Although Au/Sn solder performs like a braze in service, it can also be processed as a solder using flux. There is a caveat – not all fluxes can handle the high processing temperature required for eutectic Au/Sn solder alloy.
Ordinary rosin-based fluxes aren’t suitable as the rosin would be severely charred/caramelised by high process temperatures, in excess of 300°C. Specialized fluxes, with heat-stabilised resins that have higher breakdown temps with suitable activators, don’t char and also facilitate cleanability. An alternative to a resin flux is to use a water-soluble organic acid flux. These are available from a few specialised suppliers and can work well. Cleaning is usually easier than the resin materials, subject to the normal process constraints of water-soluble materials. Nitrogen as an inert atmosphere can be used in conjunction with the flux to enhance wetting and ease of cleaning of the flux residue. So, a good flux choice can allow the use of solder paste with its advantages - especially when handling multiple joint assemblies. The profile is shorter with a lower peak temperature (310-320°C) compared to forming gas (350°C). A flux with good wetting will have lower voiding.
Another option to using a flux is to employ formic acid. It breaks down completely, leaving no residues. As an acid it readily removes any oxide but is rapidly broken down to harmless CO2 and water during the ramp to reflow. This means we need a well-controlled nitrogen atmosphere to prevent re-oxidation, but no post reflow cleaning is needed. The acid is introduced to the oven by simply bubbling the nitrogen through it. Downside? The formic acid can be pretty fierce to the reflow oven and operators. Equipment has to be chosen accordingly. Formic acid is a kind of hybrid process between flux and fluxless. Compared to forming gas, heat transfer is not quite as efficient due to the lack of hydrogen, but no different to a fluxed process.