How to correctly set the temperature of lead-free reflow welder

The temperature setting of lead-free reflow welder must first have an appropriate time and temperature for the active phase of flux, allowing the cleaning phase to be completed when the solder particles just begin to melt. Secondly, sufficient slow heating can safely evaporate the solvent to prevent the formation of tin beads and limit the internal stress of components caused by temperature expansion, resulting in the reliability of fracture marks. The most important stage for the formation of residual solder flux is the time for the formation of residual solder flux and the complete melting of solder flux. If this stage is too hot or too long, it may cause damage to components and PCBs. Xiaobian believes that the correct setting of reflux temperature should be based on the following points:

A. Set according to the size of exhaust air volume. Generally, reflow soldering furnace has specific requirements for exhaust air volume, but the actual exhaust air volume sometimes changes due to various reasons. When determining the temperature curve of a product, the exhaust air volume shall be considered and measured regularly.

B. In addition, it is set according to the specific conditions of the equipment, such as the length of the heating zone, the material of the heating source, the structure of the reflow furnace and the heat conduction mode.

C. Set according to the temperature curve of solder paste. Solder pastes with different metal contents have different temperature curves. The reflow temperature curve of specific products shall be set according to the temperature curve provided by the solder paste supplier.??

D. The setting temperature of each temperature zone is determined according to the actual position of the temperature sensor. If the temperature sensor is located inside the heating body, the setting temperature is about 30 ℃ higher than the actual temperature.

E. It shall be set according to the density of surface assembly board components, the size of components and whether there are BGA, CSP and other special components.

F. It is set according to the material, thickness, multilayer board and size of PCB.

Several parameters affect the shape of the curve at the same time, among which the key is the conveyor belt speed and the temperature setting of each zone. The belt speed determines the duration of the board exposed to the temperature set in each zone. Increasing the duration allows more time to make the circuit assembly close to the temperature set in this zone. The total processing time is determined by the sum of the duration of each zone. The setting of solder paste reflux temperature curve is best based on the data provided by the solder paste supplier, and grasp the principle of temperature stress change inside the component, that is, the heating temperature rise rate is less than 3 ℃ per second and the cooling temperature drop rate is less than 5 ℃. If the size and weight of PCB assembly are very similar, the same temperature curve can be used. The temperature setting of each zone affects the temperature rise speed of PCB. High temperature produces a large temperature difference between PCB and zone temperature. Increasing the set temperature of the zone allows the board to reach the given temperature faster.