint by heating its one end and cooling the other. The thermal gradient in the solder joint has been found to influence the size distribution of Sn-rich and Pb-rich phases, solder composition, and fracture behavior. An explanation of these thermal gradient induced phenomena is provided.