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Effects of Thermal Aging on the Reliability of Lead Free Solder Joints
Keywords: Lead Free, Solder, Reliability
The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. The observed material behavior variation during thermal aging/cycling is universally detrimental to reliability and includes reductions in stiffness, yield stress, ultimate strength, and strain to failure, as well as highly accelerated creep. The evolution in the creep response with material aging illustrates large increases in the steady state (secondary) creep strain rate (creep compliance) and a tendency to enter the tertiary creep range (imminent failure) at ever reducing strain levels. Such effects for solder materials are extremely dramatic at the higher aging temperatures (e.g. 100-150 C) typical of the harsh environments present in automotive, aerospace, and defense applications. However, significant aging behavior is even present in lead free solder joints that remain at room temperature (25 C). In addition, the magnitudes of the material behavior evolution occurring in lead free SAC solder joints are much larger (e.g. 25X) than the corresponding changes occurring in traditional Sn-Pb assemblies. Finally, current finite element models for solder joint reliability are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena. In this work, we have explored the effects of isothermal aging on the mechanical behavior and reliability of lead free solder joints and assemblies, and then correlated the observed results. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on 95.5Sn-4.0Ag-0.5Cu (SAC405), 96.5Sn-3.0Ag-0.5Cu (SAC305), and 95.5Sn-3.8Ag-0.7Cu (SAC387) samples that were aged for various durations (0-1500 hours) at room temperature (25 C), and three elevated temperatures (100, 125, and 150 C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time.
Jeffrey C. Suhling, Professor
Auburn University
Auburn, AL

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