Detailed Explanation Of PCBA Solder Paste Reflow Process!

When the PCBA solder paste is placed in a heated environment, the PCBA solder paste reflow is divided into five stages.

1. First, the solvent used to achieve the desired viscosity and screen printing properties begins to evaporate, and the temperature rise must be slow (about 3°C per second) to limit boiling and splashing and prevent the formation of small tin beads. Also, some components have a Stress is sensitive, and if the external temperature of the component rises too quickly, it will break.

2. The flux is active, the chemical cleaning action begins, and the same cleaning action occurs for both the water-soluble flux and the no-clean flux, but the temperature is slightly different. Remove metal oxides and some contamination from the metal and solder particles to be bonded. Good metallurgical solder joints require a "clean" surface.

3. When the temperature continues to rise, the solder particles first melt individually, and begin the "light grass" process of liquefaction and surface tin absorption. This covers all possible surfaces and begins to form solder joints.

4. This stage is the most important. When the individual solder particles are all melted, they combine to form liquid tin. At this time, the surface tension begins to form the surface of the solder feet. If the gap between the component pins and the PCB pads exceeds 4mil, it is very likely due to the Surface tension separates the lead from the pad, creating an open tin point.

5. In the cooling stage, if the cooling is fast, the strength of the tin point will be slightly larger, but it should not be too fast to cause temperature stress inside the component.

Summary of reflow soldering requirements:

It is important to have sufficient slow heating to safely evaporate the solvent, prevent tin bead formation and limit internal stress on the component due to temperature expansion, which can cause breakline reliability issues.

Second, the active phase of the flux must have an appropriate time and temperature to allow the cleaning phase to complete when the solder particles are just starting to melt.

The melting stage of the solder in the time temperature curve is the most important. It must be sufficient to allow the solder particles to completely melt, liquefy to form metallurgical welding, and evaporate the residual solvent and flux residue to form the surface of the solder leg. If this stage is too hot or too long, it may cause damage to components and PCB.

The setting of the PCBA solder paste reflow temperature curve is best done according to the data provided by the PCBA solder paste supplier, and at the same time grasp the principle of the internal temperature stress change of the component, that is, the heating temperature rise rate is less than 3°C per second, and the cooling temperature drop rate less than 5°C.

If the size and weight of the PCB assembly are very similar, the same temperature profile can be used.

It is important to check that the temperature profile is correct, often or even daily.

Tecoo, as an electronics manufacturing service provider, supports turnkey PCB assembly services.