Principle of vacuum eutectic furnace

Vacuum melting system is a process welding furnace for high-end products, such as laser devices, aerospace, electric vehicles and other industries. Compared with the traditional chain furnace, it has great technical advantages. The main components of the vacuum eutectic furnace system include: vacuum system, reducing atmosphere system, heating / cooling system, gas flow control system, safety system, control system, etc.

Product principle

Compared with the traditional reflow soldering system, the vacuum welding system mainly uses vacuum to help the cavity discharge above the liquidus of solder paste / pad, so as to reduce the cavity rate. Because of the existence of vacuum system, the air atmosphere can be changed into nitrogen atmosphere to reduce oxidation. At the same time, the existence of vacuum also increases the possibility of reducing atmosphere.

Vacuum cavity removal

In atmospheric environment, air bubbles in solder paste / solder pad in liquid state / bubbles formed by flux are also under atmospheric pressure. When the outside becomes a vacuum environment, the air pressure difference between the two can increase the volume of bubbles in the liquid solder paste / pad, merge with adjacent bubbles, and finally reach the surface for discharge. Then, when the air pressure recovers, the remaining bubbles will become smaller and continue to remain in the system.

From the perspective of industrial production, the following points need to be pointed out:

In theory, absolute high vacuum can indeed reduce the void rate to a greater extent, because the pressure difference is the driving force of bubble discharge. Then pumping high vacuum takes a very long time, which needs to be considered in actual production. In addition, the time above the liquidus also needs to be considered. In fact, because the material surface of the production cavity is not completely flat, it will adsorb some gas and liquid substances to achieve absolute high vacuum, which is theoretically possible to some extent. The absolute void rate of 0% cannot be achieved, and it is impossible to ensure the complete removal of every bubble in production. Generally speaking, the requirement of the so-called low void rate is that the total void rate is less than 3% and the maximum void rate is less than 1%.

The addition of nitrogen atmosphere vacuum system can make the cavity add nitrogen atmosphere after vacuum pumping, which is also involved in traditional reflow soldering. However, the following points need to be pointed out:

The addition of nitrogen is to discharge O2 from the air to prevent oxidation. In the open environment of the reflux furnace, it is possible that O2 cannot be completely discharged. The industry believes that it is necessary to reduce O2 below 100ppm to ensure that there is no possibility of oxidation. Therefore, the closed system is a suitable system for the application of N2 environment. In addition to the presence of O2, temperature is also very important for the oxidation of metals. Therefore, when nitrogen protection is applied, the device temperature should be reduced to a certain temperature before the system can be opened and contacted with O2. For example, for the welding of DCB, it should be ensured that the oxidation can be avoided in N2 environment before the Cu surface temperature rises above 50c and the surface temperature drops to 50c after welding. Reducing atmosphere HCOOH formic acid and N2H2 syngas do not need reducing atmosphere in the use of solder paste due to the existence of flux. The vacuum step can reduce the void rate. However, considering the flux residue / cleaning cost, some manufacturers will choose to use solder pads with helpless flux. At this time, the use of reducing atmosphere is required. The reducing atmosphere can increase the wettability of the solder, so as to reduce the void rate from another angle.

For the common reducing atmosphere HCOOH formic acid and N2H2 syngas, the following points need to be pointed out:

The action principle of both is to react with metal oxides and reduce them to pure metals for further liquefaction. The reaction temperature of HCOOH formic acid and metal oxide is lower than 200C to form formic acid metal salt and water. Above 200C, formic acid metal salt decomposes into metal and H2O CO2. Therefore, HCOOH formic acid is suitable for alloy system with low melting point. The reduction reaction of H2 needs to be carried out at high temperature (250C), so it is not suitable for alloy system with low melting point. other

The main principle of vacuum eutectic furnace is to use vacuum to remove cavities. Nitrogen atmosphere and reducing atmosphere are additional conditions. Customers should choose according to their product requirements and economic level.

The existing vacuum systems are mainly divided into

The advantage of the traditional vacuum welding system is that it can not completely prevent the reflow of other modules in addition to the traditional vacuum welding system.

Single vacuum module with heating and cooling device: the advantage is that it can ensure the vacuum environment and nitrogen atmosphere. The disadvantage is that the heating and cooling are completed by the same heating and cooling plate, and the service life is short. Multiple vacuum modules, and a single module is only responsible for heating, welding and cooling steps: the advantage is that it can ensure the vacuum environment and nitrogen atmosphere. Different modules are responsible for different temperature zones, which is similar to the traditional reflow soldering multi cavity welding furnace, So as to ensure the output efficiency and quality to a greater extent. The disadvantage is that the price is high.

Online vacuum eutectic furnace: This is to realize preheating in nitrogen environment and then eutectic welding in vacuum environment. The equipment adopts plate heating, with high heating efficiency, low nitrogen consumption and oxygen content of 10ppm. It is very suitable for welding of higher quality products.