Evaluation of Thermal Stress Failure of Square Type SMD in Mounting Process on Metal Base PWB.

Abstract

Thermal stress failure of square type surface mount device (SMD) such as ceramic capacitor has been evaluated in surface mounting process on metal base printed wiring board (PWB). For the evaluation, a series of transient thermal stress analyses with FEM model and thermal stress failure experiments for the surface mount assembly were carried out with complementary relation. As a result, thermal stress failure probability of the capacitor is estimated by utilizing the distribution function of three-point bending strength and the maximum value of major principal stress in the vicinity of failure initiation point. Additionally, influence of deformation properties of solder and insulation layer, which is formed on the surface of the PWB, on the failure probability is also evaluated quantitatively by introducing two parameters Etg and σys. Etg is related to glass transition temperature and Young's modulus of the layer and σys indicates yield stress of the solder at ambient temperature. Consequently, the present evaluation has led us to appropriate choice of the solder and the layer in order to prevent the capacitor from the thermal stress failure in the surface mounting process of square type SMD on metal base PWB.