What causes braze splatter?

Why do you get braze splatter in furnace brazing?

I was asked by a customer recently why he might be experiencing braze splatter on his parts in the furnace.

I'm sure most people reading this will have experienced braze splatter for themselves and also probably noted that it only seems to occur when using "wet" braze filler metals in paste form. You do not see splatter when using dry BFM forms such as tape, wire or foils etc.

In our experience splatter is likely to be caused by one or more of a number of reasons:

• Cleanliness of joints - the first question to ask for braze issue - has the joint been cleaned sufficiently prior to brazing?

• Consistency of paste - where workshops are mixing paste by hand on site you need to ensure the right ration of powder to binder is being used and also that the paste is being sufficiently mixed to ensure equal distribution of powder particles and elimination of any air pockets. Where the powder is being inconsistently or inadequately mixed you may find pockets of the paste violently de-gassing causing splatter.

• Quality of powder - we have seen powder of variable quality with hollow cores. These hollow cores can violently de-gas at temperature and explode like popcorn. This will cause splatter.

• Allowing sufficient dry time prior to brazing - our binder is water based and so the paste needs time to be allowed to dry before entering the furnace. If the paste is still wet when it enters the furnace it may rapidly de-gas as per above. Some binders take longer to dry and some don't ever really dry.

• Thickness of bead - for the same reasons as above if the bead applied is too thick this may mean that the paste will not properly dry, melt or flow when it goes into the furnace. Again this can cause splatter.

  
  

The obvious question is what to do about this?

• We wont' lecture anyone on cleaning parts, all reputable furnace operators are doing this already.

• We recommend a ratio of between 85/15 and 90/10 when it comes to powder/binder by weight. If you are mixing at 60/40 then you have a lot more binder that needs to burn off sufficiently in your heat cycle before you hit braze temperature.

• Consider your heat rate - if you are heating too rapidly you do not allow time for any gasses to gently escape. You could be forcing any water to convert to steam too rapidly which will cause the splatter.

• Consider pre-mix pastes - pastes that are supplied by the manufacturer have been made under factory conditions and ensure consistency in blend and composition. Operator variance in mixing or formulating, especially if measured by "eyeball", can be key to issues happening in the braze cycle. Careless loading of a syringe may trap air-pockets. Likewise a heavy or inconsistent hand on the syringe will lead to an uneven or too heavy application of paste. Consider semi-automated airlines that deliver a consistent dose every time.

In summary preparation is key. Train the operators and remove the "human" element where possible for consistency. Check for any air-gaps in your syringe. Allow your paste to dry properly before going in the furnace, we recommend 30-60 minutes with our binder dependent upon thickness. Consider your heat rate - too fast and it may cause the violent de-gassing.