What is hot air reflow soldering

Hot air reflow welding is a welding method that uses a convection jet nozzle or a heat-resistant fan to force airflow circulation and achieve heating of the welded part. This type of equipment began to rise in the 1990s, and due to the use of this heating method, the temperature of printed circuit boards (PCBs) and components is close, and the gas temperature in a given heating temperature range is completely overcome. The local temperature difference and shielding effect of infrared reflow soldering are currently widely used.

1、 The principle and characteristics of hot air reflow soldering

The hot air reflow soldering furnace transfers heat energy through the laminar movement of hot air. By using a heater and fan, the air inside the furnace is continuously heated and circulated, and the welded parts are heated by the hot gas in the furnace to achieve welding. The hot air reflow soldering furnace has the characteristics of uniform heating and stable temperature. The temperature difference between the upper and lower parts of the PCB, as well as the temperature gradient along the furnace length direction, are not easily controlled and are generally not used alone. Since the 1990s, with the continuous expansion of SMT applications and the advancement of miniaturization of components, equipment development and manufacturers have improved the distribution of heaters and the circulation direction of air, and added 8 or 10 temperature zones to enable precise control of temperature distribution in various parts of the furnace, making it more convenient for ideal adjustment of temperature curves. The reflow soldering furnace with full hot air forced convection has been continuously improved and perfected, becoming the mainstream equipment for SMT welding.

2、 Hot air reflow soldering structure

1. Heating zone structure

The structure of each heating zone inside the furnace is identical. There are motors in the upper and lower heating zones that drive the impeller to rotate at high speed, generating blowing force of air or nitrogen. After being heated by heating wires or other materials, the gas is blown out of the porous board and sprayed onto the PCB board. Some reflux furnaces have programmable motor speeds, such as the Lifeng ROHS-848, which can range from 1000 to 2800RPM, while others have been fixed at the factory, such as the BTU furnace, which has been set at a high speed of about 3000 RPM. The faster the motor speed, the greater the wind force, and the stronger the heat exchange capacity. By measuring the air pressure blown out by the gas, it is possible to monitor the normal operation of the motor. Due to the volatilization of flux in the solder paste during the reflow process, it may condense on the impeller, reducing the efficiency of the wind and leading to a decrease in the temperature reflow curve. Therefore, it is necessary to regularly inspect and clean the impeller.

2. Temperature control

The temperature control of each heating zone of the hot air reflow welding machine is a vertical closed-loop control system. The temperature controller maintains the temperature at the set value through PID control. The temperature sensor uses thermocouple wires installed under the porous plate to sense the temperature of the airflow. If the temperature in the heating zone is abnormal, such as not heating or heating slowly, it is generally necessary to check whether the solid-state relay is normal, and whether the heater in the heating zone is aging and needs to be replaced (this problem is common in reflux furnaces that have been used for many years). If there is a temperature display error, it is usually due to damage to the thermocouple wire.

3. Cooling zone structure

After reflow soldering, the PCB board must be immediately cooled to achieve good soldering results. Therefore, there is a cooling zone behind the reflow soldering furnace. The structure of the cooling zone is a water circulating heat exchanger. The cooling fan blows hot air into the circulating water heat exchanger, and the cooled gas is then directed onto the PCB board. The heat inside the heat exchanger is carried away by the circulating water, which is cooled and then flows back to the heat exchanger.

Due to the tendency of flux to condense in the cooling system, it is necessary to regularly inspect and clean the flux on the flux filter. Otherwise, the decrease in thermal cycling efficiency will reduce the efficiency of the cooling system, leading to poor cooling and a decrease in the welding quality of the product. The long-term stability of PCB boards welded by overheating will decrease.