Abstract: In the field of electronic packaging, the wettability of lead-free solder and the reliability of solder joints play crucial roles in the quality of electronic devices. In this work, the effect of minor Al addition on the wettability of Sn-9Zn lead-free solder and solder joint reliability was investigated. The results indicate that 0.01 wt.% and 0.02 wt.% Al additions decrease the wetting angle of Sn-9Zn solder from 37.26° to 31.57° and 27.22°, respectively. However, 0.05 wt.% Al addition increases the wetting angle to 42.52°. This is attributed to the fact that the Al is enriched on the surface of liquid solder, preferentially forming an Al-containing oxide film with higher density and fewer defects compared to Zn. This film hinders the reaction between O and the liquid solder, thereby enhancing the oxidation resistance of the solder. The thickness of the Al-containing oxide film is thin, facilitating the spreading of the liquid solder. However, excessive addition of Al leads to rapid oxide film formation, which deteriorates the solder’s wettability. 0.01 wt.% and 0.02 wt.% Al additions improve the reliability of Sn-9Zn/Cu solder joint, but the reliability of the joint is deteriorated by the addition of 0.05 wt.% Al. The optimum addition of Al to Sn-9Zn solder is 0.02 wt.%.