I am not aware of "fluxless" soldering using metallic solder (not conductive adhesives).

One should consider the other residues potentially on an assembly besides flux residues.

In addition to the elimination of post-reflow residue related product failures, the cleaning of electronic assemblies, while intended for the removal of flux residues, actually provides for the removal of other contamination species. While the emphasis of a cleaning program is flux removal it is important to consider the many other sources of potential contamination.

The purpose of a flux is to reduce oxide and fluxless soldering could involve (a) ultrasonic soldering to scrub the oxide away; (b) forming gas / formic acid to reduce oxide.

Ultrasonic hand soldering may be possible without a flux, but will need new equipment and training

To a use a flux and eliminate cleaning for hand-soldering:

• A no-clean flux can be used. If there are concerns that the no-clean flux is not completely activated, there are no-clean rework fluxes that pass SIR unactivated. This means that even if the flux saw no heat, there would be no reliability issue.

I do not know of such a thing and there are a number of potential problems the main ones are how to prevent the Nitrogen form leaking out an depleting the oxygen and causing asfixiation of the operators.

Second is how do you ensure that you have oxide free, contaminant free surfaces going into the soldering process.

Both of your requests are handled within the selective soldering process.

All manufacturers utilize minimal flux for a no clean process which is enhanced with pure N2. Check it all out by searching Google for "selective soldering."

Ultrasonic soldering has been used without flux for many years, this certainly works with Die attach how ever not sure how useful it would be on standard PCB design (localised damage to other components).

You can get Nitrogen re-work pens these should work on PWBs where solder still remains on pads and if there is a good finish on the components. How ever as there would be substantially less solder than would be normal and as the intermetalic would not be guaranteed then it would be difficult to confirm a good joint.

To have any form of reliability it would be better to use a flux in conjunction with the solder.

I do not know of any tool that has N2 pumped around a soldering iron tip to keep it clean. However Wirebond guys keep copper wire clean with a small enclosure around the wire when they flame off and form a ball.

I suspect you could rig something up without too much difficulty. The problem is going to be dragging around a pipe for the N2. Alternatively you could make a small glove box with a window and put the N2 into the box along with a regular soldering iron.