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. 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 0.01wt.% and 0.02wt.% Al additions decrease the wetting angle of Sn-9Zn solder from 37.26° to 31.57° and 27.22°, respectively. However,0.05wt.% 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. 0.01wt.% and 0.02wt.% Al additions improve the reliability of Sn-9Zn/Cu solder joint, but the reliability of the joint is deteriorated by the addition of 0.05wt.% Al. The optimum addition of Al to Sn-9Zn solder is 0.02wt.%.