SuZhou Tonkida Electromechanical Technology Co., Ltd Professional SMT whole line supplier

Reflow process technology and precautions

Far-infrared reflow welding

The far-infrared reflow welding used in the 1980s has the characteristics of fast heating, energy saving and smooth operation, but because the printed board and various components have great differences in the absorption rate of radiant heat due to different materials and colors, the temperature of various different components and different parts of the circuit is not uniform, that is, the local temperature difference. For example, the black plastic package of an integrated circuit will overheat due to high radiation absorption, and its welding site - the silver lead, but the temperature is low to produce false welding. In addition, parts of the printed board where thermal radiation is blocked, such as welding pins or small components in the shadow of large (tall) components, will cause poor welding due to insufficient heating.

1.2 Full hot air reflow welding

All-hot air reflow welding is a kind of welding method that forces the air flow through the convection jet nozzle or heat-resistant fan to heat up the welded part. Such devices began to take off in the 1990s. Due to the use of this heating method, the temperature of the printed 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 welding, so it is widely used at present. In all hot air reflow welding equipment, the convection velocity of circulating gas is very important. In order to ensure that the circulating gas acts on any area of the printed board, the airflow must have a sufficiently fast speed. To a certain extent, it is easy to cause the jitter of the printed board and the displacement of the components. In addition, the use of this heating method in terms of heat exchange, the efficiency is poor, more power consumption.

1.3 Infrared hot air reflow welding

This kind of reflow welding furnace is based on IR furnace with hot air to make the temperature in the furnace more uniform, which is a more ideal heating method at present. This kind of equipment makes full use of the characteristics of infrared penetration, high thermal efficiency, power saving, and effectively overcome the temperature difference and shielding effect of infrared reflow welding, and make up for the impact of Hot air reflow welding on the gas flow rate too fast, so this IR+Hot reflow welding is currently widely used in the world.

With the increase of assembly density and the appearance of fine spacing assembly technology, nitrogen protected reflow welding furnace has also appeared. Welding under nitrogen protection can prevent oxidation, improve the welding wetting force, speed up the wetting speed, large correction force for non-aligned components, reduce welding beads, more suitable for no-cleaning process.

2. Establishment of temperature curve

The temperature curve is the curve of the temperature change over time at a point on the SMA when the SMA passes through the reflow furnace. The temperature curve provides an intuitive way to analyze the temperature change of a component throughout the reflow process. This is very useful for obtaining weldability, avoiding damage to components due to overtemperature, and ensuring welding quality.

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

2.1 Preheating section

The purpose of this area is to heat the room temperature PCB as soon as possible to achieve the second specific goal, but the heating rate should be controlled within the appropriate range, if too fast, it will produce thermal shock, the circuit board and components may be damaged; Too slow, the solvent volatilization is not sufficient, affect the welding quality. Due to the faster heating rate, the temperature difference in the SMA at the back of the temperature zone is large. In order to prevent damage to components by thermal shock, the general speed is 4 ° C /s. However, the rise rate is usually set at 1-3 ° C /s. The typical warming rate is 2 ° C /s.

2.2 Insulation Section

The insulation section refers to the area where the temperature rises from 120 ° C to 150 ° C to the melting point of the solder paste. The main purpose is to stabilize the temperature of the components in the SMA and minimize the temperature difference. Give enough time in this area for 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 entering the return section will cause various poor welding phenomena due to uneven temperature of each part.

2.3 Return section

The temperature of the heater is set high in this area so that the temperature of the component quickly rises to the peak temperature. In the reflux section, the peak welding temperature varies depending on the solder paste used, and it is generally recommended to add 20-40℃ to the melting point temperature 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, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature curve is the small area covered by the "zone" beyond the melting point of the solder.