Copper and brass conduct heat away from the joint faster than stainless, so the two sides reach temperature at different rates. The stainless side can look ready while the copper side is still cold, and filler fed at that moment will wet one face and sit on the other.
The metals also differ in melting range and in how they respond to a filler. Copper alloys are generally easy to wet; stainless is not, and it needs the flux to work harder on one side of the joint than the other. The joint is asymmetric before any heat is applied.
Copper and brass expand more than stainless over the same temperature rise, so a joint that fits cold will change clearance as it heats. On a joint with a long land that difference is enough to close the gap at one end while opening it at the other, and a capillary joint needs an even gap.
The mismatch also leaves residual stress behind after cooling. A brazed copper-to-stainless joint is not usually highly stressed because the filler is compliant, but a long, rigid assembly can still crack at the interface if it is restrained while it cools.
Feed the filler towards the side that is harder to wet. In practice that means directing heat and filler at the stainless, letting the copper come up by conduction, and checking that the fillet forms on both faces rather than only on the copper.
If the joint is a tube into a fitting, orient the assembly so gravity helps rather than fights the flow. Brazing upside down is possible but leaves no margin, because a filler that starts to run will leave the joint instead of filling it.
The stainless face needs the flux most, and it needs the surface prepared properly. Oxide, machining marks holding oil, and residue from an earlier process will all stop wetting on that side while the copper side appears perfectly joined, which makes the fault easy to miss.
Residue removal matters more than usual here. Flux left against stainless in contact with copper creates a small, persistent galvanic couple, and moisture trapped in the joint will attack the stainless. Clean and dry the assembly thoroughly before it goes into service.
Once joined, copper and stainless in contact form a galvanic pair, and any electrolyte between them accelerates corrosion of the less noble metal. In a wet system that puts the copper side at risk; in a dry assembly the exposure is limited to places where condensation collects.
This is where the application decides the method. A copper fitting on a dry bracket is a different proposition from a copper connection on a vessel that holds water, where the joint line becomes the place corrosion starts. Where dissimilar metals meet in our fabricated components, the interface is either kept dry or isolated.
A brazed copper-to-stainless joint suits low-pressure connections and fittings that are mechanically supported. It is not a substitute for a welded pressure joint, and it should not be the only thing holding a connection together under load.
Where the assembly is structural or pressurised, design the joint so the brazing seals rather than carries, with a mechanical feature taking the load. The aluminium water tank range follows the same rule, because a wet interface between dissimilar materials is the one thing that cannot be tolerated.
