Notice in Table 2 that there are strict requirements for the amount of oversized and undersized powder particles in each mesh size group. For example, to be classified as a -140C powder (“C” stands for “course”), the screened powder can only have a maximum of 20% -325 mesh powder. Thus, 80% of powder classified as -140C-mesh must be -140 mesh/+325 mesh, meaning it should all go through the 140-mesh screen, but 80% or more must sit on top of, and not go through, a 325-mesh screen. 

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BFM Powder Sizes

The -140 mesh powder size (either C or F; course or fine) is the size most often used for BFM powders. Notice from the chart in Table 2 that such a powder has a nominal particle size of approximately 0.004 inch (0.10 mm). The reader needs to understand that such a powder particle size is larger than the recommended joint clearance for most brazing, which ideally calls for a clearance of about 0.000-0.002 inch (0.000-0.050 mm). 

Putting such BFM powder INSIDE a joint would cause the gap clearance to become too large. There-fore, standard -140 mesh powder is typically not put directly inside a joint but is instead used in brazing pastes that are applied on the outside of joints. When the powder in that paste melts during brazing, the resultant liquid BFM can easily be pulled into, and through, the narrow joint by capillary action. 

If your process requires you to pre-place the BFM powder INSIDE a joint prior to brazing, then – looking at Table 1 – a powder size of approximately -325 mesh or finer (e.g., -400 mesh) would be preferred. Please verify this for yourself by studying Table 1. 

It is important to note that all BFM containers (powder or paste) should always show the particular BFM powder mesh size used in that product on their labels. If it doesn’t, you need to find out why that information is missing. Be sure your purchasing personnel always specify mesh size each time they order brazing powder or paste from their suppliers, and be sure it shows up clearly on the labeling and certifications received. Otherwise, brazing problems can occur in your shop. 

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Fig. 4. Two equal-size boxes with identical volumes. The one of the left is filled with -140 mesh powder, and the one on the right is filled with -325 mesh powder.