Denis Mori asked:

>     We are planning to use OCC boards in assemblies that require the
>     installation of a bulkhead. The bulkheads are normally mounted to the
>     boards with a screw and nut. The mounting tab of the bulkhead contacts
>     grounding pads on the board.

I'm not sure what OCC is, my inference is Organic ? Coating...

>     With HASL, there has not been an issue with using Zn-plated bulkheads
>     or screws. With OCC, we are concerned that the use of Zn may cause
>     electrolytic corrosion.

I'll guess you mean finished steel faceplates or enclosures for circuit
board assemblies.

Zinc-chromate has some nasty tendencies to form dendrites which can vibrate
or blow free and become conductive foreign material. This can occur in dry
air with no electrolytic acceleration. Japanese telecom manufacturers will
not use zinc-chromate finishes in enclosures, faceplates, or stiffeners for
this reason.

As to whether it is prone to corrosion, zinc is much more anodic than
copper or tin. Tin is actually more anodic than copper.

The series is like this:

ANODIC

Magnesium
Magnesium alloys
zinc
aluminum
aluminum alloys
cadmium
iron or mild steel
cast iron
stainless steels, active
tin-lead alloys
lead
tin
nickel, active
brasses
copper
bronzes
copper-nickel alloys
silver solder
nickel, passive
silver
stainless steels, passive
titanium
gold
platinum

CATHODIC

>     Has anyone tested OCC with Zn-plated bulkheads or attachments? Has
>     anyone experienced corrosion in such a case?

I have tested zinc-chromate finished CRS hardware for dendritic corrosion
at elevated temperatures (low humidity) and found great variation
apparently related to the stress of the plating. We tried several types of
zinc, and had best results with those plated with cyanide baths (not too
popular anymore for enviro reasons).

I would recommend you stick with a nickel finish if at all possible, this
gets you as close as possible to the galvanic potential of copper or
tin-lead, and it is a more attractive and durable finish. You will likely
find that it is more expensive, and that some metal finishing operations
will have difficulty maintaining the nickel bath well - but nickel is more
durable, more conductive, and offers better RF shielding than most other
finishes for steel.

Check out the faceplates on EISA or PCI bus boards. The better ones all use
nickel finished steel.

Zinc plating must have chromate conversion to offer any corrosion
protection, and the chromate is inherently dull. Zinc chromate is normally
organo-metallic, and not a very good conductor, so you may have some
difficulty with grounding (assuming you want the bulkhead to be a shield).
Many metal finishers use thin chromate finishes with organic additives to
make it "bright". This brightening will detract from the long range
protective value of the metal finish, however. IMO, if you must use zinc,
go with a low stress finish, and use as thick a chromate as your
conductivity needs will permit. Do not try to get a smooth, bright finish
unless you have little need for corrosion protection of the base metal.


regards,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com