Abstract: The wettability of the solder directly determines the performance of the solder joint in the field of microelectronic packaging soft soldering. In this work, the SAC305/Cu soldering system was employed as the research object, and on the premise that no foreign elements are added, and wetting tests of SAC305 solder on Cu substrates with different roughnesses were completed. The influence of Cu substrate roughness on the wettability of SnAgCu solder and the morphology and distribution of SnAgCu/Cu interface compounds is analyzed. The results show that the wetting angle of SnAgCu solder on the Cu substrate increases as the substrate roughness decreases, and when the substrate roughness decreases to a certain degree, the wetting angle no longer changes with the substrate roughness changing. The reaction between the SnAgCu solder and the Cu substrate produces the intermetallic compound Cu₆Sn₅. Cu₆Sn₅ grows perpendicular to the surface of the substrate in a scallop shape from the substrate to the solder direction. The thickness of the Cu₆Sn₅ layer first increases and then decreases with the decrease in roughness. And the Cu₆Sn₅ layer interdiffuses with the substrate and the solder layer.