Clean surfaces before anything else
A brazed joint is only as good as the surface preparation behind it. Both the tube and fitting need to be cleaned of oxidation and oils before assembly, since even a thin contaminant layer prevents the filler alloy from properly wetting and bonding to the base metal — leading to a joint that looks sound but has poor actual strength.
Fit-up and gap control
Brazing relies on capillary action to draw molten filler into the joint, which means the gap between tube and fitting has to fall within a narrow range — too tight and the filler can't flow in, too loose and it won't bridge the gap by capillary action alone. This is a fit-up detail that's easy to get wrong with worn tooling or inconsistent tube sizing.
Heat control and nitrogen purging
Even, controlled heating — bringing the whole joint area up to temperature rather than overheating one spot — is what produces a consistent fillet. For refrigeration and AC assemblies, purging the inside of the tube with nitrogen during brazing is standard practice: it displaces the oxygen that would otherwise form black oxide scale inside the tube, which can break loose later and contaminate the refrigerant circuit or block a filter drier or expansion valve.
Common failure signs to watch for
A poorly made joint often shows visible clues: a dull or pitted fillet, filler alloy that hasn't flowed fully around the joint, or scorching on the tube from excessive heat. Any of these on inspection is a reason to reject the joint rather than accept it — a joint that looks marginal at the bench is one that's more likely to leak once the assembly is in service.