Understanding the reflux process of Solder Paste

When the solder paste is in a heated environment, the solder paste backflow is divided into five stages,
1. First, used to achieve the required viscosity and silk screen performanceThe solvent starts to evaporate and the temperature increases slowly.(About 3 °C per second ),Limit boiling and splash to prevent small tin beadsAnd,Some components are sensitive to internal stress. If the external temperature of the component rises too fast, it may cause fracture..
2.Active flux and Chemical Cleaning, Water-soluble additive flux and washing-free additive flux both take the same cleaning action, but the temperature is slightly different. Remove metal oxides and some contamination from the forthcoming metal and solder particles. Good metallurgy Tin Solder Joints require "clean" surfaces.
3. When the temperature continues to rise, the solder particles first melt separately and start the 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 a single solder particle is completely melted and combined to form a liquid tin, the surface tension begins to form the surface of the weld pin, if the gap between the component pin and the PCB pad exceeds 4 mil, it is very likely that the PIN is separated from the pad due to the surface tension, that is, the solder point is opened.
5. In the cool-down phase, if the cool-down speed is fast, the tin point strength will be slightly larger, but the temperature stress inside the component will be caused by the non-Ethernet speed.

Summary of reflux welding requirements: important:Adequate slow heating to safely evaporate Solvents,Prevent tin beads from formingAnd restrictionsInternal Stress of components due to temperature expansion, leading to fault crack Reliability Problems. Secondly, the active phase of the flux must have proper time and temperature, allowing the cleaning phase to be completed when the solder particles are just starting to melt. In the time temperature curve, the soldering tin melting stage is the most important. It is necessary to completely melt the soldering tin particles, forming metallurgical welding by liquefaction, evaporation of residual solvents and flux, and forming the welding pin surface. This phase may cause damage to components and PCB if it is too hot or too long. It is best to set the reflux temperature curve of the solder paste according to the data provided by the solder paste supplier, and grasp the internal temperature stress change principle, that is, the heating temperature rise speed is less than 3 °C per second, and cooling temperature drop rate less than 5 °C. The same temperature curve can be used for PCB assembly if the size and weight are similar. It is important to check whether the temperature curve is correct on a regular or even daily basis.