Brazing commonly uses -140 mesh BFM powder for most applications. However, for some specialized applications, such as when powder needs to be sprayed onto heat-exchanger sheets that will be laid one on top of the other for subsequent brazing, a much finer mesh powder needs to be used, so that the powder particle size does not interfere with good joint fit-up during assembly. As can be seen from Table 1, the finer the powder, the higher the mesh size number, since finer powders require more wires per linear inch to screen them. Thus, a 325-mesh screen will produce much finer powder than a 140 mesh screen.
Note: Please be aware that the finer the mesh size of the powder, the greater will be its sensitivity to furnace atmosphere oxidation, since if you were to fill a given volume of space with -140 mesh powder, and then fill an identical volume of space with -325 mesh powder, the total surface area of all the powder particles in that volume of space will be far, far greater for the -325 mesh powder than for the -140 mesh powder. Therefore, since there will be much greater surface area exposed to the furnace atmosphere, it can be understood that in a marginal vacuum furnace atmosphere you might not be able to effectively braze with -325 mesh powder, because of the greater amount of surface area exposed to oxidation, even though you might have been able to braze okay with -140 mesh powder.
Recommendation: Check your inventory of brazing powder, or brazing paste. Does the container specify the particular mesh size for that product? It should ALWAYS do so! If it doesn’t, you need to find out why that information is missing.
Be sure your purchasing personnel are always specifying mesh size each time they order from their suppliers. Otherwise, brazing problems can occur in the shop.
As an example of this, there was a brazing job shop who did not specify mesh size when ordering their brazing powders, and therefore received different mesh sizes from their supplier over time, depending on what mesh size powder the supplier had in stock at the time they received the orders from that brazing shop. The brazing shop was upset when they got varying brazing results when using the different batches of BFM they were receiving, and complained to their supplier about the “bad” BFM they had received. When the BFM supplier explained the difference between mesh sizes, as far as surface area is concerned, and how finer mesh powders are much more sensitive to marginal atmospheric conditions, the customer learned -– by experience — that they needed to use a coarser mesh powder, such as the -140 mesh, to get better results in their particular vacuum furnace, and they needed to be sure to consistently specify that mesh size on all their subsequent orders.
Thus, if a company has a vacuum furnace with a poor leak-up rate (such as 40-microns per hour leak rate, for example), they may find it impossible to braze with -325 mesh powder because of the very high total surface area of the fine powder that is being exposed to all the oxygen and moisture in that marginal atmosphere resulting from that high leak-up rate. To greatly reduce the amount of BFM surface area exposed to the atmosphere, they need to go to much coarser powder mesh size. Obviously, they also need to find the source of their furnace leaks and fix them!
Chart is from Lucas-Milhaupt’s publication: “The Brazing Book” (courtesy of Lucas-Milhaupt).