The interaction between grain orientation evolution and thermal cycling as a function of position in ball grid arrays using orientation image microscopy

Abstract

Thermally cycled PBGA packages with a full array of 196 solder joints after various pre-conditions are examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain plan view cross sections of every solder joint, and characterized using both Polarized Optical microscopy and Orientation Imaging Microscopy (OIM). Based on observations using OIM images, we obtained a distribution map based on the Sn crystal c-axis orientation. Each precondition show its own signature distribution related to the thermal aging and thermal cycling history. Further analysis, combining the dye and pry and plan view observation, revealed the correlation between the c-axis orientations and the fatigue cracks caused by thermal cycling. A strong relationship between evolving crystal orientations where the c-axis becomes aligned with the plane of the package and cracking is identified. A combined study and observation of Polarized light images and OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling. A continuous recrystallization mechanism may account for the changes in grain orientation that lead to susceptibility to cracking.