Morphology Variations of Primary Cu6Sn5 Intermetallics in Lead-Free Solder Balls

This study focuses on the morphologies of primary Cu
₆
Sn
₅
intermetallics in small free standing SnCu (Ø 270 μm) solder balls. Those showed a large variety of different shapes and sizes ranging from facetted hexagonal rods, to partly facetted splitting crystals and parallel growing branches, to dendritic crystals without facets. The results of electron backscatter diffraction (EBSD) measurements confirm [0001] as the major growth direction and the {10I0} planes as facets of the hexagonal rods. The formation of splitting crystals parallel to the {10I0} planes may be caused by a slight deviation of the major growth direction towards <;2II0>. Morphology transition to dendritic structures can be influenced primarily by increasing the Cu content of the alloy and the cooling rate. However, strong variations occur even if the composition and the cooling rate are constant. Differences in undercooling of the Cu
₆
Sn
₅
phase have been discussed as a possible reason, since a decreasing solidification temperature promotes a faster initial phase growth due to the increasing oversaturation of the melt's Cu content.