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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