NDLI: Evolution of Lead Free Solder Material Behavior Under Elevated Temperature Aging

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Ma, Hongtao Jeffrey, C. Suhling Zhang, Yifei Lall, Pradeep Michael, J. Bozack
Copyright Year	2007
Abstract	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. In our prior work on aging effects (Ma, et al., ECTC 2006), we demonstrated that the observed material behavior variations of SAC405 and SAC305 lead free solders during room temperature aging (25 °C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are much more dramatic at the higher aging temperatures (e.g. 100–150 °C) typical of the harsh environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing 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 the current work, we have explored the effects of elevated temperature isothermal aging on the mechanical behavior and reliability of lead free solders. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on SAC405 and SAC305 samples that were aged for various durations (0–6 months) at several elevated temperatures (80, 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 and temperature. In this paper, we have concentrated our efforts on presenting the results for samples aged at 125 °C. In addition, the new elevated temperature aging data were correlated with our room temperature results from last year’s investigation. The results obtained in this work have demonstrated the significant effects of elevated temperature exposure on solder joints. As expected, the mechanical properties evolved at a higher rate and experienced larger changes during elevated temperature aging (compared to room temperature aging). After approximately 200 hours of aging, the lead free solder joint material properties were observed to degrade at a nearly constant rate. We have developed a mathematical model to predict the variation of the properties with aging time and aging temperature. Our data for the evolution of the creep response of solders with elevated temperature aging show that the creep behavior of lead free and tin-lead solders experience a “crossover point” where lead free solders begin to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. Such an effect is not observed for solder joints aged at room temperature, where SAC alloys always creep at lower rates than Sn-Pb solder.
Sponsorship	Electronic and Photonic Packaging Division
Starting Page	563
Ending Page	579
Page Count	17
File Format	PDF
ISBN	0791842770
DOI	10.1115/IPACK2007-33545
e-ISBN	0791838013
Volume Number	ASME 2007 InterPACK Conference, Volume 1
Conference Proceedings	ASME 2007 InterPACK Conference collocated with the ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference
Language	English
Publisher Date	2007-07-08
Publisher Place	Vancouver, British Columbia, Canada
Access Restriction	Subscribed
Subject Keyword	Solder joints Elastic moduli Temperature Solders Mechanical behavior Creep Alloys Accelerated life testing Mechanical properties Shear strength Stress Aerospace industry Tensile strength Lead-free solders Errors Defense industry Finite element model Materials properties Tin Reliability Shear (mechanics) Yield stress Failure
Content Type	Text
Resource Type	Article

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