Spectrum of heterogeneous nucleation modes in crystallization of Sn-0.7wt%Cu solder: experimental results versus theoretical model calculations

Abstract

In this paper we analyze both experimentally and theoretically the various possibilities of Sn-0.7wt%Cu solder crystallization and in particular we report, for the first time, low undercooling degrees (1–7 K) for this alloy. The supercooling of Sn-0.7wt%Cu alloy is measured by Differential Scanning Calorimetry experiments performed by using cyclic temperature change of the solder with total melting or with partial melting of the solder. Two different conditions for the solder alloy are used: (1) solder over Cu6Sn5 phase prepared by dipping a Cu sheet into a bath of Sn-0.7wt%Cu liquid alloy at 250 °C for 1 s providing natural interface for the heterogeneous nucleation of Sn crystals and (2) solder alloy without any specially prepared interfaces. At least three categories of undercooling degrees of Sn-Cu alloy are observed according to the experimental conditions. For each undercooling category we try to find the type of the heterogeneous nucleation site responsible for the start of the crystallization. Possible nucleation sites and nucleation modes of the crystallization are explored: (1) in the bulk (homogeneous), (2) at the planar interface liquid alloy/solid Cu6Sn5 surface, (3) as a cap over a spherical particle (a), or as a spherical shell covering a Cu6Sn5 phase sphere (b), (4) in a spherical cavity and (5) in a groove—at the step of the surface. Theoretical and experimental values of undercooling degrees are compared and the sites of the favorable nucleation are proposed for each case.