Abstract
The Pb-Sn eutectic alloy is widely used as a joining material in the electronics industry. In this application the solder acts as both an electrical and mechanical connection within and among the different packaging levels in an electronic device. Advances in packaging technologies driven by the desire for miniaturization and increased circuit speed result in severe operating conditions for the solder joint and thus solder joint reliability problems. Specifically, the mismatched thermal expansion characteristics of the materials joined by the solder and the cyclic temperature fluctuations normally encountered during service constitute a condition of thermal fatigue for the constrained solder. This is especially a problem in the surface mounting of leadless components where a shearing of the solder joint occurs with each temperature excursion. Repeated temperature cycling, such as that associated with Joule heating as the device is turned on and off, and environmental temperature changes fatigue the solder joint ultimately causing its failure. Increased solder joint reliability and the development of new more fatigue-resistant solder alloys require a fundamental understanding of the metallurgical mechanisms operative during the fatigue cycle and, hence, an understanding of the solder microstructure.
Published Version
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