Vacuum soldering of lead free alloy

A. The best way to treat the surface and to keep the substrate clean from any flux, will be to use a vacuum soldering furnace which will support a cleaning step with formic acid.

On a pressure of about 10 E-1 mbar and a substrate temperature below the liquidus of the solder, formic acid will be feed into the vacuum chamber from a bubbler using nitrogen as a tracer gas. The formic acid will react with the substrate surface and removing all oxides.

Even if you would not use any soldering past, but preforms without flux, you will get a perfect wetting of the surfaces and a voidless connection.

If there is any specific question, please let me know.

Best regards

A. Hi, Jeff. Hopefully someone experienced in this will try to help, but your question sounds rather vague to me :-)

Regards,

A. Use of formic in the way described is a normal procedure in preform soldering where post solder cleaning is an issue. The acid will clean surfaces, but is totally dissipated on soldering and broken down to H20 CO2 leaving no residues. Spatter of solder is not therefore an issue. So that's handy.
Disadvantages are that it only works where it touches (i.e. possibly not on touching surfaces), and can be pretty fierce on equipment. Fluxes do not have these problems, but may need to be removed, depending on flux application
Use of SAC on gold surfaces may be a problem depending on the gold thickness.
Hope this helps.
Mike