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Characterization of SAC Solder Microstructure Evolution During Thermal Cycling with Statistical and In-situ Methods
Keywords: crystal orientation, x-ray diffraction, internal stress
The evolution of Sn grain microstructure during thermomechanical cycling occurs due to grain boundary movement, as some single crystal solder joints gradually transform to multicrystals. This process has been documented statistically using EBSP mapping by counting the numbers of joints with different orientations and microstructures in packages with different thermomechanical cycling histories. On the basis of these measurements, the range of average stress in a solder ball can be estimated as a function of temperature. The process by which evolution occurs is examined by comparing diffraction patterns obtained from the full volume of a solder ball at different stages of thermal cycling history using synchrotron radiation. A slice from the center row of a 14x14 plastic ball grid array and a 44 x 44 ball flip chip ball grid array test vehicles were examined during in-situ thermal cycling between 0 and 100 aC. Diffraction patterns were used to identify crystal orientations present in the joint, and peak shape and peak shifts were used to assess changes in internal stress and dislocation content in several joints. Changes in internal strain and volume fraction of crystal orientations were assessed in the same joints after specimens were cycled for longer periods of time.
Bite Zhou, Research Assistant
Michigan State University
East Lansing, MI


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