Crack initiation and growth in WLCSP solder joints

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Abstract
Solder fatigue crack initiation and growth was studied in WLCSP assemblies. Samples were surface mounted to test boards, then extracted at regular intervals of thermal cycling. Crack lengths in the corner solder joints were measured using dye-and-pry technique. Four temperature cycle conditions and six solder ball alloys were evaluated. For all alloys and conditions, the cracks initiated near the component pad interface and propagated down into the solder bulk somewhat. Sn0.7Cu had some propensity toward cracks that propagated at approximately 45 degrees from the pad. 63Sn37Pb and Sn0.7Cu had much higher crack growth rates on the outboard side of the joints compared to the inboard side. SAC405, SAC305, SAC125Ni, and Sn3.5Ag had more similar growth rates on outboard and inboard sides of the joints. These differences are believed to be due to the temperature dependence of the creep deformation and damage accumulation in each alloy. 63Sn37Pb, SAC125Ni, and Sn0.7Cu showed a greater number of cycles to crack initiation compared to the other alloys. This trend is likely due to a lower creep resistance, which resulted in reduced tensile stresses at the edge of the solder joints. Sn0.7Cu and SAC405 had the lowest crack growth rates under most of the temperature cycle conditions. The estimated cycles to failure based on the present crack initiation and growth data was compared to previous data from like assemblies that were electrically monitored. It was found that the average crack growth data from individual joints over-estimates the mean fatigue life of components. However, the average + 3 a crack growth data provides a good estimate of first failure fatigue life from components. Statistical arguments are proposed to explain these results.

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