CLOSED-FORM, STRAIN-ENERGY BASED ACCELERATION FACTORS FOR THERMAL CYCLING OF LEAD-FREE ASSEMBLIES
Authors: Jean-Paul Clech, Gregory Henshall and Jian Miremadi Company: EPSI Inc. and Hewlett-Packard Company Date Published: 10/4/2009
Abstract: A closed-form Acceleration Factor (AF) model has been developed for solder joint reliability under thermal cycling conditions. The proposed AF formulation relies on a strain-energy failure criterion and an approximation of stress/strain hysteresis loop areas. The AFs are functions of temperature swings, temperature extremes, and hot and cold dwell times. The AF formulation accounts explicitly for creep of solder during hot and cold dwell periods. As a result of solder creep / stress relaxation during dwell times, the AF model includes the two material constants of a power-law creep rate equation, namely, a stress exponent and a creep activation energy. The component / assembly dependence of AFs, which can vary by a factor of 2.36 times or more according to experimental data and FEA modeling, is also accounted for via a single component correction factor. The paper presents the model formulation, its application to SAC305, SAC387/396/405 and SnPb assemblies, and discusses the type of data that is needed to extend the model to other lead-free solders or mixed assemblies. The advantages and limitations of this physically-based, closed-form model compared to other approaches in the reliability engineer’s toolbox are also discussed.