Predicting Fatigue of Solder Joints Subjected to High Number of Power Cycles

Solder joint reliability of SMT components connected to printed circuit boards is well documented. However, much of the testing and data is related to high-strain energy thermal cycling experiments relevant to product qualification testing (i.e., -55C to 125C). Relatively little information is available on low-strain, high-cycle fatigue behavior of solder joints, even though this is increasingly common in a number of applications due to energy savings sleep mode, high variation in bandwidth usage and computational requirements, and normal operational profiles in a number of power supply applications. In this paper, 2512 chip resistors were subjected to a high (>50,000) number of short duration (<10 min) power cycles. Environmental conditions and relevant material properties were documented and the information was inputted into a number of published solder joint fatigue models. The requirements of each model, its approach (crack growth or damage accumulation) and its relevance to high cycle fatigue are discussed. Predicted cycles to failure are compared to test results as well as warranty information from fielded product. Failure modes were confirmed through cross-sectioning. Results were used to evaluate if failures during accelerated reliability testing indicate a high risk of failures to units in the field. Potential design changes are evaluated to quantify the change in expected life of the solder joint. Solder Joint Fatigue Prediction Theory Degradation of solder joints due to differential expansion and contraction of joined materials has been a known issue in the electronics industry since the basic construction of modern electronic design was finalized in the late 1950’s / early 1960’s. Initial assessment of the behavior borrowed heavily from observation of structural materials in the early 1950’s, such as solder fatigue in automotive radiators and thermal fatigue of steels. The resulting Coffin-Manson relation states that the number of cycles to failure has a power law dependence on the magnitude of the plastic strain, or inelastic deformation, experienced during that specific thermal cycle.