Abstract
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 Cu6Sn5 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 Cu6Sn5 layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu6Sn5 crystals reveals a competition between the nucleation of Cu6Sn5 in the liquid versus growth of Cu6Sn5 from the existing Cu6Sn5 layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu6Sn5 layer in Sn-3.0Ag-0.5Cu/Cu joints.
Highlights
Microstructures are an important link between materials processing and performance, and microstructure control is essential for any materials processing route where the microstructure plays a major role in determining the properties[1]
In our previous study[17,18], we reported on the formation of the Cu6Sn5 interfacial layer at the liquid/Cu interface during the early stages of soldering and the effect of Ni on the growth of primary (Cu,Ni)6Sn5 in Sn-0.7Cu/Cu joints
From existing synchrotron studies of solder reactions, the solder joint experiments were conducted at low frame rates and fast reactions during the soldering process, especially in the early stages of reactive wetting and subsequent solidification, were unable to be investigated
Summary
On both figures, the central round features are shallow bubbles (artifacts) while the round features at the Sn/Cu6Sn5/Cu interface are flux voids. Open liquid regions remain when no or few primary Cu6Sn5 rods have their [0001] growth direction oriented towards the IMC layer (e.g. Fig. 3) In this situation, new Cu6Sn5 can only nucleate in the liquid ahead of the layer if the constitutional supercooling in this region exceeds the required nucleation undercooling. The elucidation of solder joint microstructure development revealed in this study could be used as a basis for the design of an optimized and controlled microstructure in solder joints for future electronic interconnects technology
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