Brazing, when performed correctly, is a joining process that produces a permanent bond between two or more materials by heating them to a temperature above 450C (840F), but lower than the melting-temperature of any of the materials being joined, and a permanent, metallurgical bond between these materials is produced when capillary action draws a molten brazing filler metal (BFM) through the clean, closely fitted faying surfaces of the joint.
The filler metal is not supposed to become fully liquid (i.e., have a “liquidus”) until the brazing temperature reaches at least 450C (840F). If the liquidus of the filler metal is below 450C (840F) then that filler metal would commonly be called a “soldering alloy”. People often wonder about the temperatures used to differentiate brazing from soldering. Why 450C (840F)? Is there some significance to these “exact” numbers?
Therefore, stop-off compounds, which are blends of metallic-oxides that are packaged in a variety of forms: liquids, pastes, powders, or tapes (to name just a few), can be very effective at preventing molten BFM flow into areas protected by the stop-off.
In reality, the answer is “no”. The numbers selected have no specific metallurgical meaning, other than that they represent a convenient demarcation point between the melting temperatures of zinc (420C/787F) and aluminum (660C/1220F).
A number of years ago, the definition of brazing specified a temperature of 800F as the demarcation point between “brazing” above that temperature, and “soldering” below that temperature. But of course, since the world continues to “get smaller”, so to speak, via closer international communications and via faster air-travel, it was decided by the brazing community that the temperature for brazing really should be specified using standard metric terms, since metric is the world standard. In so doing, they agreed to declare that the “standard” minimum liquidus temperature for a BFM would now use a conveniently rounded-up metric number of 450C, rather than the previously used 800F (420C), and in one quick step brazing jumped from 800F up to 840F (officially, 450C). Of course, it’s possible that the brazing community could have said: “Let’s use 500C as the demarcation temperature”, and that would probably have been fine, too.
Thus, this choice of temperature at which soldering ends and brazing begins is really just a number “pulled out of the air”, somewhere between the melting points of zinc and aluminum, to provide a basis for being able to differentiate between the two joining processes.
Brazing used to be known as “hard soldering”, and “soft soldering” was the term used for what we merely call “soldering” today. In many places, especially among some of the older jewelry makers, the term hard-soldering and soft-soldering is still being used.
So now you know “the rest of the story”, and will hopefully understand that the temperature used to separate soldering from brazing does not really have anything to do with a specific metallurgical reaction occurring at that 450C (840F) temperature, but was merely agreed upon for the convenience of differentiating between the two processes.