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Quantitative Assessment of Effects of Long Term Aging on Sn-Ag-Cu Solder Joint Reliability
Keywords: Isothermal Aging , Acceleration , Lead free solder reliability
The assessment of the reliability of an ‘old' solder joint by accelerated testing is a non-trivial challenge. The microstructures of common lead free solder joints are so fundamentally different from those of SnPb that any experience we have with respect to aging of the latter is not relevant to the former. Lead free solder properties have been shown to change significantly with time up to more than a year at temperatures ranging from room temperature to 125oC [1]. Needless to say changes occurred faster at higher temperatures, but so far we are not aware of anyone reporting practical acceleration factors for use in preconditioning for lead-free reliability assessment. 30 mil diameter solder joins of three common lead free solder alloys on Cu pads were considered. The state of a solder joint at any given time was characterized in terms of three different room temperature properties, shear strength, shear fatigue resistance, and micro hardness. These properties were measured before and after aging for different lengths of time at different temperatures. Isothermal aging was conducted for up to 5,000 hours at temperatures of 70oC, 100oC, and 125oC respectively. As expected, the resulting room temperature properties all decreased with aging time, and faster so for higher aging temperatures. Some of the acceleration factors extracted for the evolution of the individual properties did, however, differ greatly for a given alloy, i.e. old solder joints cannot simply be simulated by accelerated aging before testing. Different mechanical properties exhibit different trends in acceleration with changes in Ag content in the alloy, but indications are that all the properties are affected by both the Ag3Sn and the Cu6Sn5 precipitates. [1] Y. Zhang, Z. Cai, J. C. Suhling, P. Lall, and M. J. Bozack, “The Effects of Aging Temperature on SAC Solder Joint Material Behavior and Reliability”, Proc. ECTC 2008, pp. 99-112.
Vikram Venkatadri, Research Project Associate
State University of New York at Binghamton
Wilmington, MA

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