The effect of preheating is as follows.
1 Evaporate the solvent in the flux and the moisture that may be absorbed on the pcb assembly board. The solvent and moisture will boil when pass wave soldering and cause solder sputtering (commonly known as “frying tin” phenomenon). The tin is like mixed wate enter into the boiling oil, it will produce splashes, forming hollow solder joints, blisters, tin beads, virtual welding, miss welding, pores and other defects. Therefore, preheating can reduce welding defects.
2 Rosin in flux and active agent need a certain temperature to decompose and activate the reaction. The activation reaction can remove oxide film and other pollutants of the printed circuit board pad, component end and the lead surface .
3. Fully preheat the prototype pcb board and components to avoid sharp heating during soldering and thermal stress damage to the printed board and components.
The preheating temperature and time of the printed board should be determined according to the type of flux, the activation temperature range, the size and thickness of the assembled board, the size and number of components, and the number of components mounted, that is, the heat capacity of the assembled board. The length of the wave soldering machine preheating zone is determined by the yield and conveyor speed. The higher the yield, the longer the preheating zone is required to achieve the desired wetting temperature for the assembled panel.
Preheating temperature is 90~130°C (refers to PCB surface temperature), preheating temperature is upper limit for multi-layer board and more mounted components, preheating temperature is different for different PCB types and assembly forms, but must be combined specific conditions of the pcb assembly board and set after the process test or test welding. As with reflow soldering, the real-time temperature profile is also need to be measured.