Heterogeneous nucleation of βSn on NiSn4, PdSn4 and PtSn4.

Abstract: During Pb-free soldering, βSn often requires a high nucleation undercooling and there is an ongoing effort to develop nucleation catalysts. It is shown here that NiSn4, PdSn4 and PtSn4 are heterogeneous nucleants for βSn, reducing the nucleation undercooling to ∼4K when these intermetallics are present either in the bulk solder or as the interfacial layer. Nucleation catalysis occurs by βSn nucleating on the (008) facet of XSn4 crystals with an orientation relationship (OR) (100)Sn||(008)XSn4 and [001]Sn||[100]XSn4 where there is a planar lattice match of ∼5%. This OR is also the origin of well-aligned lamellar βSn–XSn4 eutectic morphologies, even though the eutectics contain less than 2vol.% of faceted NiSn4, PdSn4 or PtSn4. [Copyright &y& Elsevier]