Abstract: Diode laser soldering system was used to study and explore the ways to improve the wettability and spreadability of Sn63Pb37 paste solder and Sn96Ag3.5Cu0.5 lead-free paste solder on Cu substrate. The effects of heating time of the laser on the wettability of paste solders were investigated and it was also analyzed that the microstructures of the joints and interfacial region of Sn63Pb37 and Sn96Ag3.5Cu0.5 under different time conditions by means of SEM. The Results indicate that the wettability and spreadability of the two kinds of solders on Cu substrate are improved with the increase of heating time under the condition of selected laser output power. When the heating time is longer than 1.5 s, the spreading area and wetting angle of Sn63Pb37 solder are tending towards stability and when the heating time is longer than 2.5 s, the spreading area and wetting angle of Sn-Ag-Cu solder are tending towards stability as well.