Based on a new second-order analysis of the bimaterial interface crack, a modified crack surface displacement extrapolation method (MCSDEM) for evaluating the stress intensity factor (SIF) and mode mixity at a crack has been developed. Unlike existing methods which require extrapolation of values of SIFs with the concomitant uncertainties, the new method engenders a constant SIF over a considerable distance from the crack tip. The new method is also much less sensitive to mesh density than existing methods. The new MCSDEM is then used to investigate the effects of temperature, moisture diffusion and interface vapor pressure on the delamination of the interface between the leadframe pad and the encapsulant of a plastic quad flat pack (PQFP) during solder reflow for both eutectic solder and leadfree solder. Using a 160-leaded PQFP as a test vehicle, the effect of initial delamination size varying from 0.1 mm to 3.5 mm was studied. The results show that the relative contributions of temperature, moisture and interface vapor pressure is qualitatively different for solder reflow of leadfree soldered and eutectic soldered components.

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