Reflow soldering process technology and precautions

Far infrared reflow

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 printed circuit boards and various components, the absorption rate of radiation heat is very different, resulting in uneven temperatures of various components and different parts on the circuit, that is, local temperature differences. For example, the black plastic package of the integrated circuit will overheat due to the high radiation absorption rate, while the temperature of the silver-white lead at the welding part is low, resulting in false welding. In addition, the parts of the printed circuit board where heat radiation is blocked, such as the welding pins or small components in the shadow of large (high) components, will cause poor welding due to insufficient heating.

1.2 Full hot air reflow

Full hot air reflow soldering is a soldering method that uses a convection jet nozzle or a heat-resistant fan to force the air flow to circulate, thereby achieving heating of the welded parts. This type of equipment began to emerge in the 1990s. Due to this heating method, the temperature of the printed circuit board and components is close to the gas temperature of the given heating temperature zone, which completely overcomes the temperature difference and shielding effect of infrared reflow soldering, so it is currently widely used. In full hot air reflow soldering equipment, the convection speed of the circulating gas is crucial. To ensure that the circulating gas acts on any area of ​​the printed circuit board, the air flow must have a sufficiently fast speed. This is likely to cause the jitter of the printed circuit board and the displacement of components to a certain extent. In addition, this heating method has poor efficiency and high power consumption in terms of heat exchange.

1.3 Infrared hot air reflow

This type of reflow oven is based on the IR oven and adds hot air to make the temperature in the oven more uniform. It is currently the most ideal heating method. This type of equipment fully utilizes the strong penetrating power of infrared rays, has high thermal efficiency, and saves electricity. At the same time, it effectively overcomes the temperature difference and shielding effect of infrared reflow, and makes up for the impact of hot air reflow on the gas flow rate being too fast. Therefore, this type of IR+Hot reflow is currently the most commonly used in the world.

With the increase of assembly density and the emergence of fine pitch assembly technology, nitrogen-protected reflow ovens have also appeared. Welding under nitrogen protection can prevent oxidation, improve welding wetting force, speed up wetting speed, have strong correction force for misaligned components, reduce solder beads, and is more suitable for no-clean process.

2. Establishment of temperature curve

The temperature curve is a curve of the temperature change of a certain point on the SMA over time when the SMA passes through the reflow oven. The temperature curve provides an intuitive method to analyze the temperature change of a component throughout the reflow process. This is very useful for obtaining the best solderability, avoiding damage to components due to overheating, and ensuring soldering quality.

The following is a brief analysis starting from the preheating stage.

2.1 Preheating section

The purpose of this zone is to heat the room temperature PCB as quickly as possible to achieve the second specific goal, but the heating rate must be controlled within an appropriate range. If it is too fast, thermal shock will occur, and both the circuit board and the components may be damaged; if it is too slow, the solvent will not evaporate fully, affecting the welding quality. Due to the fast heating speed, the temperature difference in the SMA in the latter section of the temperature zone is large. In order to prevent damage to components due to thermal shock, the maximum speed is generally specified to be 4℃/s. However, the rising rate is usually set to 1-3℃/s. The typical heating rate is 2℃/s.

2.2 Insulation section

The insulation section refers to the area where the temperature rises from 120℃-150℃ to the melting point of the solder paste. Its main purpose is to stabilize the temperature of each component in the SMA and minimize the temperature difference. In this area, enough time is given to allow the temperature of the larger component to catch up with the smaller component and ensure that the flux in the solder paste is fully volatilized. At the end of the insulation section, the oxides on the pads, solder balls and component pins are removed, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise various poor welding phenomena will occur due to uneven temperatures of various parts when entering the reflow section.

2.3 Reflux Section

In this area, the temperature of the heater is set to the highest, so that the temperature of the component rises quickly to the peak temperature. The peak temperature of soldering in the reflow section varies depending on the solder paste used. It is generally recommended to add 20-40°C to the melting point of the solder paste. For 63Sn/37Pb solder paste with a melting point of 183°C and Sn62/Pb36/Ag2 solder paste with a melting point of 179°C, the peak temperature is generally 210-230°C. The reflow time should not be too long to prevent adverse effects on the SMA. The ideal temperature curve is that the "tip zone" that exceeds the melting point of the solder has the smallest area covered.