The development of microstructure during melting, reactive wetting and solidification of solder pastes on Cu-plated printed circuit boards has been studied by synchrotron radiography. Using Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu/Cu as examples, we show that the interfacial Cu6Sn5layer is present within 0.05 s of wetting, and explore the kinetics of flux void formation at the interface between the liquid and the Cu6Sn5 layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu6Sn5crystals reveals a competition between the nucleation of Cu6Sn5in the liquid versus growth of Cu6 Sn5from the existing Cu6 Sn5 layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu6Sn5layer in Sn-3.0Ag-0.5Cu/Cu joints.
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