Hi Jeff,
In my RF background we were working in the CBand 2-6 GHz and KU band approx.
15GHz frequencies and at both of those freq's we avoided the use of any
nickel plating at all. Nickel is very lossy to RF and should be avoided
according to the RF EE's I have worked with...
We actually didn't use normal solder mask either. We chose to put 'solder
dams' or small 5 mil lines of solder mask across the RF line where we were
soldering a pad to limit the wetting to the pad and to minimize any solder
mask covering the RF conductors.
We had good success using OSP and just letting the copper oxidize after
assembly... oddly enough, the copper oxide is not lossy to RF and is self
healing. If you break the oxide barrier, it just oxidizes again and seals up
the opening... Solderability for rework requires a good flux or copper
brightener chemical much like you would clean the copper bottoms on your
pots and pans at home. The discoloration is largely cosmetic. I guess that's
why people used sheets of copper for roofing material... :)
Just some food for thought...
OSP can be a good alternative for a flat finish... there are some handling
issues and shelf life issues but if you keep them individually sealed and
not stored for long periods of time then it might be good for you to try it.
ENIG works well for freqs below 1GHz so I am told... The higher the freq and
especially with lower power signals, you will need to move away from
nickel/gold preparations for the surface of the board.
One of the best substitutes, I would think, would be silver. I know we used
it in many of our high powered RF filters and such, and it's oxide has the
property of still being very electrically conductive. Perhaps, Silver
immersion would work in your application? Just a guess, as I have not done
much research in that area. Perhaps there are others who have that can
comment...
Happy Holidays...
Bill Brooks - KG6VVP
PCB Design Engineer , C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
e-mail:[log in to unmask]
http://www.dtwc.com
http://pcbwizards.com
-----Original Message-----
From: McGlaughlin, Jeffrey A [mailto:[log in to unmask]]
Sent: Thursday, December 02, 2004 5:06 AM
To: [log in to unmask]
Subject: [DC] ENIG on controlled impedance coplanar wave guide
How does a nickel-gold finish impact the way you calculate the trace and
space requirements for constructing a coplanar waveguide? I have a
calculators for these structures but they assume the conductive features
to be copper. I know, from empirical measurements, that solder plating
has very little effect on these designs, but due to mechanical flatness
requirements I need a finished surface with less than 0.0003" of
roughness which can not be produced by HAL.
Jeffrey A McGlaughlin, CID
Engineering Designer
Battelle Memorial Institute
Columbus Ohio
http://www.battelle.org
[log in to unmask]
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