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Application of reducing atmosphere in vacuum sintering furnace

The reducing atmosphere processes commonly used in vacuum sintering include nitrogen process, formic acid process and nitrogen-hydrogen mixed gas process. The industry believes that when the oxygen content in the cavity is reduced to less than 100 ppm, it can ensure that no oxidation occurs during the reflow process. Therefore, usually in the first four steps of the reflow curve, two vacuum-nitrogen backfill processes can effectively reduce the oxygen content in the cavity and prevent oxidation.

Before the end of eutectic or vacuum sintering, in order to avoid oxidation of metal solder, metal shell or metal substrate, a low-temperature environment is extremely important. Therefore, before the end of the process, the product temperature should be guaranteed to drop to a certain temperature before the chamber door can be opened. . For example, for welding of DBC plates, it should be ensured that the Cu surface temperature drops below 50°C to avoid oxidation.

In the reflow process of solder paste, due to the presence of flux, there is actually no need for a reducing atmosphere. The vacuum step can achieve the effect of reducing the void rate. However, considering the cost of flux residue/cleaning, etc., some manufacturers will choose to use flux-free soldering flakes. At this time, the use of reducing atmosphere will be necessary.
The reducing atmosphere can increase the wettability of the solder sheet and reduce the void rate from another angle. Regarding the common reducing atmospheres HCOOH formic acid and nitrogen-hydrogen mixture, the following points need to be pointed out: the principle of action of both is to react with the oxides on the solder surface to reduce the solder to pure metal for the next step of liquefaction.

①The reaction temperature of HCOOH formic acid and metal oxide is lower than 200℃, forming formic acid metal salt and water. Above 200℃, formic acid metal salt decomposes into metal, H2O and CO2, so HCOOH formic acid is suitable for low melting point alloy systems. When formic acid is injected at 180°C, the formic acid produces H2 through the dehydrogenation mechanism, and H2 hydrogenolyzes the organic matter in the large molecular chain in the solder into the organic matter in the small molecular chain through the hydrogen catalytic mechanism. The organic matter in the small molecular chain will continue to circulate during the sintering process. Evaporate out, thereby achieving densification of the solder. When formic acid is injected at 280°C, formic acid generates CO through a dehydration mechanism, and the solder does not densify significantly. The sintering mechanism of solder in a formic acid atmosphere can be divided into three stages: the volatilization stage of low-temperature organic substances such as diluents and dispersants (25-180℃), the hydrogenolysis stage of high-temperature binders of macromolecular chains (180℃), and the The volatilization of organic matter and the densification stage of silver nanoparticles (180~280℃).

②The reduction reaction of nitrogen-hydrogen mixture needs to be carried out in a high-temperature (250°C) environment, so it is not suitable for low melting point alloy systems.
The main principle of the vacuum sintering furnace is to use vacuum to remove voids. Nitrogen atmosphere and reducing atmosphere are additional conditions. Customers should choose according to their own product requirements and economic level.