Heat Transfer Method of SMT Chip Processing Reflow Soldering

In SMT chip processing, a large number of surface mount components (SMA) are soldered by reflow soldering, which can be divided into 3 categories according to the heat transfer method of reflow soldering: far infrared, full hot air, infrared/hot air reflow. Reflow soldering equipment is also called reflow soldering furnace, reflow soldering or reflow soldering furnace.

1. Far-Infrared Reflow Soldering

The far-infrared reflow soldering used in the 1980s has the characteristics of fast heating, energy saving, and stable operation. However, due to the different materials and colors of the printed board and various components, the radiant heat absorption rate is quite different, resulting in uneven temperature of various components and different parts on the circuit, that is, local temperature difference. The temperature of various components and different parts is not uniform, that is, the local temperature difference. For example, the black plastic package body of the integrated circuit will overheat due to the high radiation absorption rate, while the welding part, the silver-white lead, will have a low temperature and cause virtual welding. In addition, the parts of the printed board where the heat radiation is blocked, such as soldering pins or small components in the shadow of large (high) components, will cause poor soldering due to insufficient heating.

2. Full Hot Air Reflow Soldering

Full hot air reflow soldering is a soldering method that uses convection jet nozzles or heat-resistant fans to force the airflow to circulate, thereby realizing the heating of the welded parts. This type of equipment began to rise in the 1990s. Because in this heating method, the temperature of PCB and components is close to the gas temperature of a given heating temperature zone, which completely overcomes the local temperature difference and shielding effect of infrared reflow soldering, so it is widely used at present.

In full hot air reflow equipment, the convective velocity of the circulating gas is critical. In order to ensure that the circulating gas acts on any area of ​​the PCB, the air flow must have a fast enough speed, which is easy to cause the jitter of the printed board and the displacement of the components to a certain extent. In addition, the heat exchange efficiency of this heating method is relatively low, and the power consumption is relatively high.

3. Infrared/Hot Air Reflow Soldering

This type of reflow soldering is based on the infrared furnace plus hot air to make the temperature in the furnace more uniform, which is an ideal heating method at present. This type of equipment makes full use of the characteristics of strong infrared penetration, high thermal efficiency, power saving, and at the same time effectively overcomes the local temperature difference and shielding effect of infrared reflow soldering, and makes up for hot air reflow soldering. , so this reflow soldering is currently the most commonly used.

With the increase in assembly density, the emergence of fine-pitch assembly technology, and the emergence of nitrogen-protected reflow ovens. Soldering under nitrogen protection conditions can prevent oxidation, improve the welding wetting ability, have a large correction force for unaligned components, and reduce the number of solder beads, which is more suitable for no-clean process.