Fatigue crack growth behavior of Sn–Pb and Sn-based lead-free solders

Solder alloys of lead-rich composition have been commonly used as joining materials in electronic package. However, because of environmental concerns, lead-free solders will replace lead-rich solders more and more in the future. The fatigue characteristics of the solders used are most important in assessing the reliability of joints in electronic packaging. In the present study, the fatigue crack growth (FCG) behavior of a wide variety of solders of both lead-rich and lead-free types has been investigated under a range of mean stresses and frequencies. Both time dependent and time independent (cyclic dependent) behaviors were observed. In the cyclic dependent crack growth regime, the FCG rates could be expressed as a function of either ΔKeff or ΔJ. Further, the lead-free solders were found to have a higher resistance to FCG than did the lead-rich solders. In the time dependent crack growth regime, the FCG rates were found to be a function of C∗. The point of transition between time dependent and time independent behavior was found to depend on the homologous temperature and strength of the alloys.