There is an important guideline about brazing filler metals (BFMs) that needs to be followed when performing any kind of brazing — namely, it is very important to feed brazing filler metal (BFM) into only one side of the joint to be brazed, allow that molten BFM to be pulled completely through the joint by capillary action until the BFM is visible at the other side of the joint, and can be seen to produce a complete braze-fillet (meniscus) around (or along) that opposite end of the joint.
As shown in Fig. 1, a BFM wire is being hand-fed into one end of a joint being brazed (gap is shown much thicker than desired only for illustration purposes), and the BFM flows all the way to the other side of that joint where it can be inspected for complete pull-through. This is an example of proper brazing in which “you feed from one side, and inspect on the other”.
It is never wise to feed BFM from both sides of the joint, since by so doing you can trap air, moisture bubbles, outgassing materials, etc., inside the joint between the two incoming walls of molten BFM, thus greatly increasing the void content in that joint. Fig. 2 shows an example of a cross-section photo of a round electrical contact that was brazed onto a flat surface, where the BFM was applied completely around the disc-shaped part that was to be brazed to the substrate below it. When the BFM melted and tried to flow into the joint, it could not do so since the air and other contaminants inside the joint could not escape to the outside since the “wall of liquid BFM” was blocking it. Thus, only a fillet formed around the outside of the joint but did not flow into the joint, as shown in that photo in Fig. 2.