MICRO- AND NANO-PRECISION TESTING ON LOW TEMPERATURE SOLDERS
thesisposted on 29.04.2021, 20:37 authored by Colin GreeneColin Greene
Presently, a critical requirement in electronic assemblies is the reliability of solder joints. Accurate characterization of the mechanical behavior of solder alloys is challenging due to their micro-scale size, microstructural complexity, and complex rate-dependent mechanical behavior. This research presents two mechanical testers designed to acquire accurate mechanical response of the solder alloys. The testers allow using micro-scale test samples that replicate real solder joints in size and soldering pad metallurgy.
The first mechanical tester presented in this research is the micro-precision tester. It is capable of monotonic, creep and fatigue test profiles at testing temperatures between 25 and 75◦C. Using a closed-loop control scheme and an external capacitance sensor to minimize measurement of the load train compliance, the tester is capable of precision on the order of 0.1 µm. For load controlled tests, the tester is capable of precision on the order of 0.5 N. The design and construction processes are presented, including rationale for major design choices. Additionally, the development of custom squat-joint samples for use in this tester is presented. These samples allow for increased data reliability while maintaining realistic dimensions. Both validation and test data are presented to demonstrate the capabilities of the micro-precision tester.
A second mechanical tester, the nano-precision tester, was developed to address the need for increased accuracy as solder geometries shrink. Again, the design choices and limitations are presented, with emphasis on improvements over the micro-precision tester. The load and displacement control are approximately and order of magnitude better than that of the micro-precision tester. Example tests are presented to demonstrate the accuracy and capabilities of the nano-precision tester.
Finally, the thesis concludes with recommendations on methods to further improve the two testers. Specifically, for the micro-precision tester, thermal expansion during high-temperature testing is a significant concern. For the nano-precision tester, both validation of the tester the capability of multi-temperature testing are future work.