Preheating temperature and time
1. Function of preheating:
(1) The solvent in the flux is volatilized to reduce the gas generated during welding.
(2) Rosin and active agent in the flux begin to decompose and activate, which can remove the oxide film and other pollutants on the surface of PCB pads, component terminals and pins, and protect the metal surface from reoxidation at high temperatures.
(3) The printed board and components shall be fully preheated to avoid damage to the printed board and components due to thermal stress caused by sharp temperature rise during welding.
2. Setting of preheating temperature and time
The preheating temperature and time of the printed board shall be determined according to the size and thickness of the printed board, the size and number of components, and the number of mounted components. The preheating temperature is 90-130 ℃ (PCB surface temperature), and the upper limit is taken when there are many components mounted. The preheating is controlled by the conveyor belt speed. If the preheating temperature is too low or the preheating time is too short, the solvent in the flux volatilizes insufficiently, and the gas generated during welding causes welding defects such as pores and solder beads; if the preheating temperature is too high or the preheating time is too long, the flux is carbonized, which makes the flux inactive, and also causes welding defects such as burrs and bridges. Therefore, the preheating temperature and time should be properly controlled. The best preheating temperature is that the flux coated on the bottom of PCB before wave soldering is not sticky, but still sticky.