Fluxless Soldering using Formic Acid

Fluxless Soldering using Formic Acid


Soldering under vacuum is a technology that provides excellent results for all soldering applications. The rapid cooling of the molten solder gives extra durability to the soldered connection. Our unit is the first on the market to provide both simultaneously – i.e. you can rapidly cool your soldered connections while they are still under vacuum.

In addition, the possibility to use formic acid (HCOOH) in combination with Nitrogen results in an economic and stable soldering process, reducing oxide film formation while no fluxing agent is needed in the process. The formic acid surface activation provides good results with respect to void rate across the surface.

x-ray image of vacuum soldered DBC substrate for power device

Low void rate using vacuum during reflow. X-ray image of vacuum soldered DBC substrate for power module.

The formic acid (enriched with Nitrogen) removes oxide films, which inhibit the soldering process. The formic acid is applied by a bubbler, integrated in the machine. Nitrogen is blown through the bubbler, where it is enriched with formic acid and released into the chamber.

How does the chemical reaction work?

Metal Oxide + HCOOH -> Metal + CO2 +H2O

At temperature over 150-200°C the metal oxide film on the surface  is reduced to clean metal surface. The gaseous products CO2 and water are simply flushed with dry nitrogen and exhausted to the atmosphere.

The advantages of this process are the following:

– safety engineering is less complex compared to processes using hydrogen,
– it is cost efficient,
– it is easy to control (our programmable controller provides very high accuracy of execution),
– it is unnecessary to use fluxing agents,
– no oxidized surfaces,
– no voids.

This video demonstrates the influence of the formic acid nitrogen mixture on reducing of surface oxide film before the melting phase. The formic acid is acting like flux but is in gas phase. The result is a homogeneous soldering free of any flux residue.

An excellent void free results can be achieved with a combination of formic acid pre-cleaning and vacuum during the melting phase.