QUESTION: We are facing an outgassing issue arising from the brazing filler metal (BFM) binders in our brazing pastes and tapes. Currently, we do a separate oven baking run at a temperature of 450°C to volatilize the binders before we send the parts to our brazing furnace. Even with this preliminary bake-out, our brazing people tell me that they have to limit the number of parts they can braze in each furnace run due to the continued outgassing issue. Some binder manufacturers say their binders minimize outgassing, and we do not need any baking at all. Perhaps air baking for 4-8 hours is enough even if many parts are loaded? Is this true with your experience? Do you think we can do the brazing without a separate oven bake-out? 

ANSWER: All brazing binders contain organic compounds and water that must be outgassed during heat-up so that they will not contaminate the joint or leave lots of voids in the joint when those binders volatilize. This volatilization occurs typically between about 300-450°C, depending on the specific chemicals used by your suppliers when they make their binders.

Please note that all such binders are proprietary, and none of the manufacturers will tell you the names of each of their ingredients. Each binder system is unique, and you probably need to experiment with different binders from different manufacturers to find out which ones work out best for you in your particular furnaces.

Most of the outgassing problems depend on the ratio of BFM powder to the binder. If you specify to your supplier that you want an 80/20 ratio of metal-powder to binder, please remember that that is always specified by weight! The BFM powder is much heavier than the binder, and an 80/20 ratio of metal to binder might actually work out to be about 50-50 ratio by volume of metal and binder (it can vary considerably). 

So, if you specify a 90/10 ratio, you would have less binder to volatilize, However, if you specify a 70/30 ratio or a 60/40 ratio in order to make the paste more and more “extrudable,” then the actual volume of binder is getting larger and larger and you will have a lot of potential trouble in being able to properly outgas (volatilize) the binders from the paste during the brazing cycle.

Please note that I do NOT recommend that anyone ever get a separate oven to bake-out (volatilize) the binders before moving the product into the brazing furnace! That is very costly, time-consuming and should never be necessary!
Heat more slowly. I do NOT recommend rapid heating rates in brazing furnaces, but I DO recommend using a much slower ramp-up rate. This will allow the paste binders to volatilize completely from the paste before the parts reach the high temperatures of the brazing cycle. When done properly, you should be able to volatilize all the binder from the paste without any real problems.
Limit the amount of paste used. Brazing shops typically apply much more paste than they need to accomplish the braze, and they are therefore introducing far more binder into the furnace than they need.
CONCLUSION: Check the ratio of BFM powder to binder that you are specifying to your supplier, and see if you are asking for a paste with too much binder in it. Also, check the amount of BFM paste being applied to each of the brazed joints. Typically, people put on too much BFM paste. Then check that your furnace heating rate is slow enough to eliminate the need for any “holds” along the way up to brazing temperature. Using a slower ramp will allow the binders to volatilize much more easily and much more completely without the need for built-in “holds” in the ramp-up heating cycle.