A primary concern of any reliability engineer is whether failure in accelerated testing may be caused by a different damage mechanism than the one that determines life in service. The recent suggestion that thermal expansion mismatch induced fatigue and failure of lead free solder joints under realistic service conditions may progress without the recrystallization seen to play a major role in accelerated thermal cycling thus requires systematic investigation. A total different concern is the ever recurring question of to which extent thermal cycling can be replaced by much faster isothermal cycling tests. The observation that the damage mechanisms tend to be completely different, one leading to transgranular cracking the other to intergranular cracking of lead free BGA solder joints, would seem to invalidate such ideas completely. Importantly, most practical service scenarios involve the combination of more than one loading conditions. This may be as simple as thermal excursions to different temperatures or vibrations with two different frequencies or amplitudes, but it may also involve a combination of thermal excursions and vibration, for example. Reliability prediction almost invariably relies on the explicit or implicit assumption of some principle of damage accumulation, most commonly Miner’s rule. These assumptions do, however, break down much more often than commonly recognized. One important reason for this, albeit clearly not the only one, would be the different effects of recrystallization. All of these concerns prompted an ongoing investigation to identify the parameters determining whether recrystallization will affect lead free solder fatigue life and what would be the practical consequences.

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