Fatigue life prediction of solder joints is one of the most important areas of research in the development of reliable electronic packages. Recent trends in electronic package development indicate a shift toward smaller solder joints and larger package sizes, and temperature changes under field conditions are also becoming greater. Since reliability design of solder joints has become severer, the estimation of the crack propagation is becoming important like the estimation of the crack initiation. In the present study, a new method of estimating the crack propagation, which is based on finite element analysis without geometrical crack model, was examined, in order to ensure suitability for practical use in electronic package design. On the basis of a damage model assumed for Sn-37Pb solder, the new method called ‘damage path simulation’ was verified for solder joints in QFP (Quad Flat Package). In the case of solder joints of the gull-wing type, fatigue cracks are commonly initiated from the upper surface of the solder fillet, and propagated in the vicinity of the interface with the outer lead. It was clear that the extension of the damage path showed good agreement with the behavior of crack propagation observed in the actual thermal cycle tests. Damage path extension from a pointed end of outer lead is also simulated simultaneously with that from the upper surface of the solder fillet, and both damage paths were finally combined at a gap between outer lead and printed circuit board. The advantage of the present method is especially evident when the fatigue cracks were initiated from two or more regions. From the results of this study, it was concluded that the estimation of the crack propagation in solder joints based on the present method is satisfactory for engineering purposes.

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