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 were carried out on Pb-5Sn and Pb-63Sn alloy solders under thermal cycle conditions of -55 to 150°C/cycle/h. The thermal fatigue fracture of Pb-Sn solder joints develops when cracks, initially generated from large-scale plastic deformation of the solder, gradually propagate through the joint. On the fracture surface, striations of 500 A to 1 μm in size were 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 the Mises criterion), obtained by FEM, relates strongly to thermal fatigue life. A thermal fatigue life equation relating crack propagation to bonding size and thermal strain range is developed