A comparative study of power cycling and thermal shock tests

Thermal shock test is a traditionally used test to simulate thermomechanical stresses experienced by electronic products during their operational life span. However, because of increasing power densities and modular design of modern consumer devices, the stresses induced by thermal shocks are becoming too different from those experienced in operational environment. Inherent deficiencies in thermal shock test include the omission of strong local thermal gradients and electrical current stressing. To address these issues, power cycling was investigated as an alternative test method for high-density electronic assemblies. The thermal shock and power cycling tests were compared both experimentally and by using finite element modeling. Computationally the stresses caused by thermal shocks are almost twice as large as those caused by power cycling and therefore the life expectancy was considerably shorter. However, experimental testing yielded opposite results, with power cycled specimen failing much faster than their thermally cycled counterparts. The primary failure mechanism in the power cycling test was the same as in the thermal shock test: recrystallization-assisted crack nucleation and propagation in bulk solder interconnections. Additionally, increased growth of intermetallic reaction layers was observed in the power cycling test specimen. The tests were executed with component boards manufactured with the traditional SnPb-solder and with near-eutectic SnAgCu-solder. The results show that in both tests the lead-free interconnections are at least as reliable as those produced with the traditional tin-lead soldering. While the primary failure mechanism was the same in both tests, power cycling introduced secondary mechanisms that considerably accelerated the failure of test vehicles' solder interconnections. These secondary mechanisms, especially electromigration, are expected to become more dominant in the near future. Therefore power cycling can in many cases be regarded as a more realistic and advanced alternative to the traditional thermal shock test.