Gayatri Cuddalorepatta and Abhijit Dasgupta
CALCE Electronic Products and Systems Center
Mechanical Engineering Department
University of Maryland, College Park, MD 20742 USA
Hochschule Mannheim - University of Applied Sciences
Department of Mechanical Engineering
Paul-Wittsack-Straße 10, 68163 Mannheim, Germany
This study examines microstructural recrystallization in Sn3.0Ag0.5Cu (SAC305) solder joints due to isothermal, mechanical cycling. It is well known that after reflow SAC solder joints at length scales of 200 μm consist of only a few grains [1-3]. This coarse microstructure makes the joint mechanically inhomogeneous and anisotropic, and nonrepeatable. Creep tests conducted on modified lap-shear SAC305 solder joints therefore show significant scatter in their results, because of piece-to-piece variability in the microstructural morphology . However, results of cyclic fatigue tests of the same SAC305 solder joints show less significant scatter . One possible hypothesis is that dynamic recrystallization occurs during the cycling, resulting in a much finer (and hence more isotropic, homogeneous and repeatable) microstructure. Recrystallization of solder has been reported to occur under thermal cycling [5-6]. The objective of this study is to assess the extent of recrystallization of SAC305 solder during isothermal mechanical cycling fatigue. Focused ion beam technology is used to prepare a very clean and even surface to reveal the SAC305 grains in modified lap-shear test specimens, both before and after isothermal mechanical cycling. Polarized light microscopy, scanning electron microscopy and focused ion beam microscopy are used to reveal the microstructure of these SAC305 solder joints. The results show that mechanical cycling produces the same type of recrystallization behavior of SAC solder, as has been reported in the literature for thermally cycled specimens [5-6]. The number of grains in the SAC305 solder joint changes from a few to hundreds, during mechanical cycling. As expected, the recrystallization is observed to be localized around cracks in the solder joint, where the local stresses are the highest. The minimal grain size near the cracked region is approximately 4-6 μm and the average grain size increases significantly with increasing distance from the crack face. The transition of solder from very few (non homogeneous and anisotropic) to a homogeneous recrystallized state may be one possible explanation for differences in the extent of scatter in the data from creep tests and isothermal mechanical fatigue tests.
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