Evolution in Lead-Free Solder Alloys Subjected to Both Mechanical Cycling and Aging

Abstract

Abstract Solder joints in electronic packaging often experience failure due to cyclic thermo-mechanical loading. For example, electronic components in the automotive engine compartment or a giant turbine of a power plant may encounter both thermal cycling as well as mechanical cycling due to vibration. At the same time, electronic components can undergo thermal cycling due to frequent power switching, e.g., turning a car engine on and off. Thus, the solder materials used to attach electronic components are often subjected to cyclic stresses and strains due to temperature changes and the CTE mismatches of the assembly materials, as well as due to vibration and other time-varying loadings. In the literature, recent studies by our group have been carried out to examine the mechanical behavior and property evolutions (e.g., modulus, yield stress, ultimate tensile strength, and creep rate) occurring due to either isothermal aging or due to isothermal mechanical cycling. In the case of aging, microstructure changes are the primary reason for changes in the mechanical response. In the case of mechanical cycling, both microstructural evolution and damage accumulation (e.g. microcrack growth) lead to the observed changes. Currently, there are no prior investigations on the effects of both isothermal aging and mechanical cycling on the mechanical property and microstructural evolution of lead-free solder alloys. In this study, we have evaluated the changes in mechanical properties and microstructure that occur due to mechanical cycling followed by isothermal aging at high temperature. Uniaxial samples of SAC305 lead-free solder alloy were first prepared by solidification in glass tubes under a controlled reflow profile. The samples were first mechanically cycled under strain control for various durations (e.g., 0, 50, 100, 200, 300 cycles), and then subsequently aged for 20 days at T = 125 °C. All of the preconditioned specimens were then subjected to nanoindentation testing to evaluate mechanical properties. Results for the various tests were compared to characterize the effects of combined effects of aging and mechanical cycling on the deterioration of the elastic modulus, hardness, and creep properties relative to the results for pristine specimens. As expected, exposure to both cycling and aging greatly influenced the results, with the samples that were cycled 300 times followed by aging showing the greatest degradations. Optical microscopy was also used to evaluate the microstructural evolution which demonstrates microstructural coarsening occurred in the samples due to the combination of mechanical cycling aging.