The deformation behavior of Sn62Pb36Ag2 and its implications on the design of thermal cycling tests for electronic assemblies

The introduction of new packages as well as the ongoing miniaturization in SMT make the evaluation of the reliability of solder joints a permanent task. Since solder joints fail due to low cycle fatigue caused by cyclic thermomechanical stress passive thermal cycling is an important test to evaluate the lifetime of solder joints. Since a reliability prediction with the thermal cycle encountered in reality would take years to complete the tests, methods to accelerate the test cycle are to be used. However, due to the viscoplastical deformation behavior of tin-lead solder it is mandatory to take the metallurgical behavior of the solder into account when designing accelerated tests. Two different deformation mechanisms occur, depending on the temperatures of the test as well as the temperature gradient: grain boundary sliding (GBS) and dislocation climb (DC) each one having its own influence on the damage occurring in the solder. Therefore, one is not free in choosing the parameters of a test cycle. In this paper the deformation behavior of tin lead solder is explained, A constitutive equation extracted from experiments is presented, describing the deformation behavior of Sn62Pb36Ag2. Results of simulations of the deformation behavior of Sn62Pb36Ag2 on the base of the constitutive equation are shown. Suggestions for the design of accelerated lifetime tests for solder joints are given