Factors affecting the effect of lead-free reflow soldering

The temperature of lead-free reflow soldering is much higher than that of lead reflow soldering, and the temperature setting of lead-free reflow soldering is also difficult to adjust, especially because the window of lead-free reflow soldering process is very small, so the control of lateral temperature difference is very important. The large lateral temperature difference in reflow soldering can cause batch defects. So, how can we reduce the lateral temperature difference in reflow soldering to achieve the ideal lead-free reflow soldering effect? JEENOCE will share the factors that affect the lead-free reflow soldering effect.

1. Hot air transfer in lead-free reflow soldering furnace

At present, the mainstream lead-free reflow soldering adopts the heating method of full hot air. In the development process of reflow soldering furnaces, infrared heating has also appeared. However, due to the different infrared absorption reflectivity of devices with different colors and the shadow effect caused by adjacent original devices blocking, both of these situations can cause temperature differences and pose a risk of lead soldering jumping out of the process window. Therefore, infrared heating technology has gradually been eliminated in the heating method of reflow soldering furnaces. In lead-free soldering, it is important to pay attention to the heat transfer effect, especially for high heat capacity original devices. If sufficient heat transfer cannot be achieved, the heating rate will significantly lag behind that of low heat capacity devices, resulting in lateral temperature differences. Compared with using a fully hot air lead-free reflow soldering furnace, it will reduce the lateral temperature difference of lead-free reflow soldering.

2. Chain speed control of lead-free reflow soldering furnace

Lead free reflow soldering chain speed control will affect the lateral temperature difference of the circuit board. Generally speaking, reducing the chain speed will give larger heat capacity devices more heating time, thereby reducing the lateral temperature difference. However, since the setting of the furnace temperature curve depends on the requirements of the solder paste, it is not realistic to limit the chain speed in actual production. This depends on the usage of the solder paste. If there are many large components that absorb heat on the circuit board, it is recommended to lower the reflow soldering transport chain speed so that large surface mount components can fully absorb heat.

3. Control of Wind Speed and Airflow in Lead free Reflow Soldering Furnace

If the other conditions inside the lead-free reflow soldering furnace are kept unchanged, and only the fan speed inside the lead-free reflow soldering furnace is reduced by 30%, the temperature on the circuit board will decrease by about 10 degrees. It can be seen that the control of wind speed and air volume is important for furnace temperature control. In order to achieve control over wind speed and air volume, it is necessary to pay attention to two points that can reduce the lateral temperature difference in the lead-free reflow soldering furnace and improve the welding effect:

⑴.  The speed of the fan should be controlled by frequency conversion to reduce the impact of voltage fluctuations on it;

⑵.  Try to minimize the exhaust air volume of the equipment, as the central load of the exhaust air is often unstable and can easily affect the flow of hot air inside the furnace.

4. Lead free reflow soldering has good stability and can reduce the temperature difference inside the furnace

Even if we obtain a good lead-free reflow oven temperature curve setting, it still requires the stability, repeatability, and consistency of lead-free reflow soldering to ensure its implementation. Especially in lead production, if there is slight drift due to equipment reasons, it is easy to jump out of the process window and cause cold welding or damage to the original components. So, more and more manufacturers are beginning to demand stability testing for their equipment.