The "wave soldering flux deactivation temperature question" arises every few weeks, and the easy answer is that wave solder fluxes are designed to see the temperature of the wave.
Now, in order to pass SIR testing, the test boards with the wave fluxes are sent through the wave pattern up, not in direct contact with the molten alloy. They still see the majority of the heat from the wave.
The “deactivation” temperature varies from process to process because of the differences in board complexity, flux application, and preheat, as it is really a matter of total energy input, rather than a specific temperature.
The potential for reduced SIR is the main issue, not necessarily ECM or corrosion. With a rosin-free flux, there may be some visual clues, as well. If you see that the flux has dried to a white, powdery residue that may be a sign that it had not seen temperatures.
We get this question mostly from corporations that believe they can use the same flux formulation for wave soldering and hand assembly / rework. Wave fluxes are not designed for hand soldering, and will more than likely cause some sort of downstream issue if used as such.
Feel free to ask me any questions about your process! From One Engineer To Another!