Thermal and mechanical properties of lead-free SnZn–xNa casting alloys, and interfacial chemistry on Cu substrates during the soldering process

• Precipitates of NaSn was observed and confirmed using TEM and XRD.
• Addition Na to eutectic SnZn cussed increased the mechanical properties.
• IMCs of Na–Zn and Na–Sn increased electrical resistivity and reduced the CTE.
• IMCs layers CuZn4 and Cu5Zn8 was found at the interface.
• Na content changing the character of growth CuZn4 layer in SnZnNa alloys.

The microstructural features, thermal properties and mechanical properties of eutectic Sn–Zn alloys with varying Na content (0.1, 0.2, 0.5, 1.0 3.0 and 5.0 at. %) were examined in this study. In the scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis data, precipitates of NaSn were observed. The addition of Na to eutectic Sn–Zn alloy improved the mechanical properties and increased electrical resistivity, and reduced the coefficient of thermal expansion; however, the melting point did not change. Wettability tests carried out using Na-doped Sn–15Zn alloys on Cu substrates showed the formation of Cu–Zn phases at the interfaces. Wettability studies were performed using flux ALU33 after 60, 180, 480, 900, 1800 and 3600 s of contact, at temperatures of 230, 250, 280 and 320 °C. The experiments were designed to demonstrate the effect of Na addition on the formation and growth kinetics of Cu5Zn8 and CuZn4 phases, which were identified using XRDs and EDS. The addition of Na to SnZn causes a reduction in the thickness of the intermetallic compounds layer created at the interface between the liquid solder and the Cu substrate, and an increase in the activation energy of the Cu5Zn8 phase compared to eutectic SnZn.

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