Reflow soldering under vacuum

inside the vacuum reflow oven chamber

Soldering under vacuum is a technology that provides excellent results for all soldering applications. Vacuum is used in several stages during the process execution:

  • One or two evacuations of chamber followed by filling with pure nitrogen gas helps to remove any residual water and oxygen from the chamber.
  • A deluted atmosphere during cleaning step enables faster transport of the formic acid molecules enabling in-depth cleaning of surfaces.
  • Vacuum can be used to remove the residues after a cleaning step. This takes less time compared to purging the chamber with nitrogen.
  • Using vacuum while the solder is in molten phase helps removing the voids.
  • In some applicaitons when hermetic encapsulation is required we need to cool down in vacuum.

Void-free solder joints

Evacuating the chamber, once the solder starts melting, is critical for removing of voids. At this point the voids expand in the form of bubbles and leave the molten solder area. As soon as this bubbling out is complete we fill up with nitrogen followed by cooling. This single chamber evacuation results in minimizing the void rate to under 5%.

In applications where solder area is extremely large, e.g. IGBT, the expansion force of the bubble is not enough to overcome the capillary force of the molten solder. Bubbles can not escape the molten solder area and remain locked inside it. To minimize the void rate in this case we apply positive pressure right after the vacuum step. Pressurizing the chamber 2-3bar over the atmospheric leads to further squeezing of the bubbles. This results in a very good voids removal even for large area soldering. See "Positive pressure" in product options.

x-ray of atmospheric process

Atmospheric process

x-ray of vacuum process

Vacuum process

Fluxless reflow soldering using formic acid

oxidized copper pad

Oxidized copper pad before the treatment

oxygen reduced with formic acid

Oxygen film reduced with formic acid

The possibility to use formic acid (HCOOH) in combination with nitrogen results in a cost-efficient 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.

Formic acid vapours remove the oxide films, which inhibit wetting. The formic acid is applied by a bubbler. Nitrogen is blown through the bubbler, where it is enriched with formic acid and released into the chamber. The bubbler is integrated as standard in all the ovens in our product range.

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 purged 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

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.

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

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