A model of crack nucleation in layered electronic assemblies under thermalcycling

A model for crack nucleation in layered electronic assemblies under thermal cycling is developed in this paper. The present model includes three scale s: (i) at the microscale or the mechanism level, the damage mechanism such as diffusive void growth or fatigue cracks, determine the damage growth rat e; (2) at an intermediate mesoscale, the localized damage bands are modeled as variable stiffness springs connecting undamaged materials; and (iii) at the macroscale or the continuum level, the localized damage band growing i n an otherwise undamaged material is modeled as an array of dislocations. T he three scales are then combined together to incorporate damage mechanisms into continuum analysis. Traditional fracture mechanics provides a crack p ropagation model based on pre-existing cracks. The present work provides an approach for predicting crack nucleation. The proposed model is then utili zed to investigate crack nucleations in three-layered electronic assemblies under thermal cycling. The damage is observed to accumulate rapidly in the weakest regions of the band. Estimates are obtained for critical time or c ritical number of cycles at which a macroscopic crack will nucleate in thes e assemblies under thermal cluster, but decreases rapidly as the local exce ss damage increases [S1043-7398(00)00503-X].