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A study is made of the tendency of growth of delamination between dissimilar materials occurring in large scale integration (LSI) plastic packages under temperature cyclic loading. Two groups of delamination growth processes are considered; one along the interface between the top surface of the die pad and the die-bonding layer, and the other along the interface between the bottom surface of the die pad and the encapsulant resin. In each group several different initial patterns of delaminations are assumed. Stress intensity factors and their mode ratios at the tips of growing delaminations are calculated by combining a thermoelastic finite element method for nonlinear contact problems and a linear interface fracture mechanics approach. The effects of geometries of delamination and leadframe materials on the tendency of delamination growth are clarified.