Hi Steve,

I've never tried to use oil as a solder reflow medium, though it is certainly a thought. I think an oil bath just might give you the main benefits of vapor phase reflow (by making certain that the thermal mass of the board is sufficiently insignificant relative to the thermal mass of the oil bath) without that whole "toxic vapors" issue. You could also avoid the problem of moving air shifting the position/alignment of parts relative to their pads (something everyone who has ever tried to reflow a board with a heat gun is familiar with). My one big reservation would be the effect of oils on the chemical processes (the flux action) during reflow.

But I have another application of oil in board fabrication. When fabricating multilayer boards, in the lamination process, putting the stackup in a heated oil bath that is hydraulicly pressurized produces a much more dimensionally stable board. If you've ever dripped mayo from a burger onto your shirt you're already familiar with the problem. A standard platten press squeezes the stackup in the vertical dimension (the z-axis) only. Thus, the inner layers tend to be squeezed out, more at the edge than in the center, much like the "fixins" in a sandwich, making pad-to-pad alignment at the edges problematic. Using a hyraulic bath of heated oil pretty much eliminates this problem by squeezing the stackup in all directions equally. Also, by containing the stackup in a vacuum envelope (bag) during the lamination process one not only improves outgassing (eliminating blowouts during solder reflow) but eliminates the need to clean a lot of oil from the board later, which certainly would deliteriously impact soldering.

Joe