A Pseudo-Stress, Pseudo-Strain Methodology To Predict Lead-Free Solder Joint Reliability
Author: Jean-Paul Clech Company: EPSI Inc. Date Published: 10/13/2013
Abstract: This paper introduces the concept of solder joint pseudostresses and strains that enable the analysis of solder joint life data, and reliability predictions, when test vehicle parameters are not readily available. The latter are accounted for by means of an “inverse problem” solution, determining the “cause” (a stress-driving test vehicle parameter) from the “effects” (cycles to failure). The creep analysis part of the model is validated against real stress/strain data from a Sn-Ag-Cu (SAC) Ball Grid Array (BGA) assembly by Shirazi, 2011. Pseudo-strain energy obtained from the area of pseudo-stress/strain loops for any shape of a thermal profile is correlated to cycles to failure for several lead-free solders. Creep and fatigue constants for SAC assemblies are found to follow linear relationships versus the Ag content in the range 1 to 4% Ag (in weight). These trends open up a path for the development of life prediction models for mixed SAC assemblies.