Lead-free reflow temperature is generally affected by (hot air transmission, chain speed control, wind speed and air volume control, equipment stability) four factors, because the lead-free solder reflow process window is very small, so the lateral temperature difference control is very important.

1, the hot air delivery

At present, the mainstream lead-free reflow furnaces adopt the all-hot air heating method, and the infrared heating method also appears in the development of the reflow furnace. However, due to the difference in the infrared absorption reflectivity of different color devices due to the infrared heating, The shadow of the original device shading effect, and both of these conditions will lead to temperature differences leaving lead-free solder the risk of jumping out of the process window, so infrared heating technology in the reflow oven has been gradually eliminated. In the lead-free soldering, the importance of heat transfer effect, especially for large heat capacity of the original device, if not get enough heat transfer, it will lead to significantly lower the rate of temperature rise caused by small heat capacity of the device resulting in lateral temperature difference.

2, the control of the chain speed

Chain speed control will affect the lateral temperature difference of the circuit board. In general, reducing the chain speed gives more heating time for the device with larger heat capacity, thereby reducing the lateral temperature difference. However, after all, the setting of the temperature curve depends on the requirements of the solder paste. Therefore, it is unrealistic to reduce the chain speed without restriction in actual production.

3, the wind speed and air volume control

If you keep the other conditions in the reflow oven constant and only reduce the fan speed in the reflow oven by 30%, the temperature on the board will drop by about 10 ° C. Visible wind speed and air volume control of the importance of furnace temperature control. In order to achieve the control of wind speed and air volume, need to pay attention to two points:

(1) The speed of the fan should be frequency-controlled to reduce the impact of voltage fluctuations on it;

(2) Minimize the exhaust capacity of the equipment, because the central load of exhaust ventilation is often unstable and easily affects the flow of hot air in the furnace.

4, the stability of equipment

Even if we get an optimal oven temperature profile, we still need the stability, repeatability and consistency of the device to achieve it. In particular, lead-free production, if due to equipment, a slight drift, it is easy to jump out of the process window lead to cold welding or the original damage. Therefore, more and more manufacturers begin to put forward the requirement of stability test to the equipment.