Abstract:

Predicting life of SAC305 solder joints in service is difficult as many Sn-based solder materials may undergo microstructural evolution depending on the loading conditions. Aging of solder joint at even room temperature can change the microstructure of the solder material, affecting its properties and fatigue durability of solder joints. SAC solder joints are well known to contain few grains (oligocrystalline). Due to this, accurate estimation of thermo-mechanical properties of single SAC crystal and grain boundaries is required, to model grain-scale solder joint response under life-cycle loading. Multitier microstructure-based crystal-viscoplasticity (CV) modelling of oligocrystalline SAC solder joint, based on fundamental dislocation mechanics, developed by this research group, is used for predictive modeling. During isothermal aging of solder joints, both the pro-eutectic β-Sn dendrites as well as the eutectic regions undergo thermal evolution. In addition, under the presence of cyclic mechanical stresses at temperature, grain and grain boundaries may also continuously evolve. In microstructure-sensitive modeling, it is important to determine the contribution of each of these microstructural features so that accurate modelling of anisotropic behavior of oligocrystalline solder joints can be conducted for more accurate prediction of their fatigue durability.In this paper, single-crystal high aspect ratio (HAR) SAC305 solder joints are fabricated with appropriate cooling rate, and then subjected to isothermal aging, to capture the effects of such microstructural aging on microstructural evolution and resulting evolution of creep resistance of single SAC crystals. Monocrystalline morphology and grain orientation are determined from microscopy and electron backscatter diffraction (EBSD). Features being tracked within the grain are Ag 3 Sn IMC particle size and spacing in the eutectic region and the area fraction of the pro-eutectic β-Sn dendritic region.Samples are aged at 125°C for 0, 100 and 300 hrs., for studying the effect of isothermal aging. Scanning electron microscope and image processing are used to study microstructural evolution. Room temperature creep tests are conducted on single crystal solder joints. Future paper will report the associated electron backscatter diffraction (EBSD) for grain orientation measurement and the creep modelling (using CV model), to determine the effect of the microstructural aging on anisotropic creep (Hill-Garofalo) model constants. These calibrated continuum model constants will be used to conduct thermomechanical simulations of QFN package to investigate the effect of isothermal aging on the fatigue durability of solder interconnects.

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