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Grain Orientation and Microstructure Evolution in Sn-Ag-Cu Solder Joints as a Function of Position in Ball Grid Array Packages
Keywords: Grain orientation, microstructure, Pb free
The microstrcuture evolution of Sn-Ag-Cu solder joints during aging and thermal cycling are studied, focusing on Sn grain orientation of Plastic Ball Grid array (PBGA) packages. Thermally cycled PBGA packages with 196 full array solder joints after various pre-conditions are examined. Each PBGA package was polished to obtain plane view cross sections of every solder joint, and characterized using both Polarized Optical microscopy and Orientation Imaging Microscopy. Based observations using polarized optical images, a single grain solder joint distribution map shows that thermal aging has a relatively small impact on the overall single grain solder joint fraction, but with thermal cycling, many single crystal joints transform to multi grained solder joints. Combined thermal aging and thermal cycling further increases the fraction of multi-grained joints. Single and multicrystal distribution maps are just part of the evolution process that occurs during thermal cycling. A combined study and observation of Polarized light images and OIM provide further understandings are discussed about deformation and microstructure evolution processes that occur during thermal cycling.
Tae-Kyu Lee,
Cisco Systems, Inc.
San Jose, CA

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