Electronic packages are frequently exposed to thermal cycling during their service life between low to high temperature extreme.&nbsp; Similar phenomena can be observed in solder joints during the characterization of thermal-mechanical fatigue behavior.&nbsp; This variation in temperature causes the evolution of mechanical and microstructural behavior of solder joints.&nbsp; In our prior study, degradation of mechanical properties of SAC305 lead-free solder material under different thermal cycling exposures have been reported for up to 5 days of thermal cycling.&nbsp; In the current investigation, our prior study has been extended to examine several additional thermal profiles (slow thermal cycling, thermal shock, thermal ramping, and thermal aging).&nbsp; Finally, longer exposure times up to 50 days have been considered in this new work. &nbsp;Uniaxial miniature bulk specimens and BGA solder joints were prepared for both solder alloys.&nbsp; A controlled reflow profile was used to prepare both types of solder samples.&nbsp; After preparation, the samples were then preconditioned by exposure to thermal cycling from T = -40 to +125 oC or thermal aging at T = 125 oC for various durations up to 50 days.&nbsp; The cycling was performed inside an environmental chamber under no load condition (stress-free).&nbsp; Several thermal cycling profiles were examined including: (1) 150 minute cycles with 45 minutes ramps and 30 minutes dwells (slow thermal cycling), (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps (thermal shock), and (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramping).&nbsp; The results for the three cycling profiles were compared to those for pure isothermal aging at the high temperature extreme (aging).After preconditioning via thermal exposure, the samples were tested to characterize their mechanical behavior and microstructure evolutions.&nbsp; The uniaxial bulk specimens were subjected to stress-strain testing to measure their mechanical properties including effective elastic modulus, yield stress, and Ultimate Tensile Strength (UTS).&nbsp; The mechanical behavior of the BGA solder joint samples was studied using nanoindentation.&nbsp; Single grain solder joints were tested since they had large regions of solder material with equivalent mechanical behavior, which could then be indented several times after various durations of thermal cycling/aging.&nbsp; Using the nanoindentation technique, the mechanical properties (modulus and yield stress) were extracted.&nbsp; For both the miniature bulk and solder joint samples, the evolutions of the mechanical properties for each thermal profile were characterized as a function of the time the samples were preconditioned before mechanical testing.&nbsp; Several comparisons were then made including: (1) comparing the observed evolutions for SAC305 mechanical properties for each thermal cycling/aging profile, and (2) comparing the relative severity of degradations occurring for the four different thermal profiles.

Evolution of Mechanical Properties and Microstructure in SAC Bulk Solder and Solder Joints During Thermal Cycling Exposures

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2021/volume/85505

ASME 2021 International Electronic Packaging Technical Conference and Exhibition (InterPACK2021)

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