Zinc from the brass diffuses into the solder and reacts with tin causing dezincification.
Dezincification is a specific type of de-alloying, or selective, leaching corrosion of brass fittings. This type of corrosion selectively removes zinc from the brass alloy, leaving behind a porous, copper-rich structure that has little mechanical strength. This weakens the integrity of the brass fitting. [2nd paragraph excerpted from and editing of a USEPA report]
So, you need a barrier plate between the brass and the tin of the solder. Use a flash of copper or nickel-plating as a barrier between the brass and tin. Nickel has a longer shelf life, but difficult to solder. Copper is easier to control and solder. So, we suggest that you use copper.
IPC-HDBK-001, 5.1.2 Barrier Layers: Brass is an alloy consisting mainly of copper and zinc. Without a barrier layer of copper or nickel over the brass, the zinc will dissolve into the molten solder during soldering, zinc contamination of the solder joint is evident by a rough, grainy, frosty, porous surface with a high dendritic structure. Also, solder wetting to the terminal would be inhibited.
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