The correct setting of reflow welder temperature should be based on the following points:

1. 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 soldering furnace and the mode of heat conduction, etc.

2. Set according to the temperature curve of the solder paste used. The solder paste with different metal content has different temperature curve, so the reflow temperature curve of specific product should be set according to the temperature curve provided by the solder paste supplier.

3. Set according to the size of the exhaust air volume. General 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 should be considered and measured regularly.

4. According to the actual position of the temperature sensor, the set temperature of each temperature zone is determined. If the temperature sensor is located inside the heating body, the set temperature is about 30 ℃ higher than the actual temperature.

5. According to the PCB material, thickness, whether multi-layer board, size settings.

6. According to the density and size of the surface assembly board components, as well as BGA, CSP and other special components.

For hot air reflow welding, the solder paste in the welding process needs to go through the following processes: solvent volatilization, flux removal of oxide on the surface of welding parts, melting of solder paste, flow and cooling and solidification of solder paste. In terms of temperature curve, we can divide it into preheating area, heat preservation area, reflux zone and cooling zone, Below, guangshengde gives a brief introduction to these areas of its temperature curve.

In terms of preheating area, the main purpose is to make PCB and components preheat to achieve balance, and at the same time, remove water and solvent from solder paste to prevent solder paste from falling and splashing around. The heating rate of the system should be strictly controlled within a suitable range. Generally, we stipulate that the high temperature rise rate is 4 ℃, the rising rate is set to 1-3 ℃ and the standard low level of ECS is 3 ℃.

The insulation area of reflow welding generally refers to the area where the temperature is from 120 liters to 160 ℃, the main purpose is to make the temperature of each component of PCB tend to be uniform, minimize the temperature difference, and ensure that the solder can be completely messed up before reaching the reflow temperature. At the end of the heat preservation zone, the oxide on the welding pad, solder paste ball and element pin shall also be removed, The temperature of the entire circuit board will reach a balanced level at this time.

For the reflow region, the temperature of this region will reach high, and the welding value will vary with the solder paste, generally it will be higher than the melting point of solder paste 20-40 ℃. At this time, solder in solder paste has begun to melt, showing flow state. Sometimes we can also divide the reflow of reflow into two areas, namely, melting zone and reflow zone. The ideal temperature curve should be that the end area covering area of the solder melting point is small, and the left and right sides are symmetrical with each other. Generally, the time between the temperature of the reflow welding and the temperature of over 200 ℃ is 30-40 seconds.

The rapid cooling after reflow welding is completed helps to get a bright solder joint, with full shape and low contact angle. If the slow cooling is slow, it will easily lead to more decomposition of pad into tin, and some gray and dark and impetuous solder joints will be produced, and even the bad tin dip and weak solder joint bonding force will be caused. Generally speaking, the cooling rate of cooling zone is within -4 ℃, The cooling temperature is 75 ℃, and it is also required to use cooling fan to cool it forcibly.