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 places during the process execution:

  • One or several evacuations of chamber followed by filling with pure nitrogen gas help 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 cleaning step that goes faster compared to purging the chamber with nitrogen.
  • Using vacuum while the solder is in molten phase helps for removing the voids.
  • In some applicaitons when hermetic encapsulation is required we need to cool down in vacuum.

Void-free solder joints

Critical for removing of voids is that we evacuate the chamber once solder starts melting. At this point the voids start to expand in the form of bubbles and leave the molten solder.

Once this bubbling out is complete we fill up the chamber with nitrogen that presses both surfaces against each other.

Our positive pressure configuration of VSU28 model enables even further pumping the chamber. Thus the voids' size is even extra reduced.

xray image of void free soldered power semiconductor

Fluxless reflow soldering using 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.

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 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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