Fatigue crack growth behavior of 96.5Sn–3.5Ag lead-free solder

The fatigue crack growth tests of 96.5Sn–3.5Ag solder have been carried out at frequencies from 0.1 to 10 Hz and at stress ratios from 0.1 to 0.7. Fatigue cracks propagated in a transgranular manner for the specimens tested under low stress ratios and high frequencies (R=0.1 and 0.3 at 10 Hz, R=0.1 at 1 Hz), where the fatigue crack growth behavior was dominantly cyclic dependent. Intergranular crack growth was observed for the specimens tested at high stress ratios (R≥0.5) and low frequencies (f=0.1 Hz), where the fatigue crack growth behavior was dominantly time-dependent. Irregular eutectic structure of Sn and plate-like Ag3Sn intermetallics was formed in the material. Plate-like Ag3Sn intermetallics behaved as a strengthening phase and improved the transgranular crack growth resistance in the cyclic dependent regime, while it had no significant influence on the intergranular crack growth resistance in the time-dependent regime. The formation of small grains in the Sn matrix was observed during the fatigue crack growth tests, which might result from polygonization in the Sn matrix.