Control system of reflow welder

The reflow welding machine is composed of control system (the control system adopts PC + PLC + HMI (man-machine interface) mode), hot air system (pressurized forced circulation hot air heating system, front and rear return air to prevent the influence of air flow in the temperature range and ensure temperature uniformity and heating efficiency; special high-temperature motor, adjustable speed frequency conversion), cold air system (forced air cooling and water-cooling structure, adjustable temperature display in the cooling zone), machine body and transmission system. Here I share the control system of the reflow welder.

1. Reflow transport system

PCB products are transported through the transportation system and through the reflux furnace. The transportation system is mainly composed of transportation guide rail, transmission sprocket, chain and transmission mesh belt. The transmission mesh belt is a standard 440mm (max 560mm) stainless steel mesh belt, and the transmission speed is stepless regulated by the control software. The width of the transport guide rail can be adjusted freely through the transmission of the deflector, screw rod and nut according to the size of the PCB board, and the width can be adjusted to 400mm (maximum 460mm).

Chain network transmission is adopted, and the driving device is installed at the outlet of the furnace. The drive device includes drive motor, drive shaft, drive chain and sprocket, guide screw and other guide rail transportation elements.

2. Composition of temperature zone in reflow furnace

The reflux furnace body is mainly composed of inlet, preheating area, insulation area, reflux area and cooling area. With the guide rail as the center, the upper and lower temperature zones circulate independently and control the temperature independently. After accelerated pressurization and heating, the air flow in each independent temperature zone acts evenly on the upper and lower sides of PCB through rectifier board. All heating areas of ser series reflux furnace have two heating modules (see Section 3.3 reflux furnace structure diagram for details), independent circulation and independent temperature control. Each temperature zone adopts the same modular structure, which is easy to maintain and replace.

3. Reflow cooling zone

After cooling the module, it can be connected to the cooling area outside the furnace to realize high-efficiency cooling.

When the nitrogen device is selected, double cooling zones are used for cooling: the gas cooled by heat exchange is acted on the PCB board from above through water-cooled heat exchanger, cooling fan and air curtain at the outlet. After forced cooling in the cooling area, the PCB board is sent to the outside of the furnace by the transportation system to complete reflow welding.

The heat exchanger can filter the flux and ensure the circulation of clean gas in the furnace; The air volume of the cooling fan is appropriate, which can maintain the stable inert gas content in the cooling area.

Modular structure, heat exchanger, cooling fan and rectifier orifice plate are adopted for convenient maintenance.

4. Reflow transport speed control

The speed of reflux furnace transportation system can be set according to the needs of users and controlled by software. The control accuracy can reach ± 2mm / min.

5. Reflow temperature control

The set temperature of each temperature zone is set through software operation. The actual temperature of each temperature zone is transmitted through the thermocouple installed in each temperature zone and displayed on the PC monitoring screen. At the same time, the control signal is also fed back to the solid-state relay in the electric control box to provide sufficient power to maintain the temperature at the set value.

6. Reflow soldering temperature measuring line interface

The temperature measuring line interface of reflux furnace is installed on the operation control panel in front of the machine. The input end of the temperature measuring line is connected with the temperature measuring card on the industrial computer. The temperature curve during welding can be captured through the reflux furnace control software, and the temperature curve can be displayed, printed, stored and called.

7. Reflow soldering temperature out of tolerance alarm protection

When the actual temperature exceeds the upper and lower limits of the set temperature, the system will send out alarm signals and display alarm information through software control. At the same time, all heating areas will stop heating (if the temperature in the temperature area is set to 200 ℃, when the upper and lower limits are set to 10 ℃, an alarm will be sent when the actual temperature exceeds 210 ℃ or is lower than 190 ℃), and the transportation system will continue to operate, Send the PCB board out of the reflow furnace safely. After troubleshooting, turn on the heating switch again for heating.

8. Reflow gas control

The reflux furnace requires a gas control device because it needs to supply clean nitrogen or air to the furnace. Gas control devices include:

【1】 Entrance and exit air curtain

【2】 Intake and exhaust devices and control software

【3】 Flowmeter and oxygen concentration analyzer (optional for nitrogen type)

【4】 Optional flux recovery device

9. Reflow control system

The reflux furnace control system is composed of PC + PLC and reflux furnace control software. It can realize the setting of the system, accurate control of the process (such as speed, temperature and working time), real-time monitoring of the state (temperature curve, overtemperature, power phase loss) and other functions. See the structure diagram in Section 3.3 for the installation of PC control system. The display is installed on the cantilever frame at one end of the furnace, and the cantilever can be rotated according to the use needs to adjust the position of the display and keyboard; The PC is installed in the rack under the display, surrounded by machine door protection, and connected with PLC through network cable.