Authored By:
Pradeep Lall, Vishal Mehta, Mrinmoy Saha
Auburn University
NSF-CAVE3 Electronics Research Center
Auburn, AL 36849
Summary
The use of constitutive models is essential in ensuring the reliability and safety of high-reliability electronics systems, particularly those designed for use in harsh environments. These systems can experience extreme temperatures, ranging from -65°C to +200°C, that can cause significant out-of-plane deformation of board assemblies when exposed to shock, vibration, or transient loads. In applications such as automotive and aerospace, electronics may require prolonged high-temperature exposure in the range of 150-200°C and random exposure to mechanical shock and vibration. Additionally, electronics may be stored for extended periods in non-climate-controlled enclosures before deployment. To address the negative effects of sustained high-temperature exposure, the industry has experimented with the addition of dopants to SAC solders.
This study examines various doped alloys, including SAC-Q, SAC-R, M758, and SAC305 solder, in a strain-rate range of 1-100 per second in the temperature range of -65°C to +200°C after one year of sustained exposure to 50°C and 100°C. The study also involves the use of dopants such as Nickel and Bismuth. The Anand parameters have been computed and used to quantify the accuracy of the Anand model by comparing the experimentally measured data with the predicted data with the computed model constants. Moreover, the Anand parameters have been integrated into an FE framework to simulate drop events for a ball-grid array package on a printed circuit board assembly, aiming to determine the hysteresis loop and plastic work density. The study also examines the evolution in hysteresis loops and PWD.
Conclusions
This research delves into the effects of low working temperatures and high strain rates on mechanical parameters, including ultimate tensile strength (UTS) and elastic modulus (E), in un-doped and doped solders. Samples of unaged and isothermally aged solder were tested using high strain rate tensile tests across a range of temperatures (-65 to 200°C). The results showed that the elastic modulus and UTS of all SAC solders are dependent on strain rate and increase as strain rate increases. Interestingly, all SAC solders exhibit increased mechanical characteristics at -65°C. The study also examined the impact of thermal aging on various solder alloys at low working temperatures.
The results showed that isothermal aging has less of an impact on the material characteristics of certain solders compared to others. In addition, the study found that the impact of operating temperature is greater than that of thermal aging for all the solder alloys examined. To predict stress-strain curves, the study derived 9 Anand parameters from the stress-strain data and fitted the experimental material data to the Anand viscoplasticity constants using the non-linear least squares technique. The stress-strain curves from Anand constants had a good correlation with the experimental data.
The study also used ANSYS™ software to evaluate a drop event based on FEA. The results showed that the solder junction and copper pad experienced the greatest amount of stress during the drop event. Plastic work in the solder joints was evaluated using an FEA-based drop simulation at 1500g for extremely high ambient temperatures. The study also determined the evolution of the hysteresis loop and plastic work density due to thermal aging.
Initially Published in the SMTA Proceedings
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