In situ imaging of microstructure formation in electronic interconnections

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 Cu(6)Sn(5) layer is present within 0.05 s of wetting, and explore the kinetics of flux void formation at the interface between the liquid and the Cu(6)Sn(5) layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu(6)Sn(5) crystals reveals a competition between the nucleation of Cu(6)Sn(5) in the liquid versus growth of Cu(6)Sn(5) from the existing Cu(6)Sn(5) layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu(6)Sn(5) layer in Sn-3.0Ag-0.5Cu/Cu joints.