Isothermal Aging Effects on the Harsh Environment Performance of Lead-Free Solder Joints
Authors: Jiawei Zhang, Zhou Hai, Sivasubramanian Thirugnanasambandam, John L. Evans, M. J. Bozack, and Richard Sesek Company: Center for Advanced Vehicle and Extreme Environment Electronics (CAVE3) Auburn University Date Published: 10/14/2012
Abstract: This paper presents the latest results from an on-going study of Pb-free solder joint under harsh environment. Electronics assemblies containing solder joints are often exposed to elevated temperatures for prolonged periods of time. The time-at-temperature stress impacts the overall package reliability of the assembled circuitry due to evolving materials, microstructural, and mechanical properties. It is especially important to understand the impact of isothermal aging on the long term behavior of lead (Pb)-free solder joints which operate in harsh environments. In this study, we have explored the effects of elevated temperature isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies on board level packages. As the isothermal aging temperature increases, the Weibull characteristic lifetime for SAC 105 and 305 solder joints is drastically reduced compared to Sn-37Pb.In parallel mechanical studies on bulk solder specimens, the creep rate for SAC105, 305 rapidly increases with aging. A full test matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch µBGAs. The test structures were built on three different board finishes (ImSn, ImAg and SnPb). Storage condition temperatures were 25C, 55C, 85C and 125C with aging over time periods of 0, 12 months. Subsequently, the specimens were thermally cycled from -40C to 125C with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 55-60% after aging at elevated temperature.