Temperature cycle-induced die metal shearing in surface mount devices after infra red/vapour phase reflow

Moisture-induced “popcorn” effect in surface mount devices during infra red or vapour phase reflow manifests itself as visible cracks originating from the edges of the die or the leadframe mount pad. More robust packages often give rise to only microcracks after reflow which subsequently require further thermal stresses to induce device or package failure. This paper analyses the secondary effect of die metal shearing in SOJ packages subjected to temperature cycling after an infra red reflow. This phenomenon has significant bearing on the current practice of preconditioning packages using infra red or vapour phase reflow before subjecting them to reliability tests like temperature cycling. A model is developed to explain the relationship between package microcracks after reflow and subsequent metal shearing when subjected to temperature cycling. The metal shear lines are characterised using failure analysis techniques to validate the model. Various methods of improving the SOJ package resistance to microcracking during reflow are discussed