Creep strains in an elongated bond layer

Thermally induced shear stresses and strains in a bimaterial assembly are studied as functions of two variables, position and time. It is shown that the initial uneven distribution of shear stress will first relax such that stresses are distributed uniformly along the bond without change of total shear force; during this initial stress redistribution, a significant magnitude of creep may occur in the region of high stresses. Numerical procedures for calculating possible failure indicators, including peeling stresses and several kinds of creep strain, are developed. For some of these failure indicators, approximate formulas are presented. The results are applicable to any bimaterial assembly, but particular examples from electronic packaging applications are used. Detailed numerical results are presented for an example case of the 'lead-foot' structure.<<ETX>>