THERMAL FATIGUE LIFE OF PB-SN ALLOY INTERCONNECTIONS

A highly accurate estimation of Pb-Sn alloy solder thermal fatigue life is made for surface mount solder joints. Experiments are carried out on Pb-5Sn and Pb-63Sn alloy solders under thermal cycle conditions of -55 to 150-degrees-C/cycle/hr. The thermal fatigue fracture of Pb-Sn solder joints develops when cracks, initially generated from large scale plastic deformation of the solder, propagate gradually through the joint. On the fracture surface, striations of 500 angstrom to 1-mu-m in size are observed and crack propagation rates (da/dN) are obtained for these solders. Additionally, thermal stress and strain in solder joints is studied by the finite element method (FEM) using three-dimensional and thermoelastoplastic models. The equivalent strain range based on Mises criterion, obtained by FEM, relates strongly to thermal fatigue life. A thermal fatigue life equation considering crack propagation related to bonding size and thermal strain range, is developed.