What should be paid attention to when using reflow soldering? In the process of reflow soldering, what kind of change will happen?
1、 When the solder paste reaches the heating zone, the temperature begins to rise. The temperature rise must be slow (about 2 ℃ ~ 3 ℃ per second) to prevent boiling and splashing and to avoid the formation of small solder beads; Some components are sensitive to internal stress, which may cause fracture when the external temperature of components rises too fast（ At this time, the solvent used to improve the screen printing performance and viscosity of solder paste begins to evaporate.)
2、 The flux inside the solder paste starts to be active, and the chemical cleaning action begins. The same cleaning action will take place for both water-soluble flux and free cleaning flux, but the temperature required to start the action is slightly different（ At this time, the chemical cleaning action will remove the metal oxide and some contamination from the solder particles and metal to ensure that the metallurgical requirements of the "clean" solder joint surface are met
3、 When the temperature in reflow soldering furnace continues to rise, a single solder particle begins to melt, liquefy and absorb tin on the surface. In this way, all the surfaces that may be covered begin to cover and solder spots gradually form.
4、 This stage is the most important in reflow soldering. When all the individual solder particles are melted and combined with the liquid tin formed due to high temperature, the surface of solder pin begins to form under the action of surface tension. If the gap between component pin and PCB pad exceeds 0.1mm, the pin and pad may be separated under the action of surface tension, As a result, the solder joint is open.
5、 When the solder joint formed by solder paste enters the cooling zone, the solder joint begins to cool. If the cooling speed is a little faster, the strength of the solder joint will increase, but it should not be cooled too fast, otherwise it will cause the internal temperature stress of the component.
