Hi, Bill,
I may be wrong, copper is one of those metal produce un-stable oxide under
normal environmental condition (just look at the green roof of the old
building). If the PWA is not protected (coating or upper level assembly),
the environmental effect of corrosion may occur. On top of that, Cu is high
on the electrochemical activity side. Any coupling (assume your connector
mate with some other surfaces, hopefully, not gold) may cause autocatalytic
corrosion or anodic corrosion with presence of moisture. Copper's surface
diffusivity is very fast, I have seen some exposed copper migrated along the
edge of the gold pads (poor Ni/Au coverage), and "ate" the gold area during
reliability testing. Plated surface always not as clean (ionics allowed
according to IPC is 10 ug/in2?) as the PVD deposited Cu surface. I would be
careful about the exposed Cu under plated surfaces(not worry too much about
the PVD deposited Cu, such as used on chips/ceramic substrates)....Getting
old, can't handle any risky excitment.
jk
At 09:29 AM 5/30/00 -0700, you wrote:
>David,
>
>At the risk of creating a debate.... (which I think is always a good
>thing)... I have made PCB's without any protection for the surface of the
>copper clad... and had them last for many years. The issue of having exposed
>copper on a PCB is perhaps overzealously promoted by many designers and QA
>inspectors.. Copper oxidizes when exposed to moisture and air and seals
>it's surface with copper oxide which provides a barrier against further
>oxidation. Having an exposed strip of copper at the edge of the gold plated
>fingers on a card edge connector present no risk to the PCB in most
>environments. The copper at the edge will oxidize and seal itself. Obviously
>if the environment into which the PCB is placed is caustic or harsh,
>protections will be needed. However, gold plated edge contacts would not be
>my choice of contact, in that event.
>
>The real reason for the gold plating is to have a clean mating surface for
>good electrical connections in a repeated fashion. Obviously the gold over
>nickel plating does this job superbly. The copper at the edge where the
>chamfer is... plays no actual roll in the connection process. There is no
>violation to any IPC spec that I am aware of.
>
>Clean pristine copper is only necessary to provide good wetability of
>solder. Many companies are using OSP (organic solder preservative) instead
>of solder coating the boards to keep the surface ready for solder. Even in
>RF applications, a copper board with no solder or mask will present no
>degradation to the RF signal just because the copper has been allowed to
>oxidize on it's surface. I have seen this demonstrated in the CBAND
>frequencies with no problems. I can't vouch for higher frequencies, because
>I have not demonstrated that yet...
>
>So, I would suggest, that the copper that you are seeing at the edge of the
>connector is really not a problem. Of course, I welcome other opinions...
>
>
>
> -------------------------------------
>| Bill Brooks, Sr. PCB Designer
>| Zoneworx, Inc.
>| 40925 County Center Drive, STE 200
>| Temecula, CA 92591
>| [log in to unmask]
>| www.zoneworx.com
>| Tel: (909) 296-1226 x 1037
> -------------------------------------
>
>
>-----Original Message-----
>From: ROMERO, DAVID [mailto:[log in to unmask]]
>Sent: Thursday, May 25, 2000 1:56 PM
>To: [log in to unmask]
>Subject: [TN] IPC standards
>
>
>I was hoping to get some help from you IPC standards experts out there.
>
>I was looking for the IPC standard that is violated when copper is exposed
>on a gold finger card edge connector. The copper that I am seeing is a
>result of a chamfer that cuts an internal layer of the PCB. The spec is for
>.035 material removed along gold finger and .020 removed in the PCB's
>thickness. I expect analysis will prove that this spec is being violated.
>I know that exposed copper is a violation but I cannot find a standard that
>proves this in this scenario.
>
>On a similar note, when ordinary gold fingers are made correctly and have a
>chamfer, copper is exposed when the chamfer cuts into the gold finger
>plating. If this is a correct statement, why is this not a violation due to
>corrosion of the exposed copper?
>
>Thanks in advance
>
>David Romero
>
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