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Solderability problems gold over nickel over copper

We barrel plate some brass contact pins with 50 microinches hard gold over 100 microinches min. sulfamate nickel(non-brightened) over a cyanide copper strike. The plated part is required to pass a Solderability test per MIL-STD-202 method 208. We fail this test periodically due to De-wetting. The nickel bath is dummy plated continuously to keep down the copper, and has a carbon cartridge(10") running continuously, replaced every 3 weeks. The gold bath also is continuously carbon treated.

Can anybody provide some clues as to the possible causes for this problem and/or some good information source. Please note that the parts show excellent adhesion and pass nitric acid vapor porosity tests. Can organic contamination via a treated closed loop recycle rinse system cause de-wetting on nickel? Any help would be great.

David Muliyil
- Mesa, Arizona
1999

Hey David,

Your process (step-wise) is good. Let's look at the steps. The cyanide copper strike is the perfect first step for adhesion and sealing in the zinc (from the brass). But, the word strike means color (coverage) and not thickness. This step should be at least 50 microinches thick (measured at the thinnest point) to prevent the zinc from impacting the solder test. Organics in the plating solution at this point can also cause de-wetting (add carbon). Make sure that the nickel plating is thick enough 100 microinches plus 2x the standard deviation (about 130-150 microinches). In the nickel, continuous carbon treatment is a very good idea but one 10" tube changed every three weeks will not get the job done. Closed loop systems can cause a build-up of contaminates (both metallic and organic) in the nickel. Dummy plating is also a very good practice. Check your copper PPM (send out samples for testing) to be sure you have enough area and/or amps to remove the copper (from time to time get the parts up from the bottom of the tank). (You can test for de-wetting after each process by removing a few parts after each step to pin-point the problem). One last note: the carbon tube used in the gold bath can hold up to one oz of gold, so reclaim. And, the old solder also has gold in it, so send it out for reclaim (old solder and flux can be a (rare) source of de-wetting).

Regards,

Fred Mueller, CEF
- Royersford, Pennsylvania
1999