A unified creep plasticity damage (UCPD) model for eutectic Sn-Pb and Pb-free solders was developed and implemented into finite element analysis codes. The new model will be described along with the relationship between the model's damage evolution equation and an empirical Coffin-Manson relationship for solder fatigue. Next, developments needed to model crack initiation and growth in solder joints will be described. Finally, experimentally observed cracks in typical solder joints subjected to thermal mechanical fatigue are compared with model predictions. Finite element based modeling is particularly suited for predicting solder joint fatigue of advanced electronics packaging, e.g. package-on-package (PoP), because it allows for evaluation of a variety of package materials and geometries.
Decisions on material selections for electronics packaging can be quite complicated by the need to balance the criteria to withstand severe impacts yet survive deep thermal cycles intact. Many times, material choices are based on historical precedence perhaps ignorant of whether those initial choices were carefully investigated or whether the requirements on the new component match those of previous units. The goal of this program focuses on developing both increased intuition for generic packaging guidelines and computational methodologies for optimizing packaging in specific components. Initial efforts centered on characterization of classes of materials common to packaging strategies and computational analyses of stresses generated during thermal cycling to identify strengths and weaknesses of various material choices. Future studies will analyze the same example problems incorporating the effects of curing stresses as needed and analyzing dynamic loadings to compare trends with the quasi-static conclusions.
A unified creep plasticity damage (UCPD) model for Sn-Pb solder is developed in this paper. Stephens and Frear (1999) studied the creep behavior of near-eutectic 60Sn-40Pb solder subjected to low strain rates and found that the inelastic (creep and plastic) strain rate could be accurately described using a hyperbolic Sine function of the applied effective stress. A recently developed high-rate servo-hydraulic method was employed to characterize the temperature and strain-rate dependent stress-strain behavior of eutectic Sn-Pb solder over a wide range of strain rates (10{sup -4} to 10{sup 2} per second). The steady state inelastic strain rate data from these latest experiments were also accurately captured by the hyperbolic Sine equation developed by Stephens and Frear. Thus, this equation was used as the basis for the UCPD model for Sn-Pb solder developed in this paper. Stephens, J.J., and Frear, D.R., Metallurgical and Materials Transactions A, Volume 30A, pp. 1301-1313, May 1999.
Legislated requirements and industry standards are replacing eutectic lead-tin (Pb-Sn) solders with lead-free (Pb-free) solders in future component designs and in replacements and retrofits. Since Pb-free solders have not yet seen service for long periods, their long-term behavior is poorly characterized. Because understanding the reliability of Pb-free solders is critical to supporting the next generation of circuit board designs, it is imperative that we develop, validate and exercise a solder lifetime model that can capture the thermomechanical response of Pb-free solder joints in stockpile components. To this end, an ASC Level 2 milestone was identified for fiscal year 2010: Milestone 3605: Utilize experimentally validated constitutive model for lead-free solder to simulate aging and reliability of solder joints in stockpile components. This report documents the completion of this milestone, including evidence that the milestone completion criteria were met and a summary of the milestone Program Review.