The lead-free reflow soldering temperature is much higher than the lead-based reflow soldering temperature. The temperature setting of lead-free reflow soldering is also difficult to adjust. Especially because the lead-free soldering reflow process window is very small, the control of the lateral temperature difference is very important. A large lateral temperature difference in reflow soldering will cause batch defects. So how can we reduce the lateral temperature difference in reflow soldering to achieve the ideal lead-free reflow soldering effect? Chengyuan Automation starts from four factors that affect the reflow soldering effect.
1. Hot air transfer in lead-free reflow soldering furnace
At present, the mainstream lead-free reflow soldering adopts the full hot air heating method. In the development process of the reflow soldering oven, infrared heating has also appeared. However, due to infrared heating, the infrared absorption and reflectivity of different color components are different and due to the adjacent original The device is blocked and produces a shadow effect, and both situations will cause temperature differences and put lead soldering at risk of jumping out of the process window. Therefore, infrared heating technology has been gradually eliminated in the heating method of reflow soldering ovens. In lead-free soldering, it is necessary to pay attention to the heat transfer effect, especially for original devices with large heat capacity. If sufficient heat transfer cannot be obtained, the temperature rise rate will significantly lag behind devices with small heat capacity, resulting in lateral temperature differences. Compared with using a full hot air lead-free reflow oven, the lateral temperature difference of lead-free reflow soldering will be reduced.
2. Chain speed control of lead-free reflow oven
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 devices with large heat capacity more time to heat up, thereby reducing the lateral temperature difference. But after all, the setting of the furnace temperature curve depends on the requirements of the solder paste, so the limited chain speed reduction is unrealistic in actual production. This depends on the usage of the solder paste. If there are many large heat-absorbing components on the circuit board, For components, it is recommended to lower the reflow transportation chain speed so that large chip components can fully absorb heat.
3. Control of wind speed and air volume in lead-free reflow oven
If you keep other conditions in the lead-free reflow oven unchanged and only reduce the fan speed in the lead-free reflow oven by 30%, the temperature on the circuit board will drop by about 10 degrees. It can be seen that the control of wind speed and air volume is important to furnace temperature control. In order to control the wind speed and air volume, two points need to be paid attention to, which can reduce the lateral temperature difference in the lead-free reflow furnace and improve the soldering effect:
⑴The fan speed should be controlled by frequency conversion to reduce the impact of voltage fluctuations on it;
⑵ Reduce the exhaust air volume of the equipment as much as possible, because the central load of the exhaust air is often unstable and can easily affect the flow of hot air in the furnace.
4. Lead-free reflow soldering has good stability and can reduce the temperature difference in the furnace.
Even if we obtain an optimal lead-free reflow oven temperature profile setting, achieving it still requires the stability, repeatability and consistency of lead-free reflow soldering to ensure it. Especially in lead production, if there is a slight drift due to equipment reasons, it is easy to jump out of the process window and cause cold soldering or original device damage. Therefore, more and more manufacturers are beginning to require stability testing of equipment.
Post time: Jan-09-2024