A MECHANISTICALLY JUSTIFIED MODEL FOR LIFE OF SnAgCu SOLDER JOINTS IN THERMAL CYCLING
Authors: P. Borgesen, L. Yang, A. Qasaimeh, L. Yin, and M. Anselm Company: Binghamton University and Universal Instruments Corporation Date Published: 1/22/2013
Pan Pacific Symposium
Abstract: We have shown the life of a SnAgCu solder joint in a typical BGA or CSP assembly in thermal cycling to scale with the time to completion of a network of high angle grain boundaries across the high strain region of the joint. This provides for a credible materials science based model. In-depth studies did however show this to require significant temperature variations. Isothermal cycling may also lead to recrystallization, albeit at a much lower level depending on alloy, processes, and cycling parameters, but a quantitative model would need to be completely different. The question therefore arises as to how large a cycling temperature range is required for our model to apply. We present results indicating that repeated cycling between 20oC and 60oC should be sufficient, i.e. the model should allow for extrapolation of accelerated test results to realistic service conditions. Many practical applications involve a combination of thermal excursions and mechanical cycling, and there is little doubt that thermal cycling induced recrystallization will tend to lead to much faster crack growth through the solder in subsequent vibration, etc. We discuss how this greatly complicates the definition of a conservative but still practical accelerated test protocol.
Reliability, thermal cycling, recrystallization, fatigue, service conditions, Pb-free solder.