Creep strains in an elongated bond layer

Thermally induced shear stresses and strains in a bimaterial assembly are studied as functions of position and time. It is shown that the initial uneven distribution of shear stress will first relax so that the stresses are distributed uniformly along the bond without a change in the total shear force. During the time of this initial stress redistribution, a significant magnitude of creep may occur in the region of high stresses. Numerical procedures to calculate possible failure indicators, including peeling stress and several kinds of creep strain, are developed. Approximate formulas are presented for some of these failure indicators. While the results are applicable to any bimaterial assembly, examples from electronic packaging applications are used