# Effects of Al Element on the Microstructure and Properties of Sn-9Zn Solder Alloy

## TL;DR

Adding aluminum to a Sn-9Zn solder alloy improves its strength, oxidation resistance, and corrosion resistance, but reduces ductility.

## Contribution

The study demonstrates that Al microalloying enhances mechanical and corrosion properties of Sn-9Zn solder through the formation of intermetallic compounds and protective oxide layers.

## Key findings

- Al addition forms AlZnSn intermetallic compounds that refine the microstructure and increase tensile strength by 37%.
- In situ-formed Al2O3 passive film improves oxidation and corrosion resistance of the solder alloy.
- A trade-off between wettability and ductility is observed with Al microalloying.

## Abstract

The application of microalloying technology has significantly improved the mechanical properties, oxidation resistance, and corrosion resistance of the Sn-9Zn-xAl-series solder. The effects of Al addition on microstructural evolution and service-related performance of the solders were systematically investigated using a combination of characterization techniques, including scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), differential scanning calorimetry (DSC), tensile testing, spreading testing, thermogravimetry (TG), and potentiodynamic polarization measurements. Microstructural characterization reveals that an optimal content of Al reacts with the Sn-Zn matrix to form AlZnSn intermetallic compounds (IMCs), which effectively refines the Zn-rich precipitates and eutectic lamellar structure. Concomitantly, the formation of second-phase strengthening contributes to a significant enhancement in the tensile strength of the solder alloys. Specifically, the Sn-9Zn-0.8Al solder exhibits a tensile strength of 87 MPa, corresponding to a 37% increment compared to the base Sn-9Zn alloy, whereas the elongation is reduced to 14.1%. Moreover, the in situ-formed Al2O3 passive film provides effective protection for the solder matrix, inhibiting oxidation induced by oxygen atoms and corrosion caused by chlorine ions, thereby remarkably improving the oxidation and corrosion resistance of the alloy. Collectively, these findings demonstrate that Al microalloying can substantially enhance the strength, oxidation resistance, and corrosion resistance of Sn-9Zn solder; however, a trade-off between wettability and ductility needs to be carefully considered for practical applications.