The twinning phenomenon in SnAgCu solder balls

The study focuses on a particular solidification phenomenon found in SnAg3.0Cu0.5 solder balls. Investigations on the microstructure of as cast solder spheres showed, that there is an abrupt change in grain structure during solidification. The microstructure of these specimen consists of areas with small grains that are surrounded by large areas with major grain orientations. In order to characterize this phenomenon Orientation Imaging Microscopy (OIM) done by Electron Backscatter Diffraction (EBSD) has been carried out. It will be shown that the fine grains as well as the large grains show three major, distinct orientations. These measurements indicate the existence of a phenomenon, which is supposed to be caused by a twinning mechanism that creates a misorientation angle of approx. 60deg between adjoining grains. In further investigations multilayer cross-sections have been carried out, in order to get a 3-dimensional overview of this phenomenon.