Dendrite Growth in Single-Grain and Cyclic-Twinned Sn–3Ag–0.5Cu Solder Joints

Abstract

The microstructure of electronic solder joints is generated by the solidification of a small volume of bulk undercooled liquid. Here, we study β-Sn dendrite growth in Sn–3Ag–0.5Cu in the specific geometry and nucleation conditions of ball grid array (BGA) solder joints by combining electron backscatter diffraction and imaging of microstructures. It is shown that, while ⟨110⟩ is the preferred dendrite growth direction, out-of-plane branching and growth with ⟨11W⟩ directions are important for allowing dendrites to fan out into the spheroidal volume of BGA joints due to the low symmetry of β-Sn. We find that the crystallographic orientation of β-Sn at the nucleation point plays a strong role in subsequent dendrite growth. In single-grain joints, dendrites are often unfavorably oriented for growth, resulting in different types of zig-zag dendrite growth. In cyclic-twinned joints, it is shown how competitive out-of-plane trunk growth between three dendrite orientations produces {101} boundaries and the characteristic beachball microstructure.