The analysis of creep data for solder alloys.
Soldering and Surface Mount Technology, 15(1) pp. 26–30.
With the continued miniaturisation of electronics equipment, a more detailed examination of the mechanical behaviour of solders is required to ensure reliability in performance. The paper reviews various aspects of the interpretation of the creep response of lead-containing and lead-free alloys. It demonstrates the necessity of acquiring stress-rupture data over as long a period as possible to avoid non-conservative extrapolation. For example, at 75°C, the transition in slope of the applied stress vs time to rupture plot occurs after about 1,000?h for Sn-37Pb, although for the lead-free alloys examined no such transition is observed within this timescale. Sometimes, deformation may be a more appropriate failure criterion than rupture, and it is shown that for Sn-37Pb this may result in substantially shorter failure times than utilising a rupture criterion. The quality of life estimation methods then depends upon the extent of this stage when the creep rate is a minimum. The Y factor (tm?:?tr) where tm is the time spent in steady state or within 10 per cent of the minimum creep rate, for all the solders examined at 75°C generally falls into the 20-30 per cent range. Estimations of creep life may substantially under predict because of this.
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