We're having trouble soldering brass parts plated with electro-tin plate, bright. Is the "bright" finish part of the problem? What is the best finish to call out? Thanks!J. Willis
- - MDS
Dull Tin (no organics added) remains solderable longer, but fingerprints badly, so it depends.
Other possibilities: The bright tin bath may need a carbon treatment, or your preplate cycle may not be leaving a clean base metal. Do you strike first with cyanide copper ? Are you etching the brass before plating? I was going to write "overetching", but you should not etch at all, the brass should stay bright in the preplate cleaning step, the cleaners could be too hot, the acid could be too concentrated.
You may try analyzing the tin plate bath for Copper, Zinc, Iron, etc. contamination. These contaminants are usually detrimental if over 500-1000 PPM.
How long between plating and soldering? If parts have been on the shelf for an extended period, especially if at elevated temperature or for more than six months, or if there is no barrier plate between brass and tin, zinc and/or copper diffusion can poison the tin. This usually shows as a dark disoloration of the tin deposit, but might also affect solderability. If freshly plated, ignore this line of reasoning.
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Another problem you may have is conversion of the metal tin plating to intermetallic. If the plating is mostly intermetallic, solderability is greatly diminished.
Intermetallic conversion is a time and temperature dependent process. Has the material been plated for a long time, i.e. many months? Have the parts been through any elevated temperature processing after plating? Either of these factors will contribute to formation of intermetallics in the plating. Intermetallic formation can be prevented in most cases by a nickel flash between the copper or brass and the tin.
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