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The principle of vacuum soldering system is explained in this paper

Dec 16, 2019

Vacuum soldering system is a kind of process welding furnace for high-end products, such as laser devices, aerospace, electric cars and other industries, compared to the traditional chain furnace, has great technical advantages.

The main components of the vacuum eutectic furnace system include: vacuum system, reduction atmosphere system, heating/cooling system, gas flow control system, safety system, control system, etc.

The product principle

Compared to traditional reflow welding systems, vacuum welding systems mainly use vacuum above the liquidus to help the cavity discharge, thus reducing the void rate.

Because of the vacuum system, the air atmosphere can be changed to nitrogen atmosphere, reducing oxidation.

At the same time, the existence of vacuum also increases the possibility of reducing atmosphere.

Vacuum removal cavity

In the atmosphere, air bubbles in the solder paste/solder sheet in the liquid state/air bubbles formed by the flux are also at atmospheric pressure.

When the outside becomes a vacuum, the air pressure difference between the two allows the bubbles in the liquid solder paste/solder sheet to increase in volume, merge with the adjacent bubbles, and finally reach the surface to discharge.

Then the pressure recovers, and the remaining bubbles become smaller and remain in the system.

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

The absolute high vacuum (10-n mbar, as some manufacturers claim) does, in theory, reduce the void rate even more, since the pressure difference is the driving force of the bubble.

Then it takes an extremely long time to pump the high vacuum, which needs to be considered in the actual production.

The time above the liquidus also needs to be considered.

And the fact that the material surface of the production chamber is not completely smooth, will absorb some gas and liquid phase substances, and achieve an absolute high vacuum is theoretically possible to some extent.

An absolute 0% void rate cannot be achieved, and complete removal of each bubble cannot be guaranteed in production.

Generally speaking, the requirements for the so-called low void rate are the total void rate <3% and the maximum void rate <1%.

Nitrogen atmosphere

The addition of a vacuum system allows the cavity to be filled with nitrogen atmosphere after vacuumization, which is also involved in traditional reflow welding.

However, the following points need to be pointed out:

1 Nitrogen is added to remove O2 from the air to prevent oxidation. In the open environment of the reflow furnace, O2 cannot be completely discharged.

The industry says it needs to get O2 below 100ppm to ensure no oxidation is possible.

So a closed system is one that applies N2 environment relative to the appropriate system.

2 The oxidation of metals, in addition to the presence of O2, is also extremely important.

Therefore, in the application of nitrogen protection, the device temperature should be guaranteed to drop to a certain temperature before the system can be opened to contact with O2.

For example, for the welding of DCB, it is necessary to ensure that the surface temperature of Cu rises above 50C and the surface temperature drops to 50C after welding before completely avoiding oxidation in N2 environment.

Reductive atmosphere HCOOH formic acid and N2H2 syngas

In the use of solder paste, there is virtually no need for a reducing atmosphere due to the presence of flux.

The vacuum step can reduce the void rate.

However, due to the residual flux/cleaning cost, some manufacturers may choose to use a solderless flux.

This requires the use of a reductive atmosphere.

The reducing atmosphere can increase the wettability of the weld, thus reducing 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:

1 Both work by reacting with metal oxides and reducing them to a pure metal for further liquefaction.

2 The reaction temperature of HCOOH formic acid and metal oxide is lower than 200C, forming metal salt and water of formic acid. Metal salt of formic acid above 200C is decomposed into metal and H2O CO2, so HCOOH formic acid is suitable for alloy system with low melting point

3 H2 reduction reactions need to be performed at high temperatures (250C) and are therefore not suitable for alloys with low melting points.

Other

The main principle of the vacuum eutectic oven is to use the vacuum to remove the void, nitrogen atmosphere and reducing atmosphere are additional conditions, the customer should choose according to their own product requirements and economic level.

The vacuum system that exists today is divided into

1 Traditional reflow soldering + vacuum module: the advantage is that theoretically it can be improved in the existing reflow soldering production line, but the disadvantage is that other parts except the modules of the vacuum system cannot completely prevent oxidation.

2 Single vacuum module with heating and cooling device: the advantage is to ensure the vacuum environment and nitrogen atmosphere, the disadvantage is that the heating and cooling by the same heating, cooling plate, short service life

3 Multi-vacuum module, single module is only responsible for heating, welding and cooling steps: the advantage is to ensure the vacuum environment and nitrogen atmosphere, different modules are responsible for different temperature zones, similar to the traditional reflow multi-cavity welding oven, so as to maximize the output efficiency and quality, the disadvantage is that the price is higher.

In-line vacuum eutectic oven: this is fully realized in nitrogen preheating environment, and then in vacuum eutectic welding environment.

The equipment adopts plate heating with high heating efficiency, low nitrogen consumption, and oxygen content can be controlled to 10PPM.

Very suitable for welding of higher quality products.

Domestic vacuum eutectic oven manufacturers are few.

At present Beijing TORCH do well, and imported equipment gap is not big.

Many listed companies use their vacuum eutectic furnaces, and the overall market feedback is good.

Also obtained the national torch program industrialization demonstration project.

If you are interested in our products, please visit our website(www.torch.cc) or search vacuum soldering system on baidu to learn about our products.

 

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