Interfacial reactions and microstructural evolution of Sn-based solder joints prepared with reactive Fe nanoparticles

Abstract

The aim of this study was to gain a better understanding of the strengthening mechanisms in reactive metal-nanocomposite solder joints. To achieve this, Sn-3.5Ag/Cu solder joints were produced using Fe nanoparticle (NP)-doped flux with up to 2 wt.% NP additions. The concentration of the NPs between the solder and the substrate prevents the excessive growth of intermetallic phases (IMC) at the interface without impairing the properties of the solder joint such as toughness. Furthermore, this method can be used in surface mounting technology without an additional step in the production line. Advanced high-resolution electron microscopic investigations in conjunction with elemental analysis by energy dispersive X-ray spectroscopy allowed for detailed observations and analysis of the changes in the chemistry and microstructure of the substrate/IMC layer/solder joint region, prior and after long-term aging. Atomic-scale characterization of the IMC layer was performed using atom probe tomography analysis. The microstructural evolution of the interfacial region and the bulk of the solder during the reflow process and the subsequent long-term aging at elevated temperatures of up to 180°C through interaction with the Fe nanoparticles can be described as follows: It is assumed that the diffusion of iron into the Cu6Sn5 IMC phase during the liquid-solid reactions can lead to changes in the crystal lattice stoichiometry to form (Cu, Fe)6Sn5, whereby the Cu3Sn structure seems to remain unchanged. In addition, the dissolution of iron nanoparticles in the Sn-Ag solder, followed by the solid-state interfacial diffusion during long-term aging, can result in the formation of a FeSn2 phase and partial iron segregation near the interface. Analysis of the growth kinetics of the interfacial layers after high-temperature storage revealed an increase in the activation energy of the formation of IMCs and suppression of their growth with the addition of Fe-NPs to the solder alloy. The results of isothermal shear tests and nanoindentation mapping of the joint area confirmed that incorporation of optimized amounts of reactive Fe-nanoparticles to the flux can significantly improve the mechanical performance of the Sn-based solder joints.