Morphologies of Primary Cu6Sn5 and Ag3Sn Intermetallics in Sn–Ag–Cu Solder Balls

This article focuses on the morphologies of primary Cu
₆
Sn
₅
and Ag
₃
Sn intermetallics in small Sn-Ag-Cu (Ø 270 μm) solder balls. The Cu
₆
Sn
₅
phase 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 {101̅0} planes as facets of the hexagonal rods. The formation of crystals splitting parallel to the {101̅0} planes may be caused by a slight deviation of the major growth direction toward (21̅1̅0). Primary Ag
₃
Sn intermetallics showed less morphology variations and solidified as typical plate structures with a planar and an irregular side. Sixfold starshaped formations of multiple plates arranged around a common center have been investigated by EBSD. All plates appear to have the same hexagonal orientation and possible growth directions are parallel to the {101̅0} planes. However, these pseudohexagonal structures showed color differences between plates of different growth directions, if investigated with polarized light microscopy. Those orientation differences were not detectable by EBSD. Possible interpretations are discussed on the basis of the orthorhombic distorted lattice and by possible twinning mechanisms.