The SMT wave soldering process

The SMT wave soldering process mainly consists of three stages: preheating, wave soldering and cooling. The specific process includes key steps such as fixture installation, soldering agent application, and double-wave soldering.

SMT wave soldering is a key process in surface mount technology (SMT) used for welding surface-mounted components (THT) or mixed-assembly circuit boards. Its core lies in achieving batch welding through the wave of molten solder. The main process is as follows:

1‌.Fixture installation‌.
Fix the printed board with components installed on it on a dedicated fixture to prevent the substrate from deforming due to heat and prevent solder spatter, ensuring welding stability.‌‌

‌2.Spraying flux‌.
Uniformly apply flux to remove oxides on the surface of the pads and component pins to prevent secondary oxidation during the welding process. The type of flux needs to be selected based on the PCB material and component density.‌

3.Preheating. 
Temperature control: The preheating temperature is usually 90 to 130℃. For multi-layer boards or high-density surface mount boards, the upper limit (115 to 125℃) should be adopted. 
Function: Activates the flux, reduces thermal shock, and prevents tin splashing during welding due to solvent evaporation.

4.Wave soldering. 
Double peak structure: The first peak (disordered wave) ensures that the solder fully penetrates the solder joints, while the second peak (wide flat wave) eliminates bridging and tip pulling defects.

Parameter Settings: The temperature of the soldering furnace is usually 245 ± 5℃, and the welding time is 3 to 5 seconds. If it lasts too long, it may damage the substrate.

5.Cooling.
Quick cooling is used to enhance the strength of solder joints, usually through air cooling or water cooling methods. The cooling rate must be controlled to avoid thermal stress cracks.

Process Variants and Optimization 
1.Slope Peak Soldering: The conveying guide rail is designed in a slope shape, which prolongs the immersion time of the solder joints, improves efficiency and reduces air entrapment at the solder joints. 
2.High Peak Soldering: Suitable for long-leg surface mount welding processes. Used in conjunction with a foot-cutting machine, the nozzle height can be adjusted.

3.Online process: Integrates post-processing steps such as cutting and cleaning, suitable for automated production lines.