Effects of solder joint dimensions and assembly process on acceleration factors and life in thermal cycling of SnAgCu solder joints

Although far from always obvious to the practitioner the interpretation of accelerated thermal cycling results is often confounded by complex effects of the evolving solder microstructure. One example is the variation of not only life times but also acceleration factors with SnAgCu solder joint dimensions, contact pad finishes and assembly process parameters. This may be critical as most testing relies implicitly on the assumption that the material, design or process leading to the best performance in testing will also ensure the best performance in service. A unique `deconstructed thermal cycling' experiment has led to a quantitative understanding of the mechanisms controlling the life of area array assemblies with SnAgCu solder joints in thermal cycling. In the absence of precipitate coarsening the life can be approximated as determined by the accumulation of a certain amount of work during the high temperature dwells. This does, however, vary with precipitate spacing unless that and the rate of work are high, and cycling usually leads to significant coarsening of the secondary precipitates. Together with variations in the initial precipitate distributions this explains the abovementioned effects.