Effect of cooling rate on interfacial fatigue-crack growth in Sn-Pb solder joints

The effect of the cooling rate after the reflow on the behavior of fatigue crack growth along Cu/Sn-Pb interfaces was examined in solder joints. Flexural peel specimens made from eutectic Sn-Pb solder alloy and Cu were cooled down from reflowing to room temperature using furnace cooling, air cooling, and water quenching. Kinetics of fatigue crack growth along the solder/Cu interface were measured from the flexural peel specimens as a function of strain energy release rate. The effect of cooling rate was found to depend on the level of strain energy release rate. Increasing the cooling rate from furnace cooling to water-quenching enhanced the fatigue crack growth resistance by up to more than 50% at low strain energy release rates, but reduced the fatigue crack growth resistance by up to 100% at high strain energy release rates. The enhanced fatigue crack growth resistance at the low strain energy release rates is shown to result from roughening of the interface with increasing cooling rate, while the reduction in the fatigue crack growth resistance at very high strain energy release rates followed the change in crack growth mechanisms from cohesive to interfacial.