Microstructural Indicators for Assessment of Effect of Prolonged and Intermittent Storage on Reliability of Lead-free Electronics

Electronic systems may be subjected to prolonged and intermittent periods of storage prior to deployment or usage. Prior studies have shown that the lead-free solder interconnects show measurable degradation in the mechanical properties even after the brief exposures to high temperature. In this paper, a method has been developed for determining the equivalent storage time to produce identical damage at a different temperature. Electronics subjected to accelerated tests often have a well-defined thermal profile for a specified period of time. Quantification of the thermal profile in field-deployed electronics may be often difficult because of the variance in the environment conditions and usage profile. There is a need for tools and techniques to quantify the damage in deployed systems in the absence of macroindicators of damage without the knowledge of prior stress history. The approach for mapping damage in the lead-free second-level interconnects between different thermal conditions is new. High-reliability applications, such as avionics and missile systems, may be often exposed to long periods of storage prior to deployment. The effect of storage at different temperature conditions can be mapped using the presented approach. A framework has been developed to investigate the system state and estimate the remaining useful life of the solder ball subjected to a variety of isothermal aging conditions, including 60 °C, 75 °C, and 125 °C for periods of time between 1 and 4 weeks. Data on damage precursors, including the rate of change in the normalized phase growth and the normalized IMC thickness, has been collected and analyzed to derive physics-based damage mapping relationships for aging. Mathematical relationships have been derived for the damage mapping to various thermal storage environments to facilitate determining an appropriate time-temperature combination to reach a particular level of damage state. Activation energy for the leading indicators of failure is also computed. Specific damage proxies examined include the phase-growth indicator and the intermetallic thickness. The viability of the approach has been demonstrated for the lead-free test assemblies subjected to multiple thermal aging at 60 °C, 75 °C, and 125 °C. Damage mapping relationships are derived from the data based on the two separate leading indicators.