RE>>Fabrication 9/18/96
I think that the key here is that the OSP is being applied to a board with *electroplated* gold rather than *immersion* gold. (At least, this is the implication that I get!) I'd find it hard to justify using an OSP on an i.Au/e.Ni board, too.
Consider a board that's built with OSPs that has electroplated gold on the fingers. This combination makes sense to me. Someone earlier in this forum had a good warning with regard to avoiding a copper/gold galvanic couple during the OSP application process. I've also heard that the major OSPs differ in performance: some require masking of the gold fingers (or else there will be a "tarnishing effect") while others don't.
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]
--------------------------------------
Date: 9/18/96 11:23 AM
To: Greg Bartlett
From: [log in to unmask]
Scott
I cannot answer your question about the Enteck and the tarnishing, but I am
wondering why you have selective gold plating and the Enteck process.
The electroless gold plating can be used to ensure solderability of SMT
pads. We presently populate and reflow a PCB that is electroless gold over
electroless nickle. The gold plate ensures the solderability of the nickle
and the thickness of the gold plate is such that you do not have a gold
embrittled joint.. We have been running this in production for well over a
year with no solderability problems.
This process was chosen because we needed a nickle pad on the PCB that
contacted a nickle terminal within the housing(like metals) for wear
purposes. Originally the PCB was going to be the nickle plate with HASL
and the one pad would be masked off so it would stay nickle. The bare
board fabrication cost was less by using the nickle plate and then the gold
plate. The other process was lengthier and required masking of the one pad
during the HASL step and then removal of the mask. Since gold is a noble
metal, we can leave it on the contact pad with no problems and the gold
ensures the solderability of the nickle.
To re-ask my question--Why have a PCB with some pads gold plated and the
other Enteck, thus requiring more steps in the PCB fabrication when you
could have the entire PCB electroless gold plated and still have proper
solderability.
-------------
Original Text
From: C=US/A=INTERNET/DDA=ID/TechNet-request(a)ipc.ipc.org, on 9/18/96 7:30
AM:
From: Scott Westheimer
Date: 9/17/96
Sorry if my message of last night was not very clear, but what can you
expect at 2:30 am.
The issue is with entek coating not the gold plating. The panels have
selective gold plating on some pads and the others have entek. The problem
is that when the panels are first gold plated( electrolydicly ), then S/M
the entek pads are fine. However, when the panels are first S/M and then
gold plated the entek pads are tarnished around the edges. Does anyone have
experience with this problem.
Thanks,
PS Before anyone makes the comment, the boards are enticed after all the
processes have been completed.
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