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May 2005

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Subject:
From:
"Brooks,Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brooks,Bill
Date:
Mon, 16 May 2005 08:34:44 -0700
Content-Type:
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You comments confirm my experiences as well.
 Working in the 2 to 6GHz range we used OSP to protect the board until
soldering time, and afterward the bare copper was the only finish. This was
on Rogers' 4003 material. Solder mask is also very lossy to RF so we reduced
the amount of masking to a small strip of soldermask we called a 'solder
dam' that prevented the wicking effect of the trace to pad interface during
reflow but did not cover the whole board with soldermask. After assembly the
board was cleaned and allowed to oxidize normally, which of course protects
the copper after it oxidizes, and is a self healing natural protectant for
copper. Why do you suppose they used it for roofing and piping in homes?
 The EE that was doing the simulation for the circuit remarked that the
nickel under plating was very lossy to RF and that copper oxide on the
surface would not affect the RF signal at all. The signals we were working
with were very low power and would disappear all together if they were run
down a nickel plated transmission path.
 We made a lot of high powered RF filters and they were constructed of
Silver plated Aluminum. Silver is a better conductor than gold, contrary to
some people's assumptions, and silver oxide is still very conductive, not
like most metals that have insulative properties when they oxidize.

The only downside to silver was it's sensitivity to being exposed to sulphur
which made a silver sulfide when exposed which turns black and is very
non-solderable... So we had to make sure there were no natural rubber
compounds in the package that would outgas the sulphur compounds that remain
in them during processing and ruin the silver plating.

http://en.wikipedia.org/wiki/Silver_sulfide

Interesting thread...


Best regards,

Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
_______________________________________
Member of the San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com


-----Original Message-----
From: Stadem, Richard [mailto:[log in to unmask]]
Sent: Monday, May 16, 2005 7:33 AM
To: [log in to unmask]
Subject: [TN] FW: [TN] FW: Possible Lead



-----Original Message-----
From: Stadem, Richard
Sent: Monday, May 16, 2005 8:18 AM
To: [log in to unmask]
Subject: RE: [TN] FW: Possible Lead

The actual answer is silver, or rather, copper, which is what you are
really soldering to with a silver finish.
The reason for this is because at these extremely high frequency/high
impedance ranges the signals tend to migrate to the surface due to the
skin effect (Hall effect).
The final finish itself has nothing to do with the conductance, as that
is quickly absorbed into the solder joint itself. What affects the
overall impedance is the conductivity of the basis metal. In an
immersion silver finish, the silver is absorbed into the solder joint,
and copper is the conduit. In an immersion gold finish, nickel is the
underlying conductor. Copper's resistivity is 1.67 uohms per centimeter.
Pure nickel's resistivity is 7.4 uohms per centimeter. However, due to
the phosphorus that is added to the nickel surface as part of the
plating process, its actual resistance can climb as high as 90 uohms per
centimeter.
It has been documented in various papers that the signal loss is around
.0015 Db per inch for copper, and about .012 Db per inch for nickel.
Note that nickel has nearly 10 times the signal loss.
The circuit's accumulated resistance through all of the solder joints
where nickel is the basis metal adds up to the net loss factor.

Also, I believe the immersion silver finish is less costly than that of
immersion gold over electroless nickel.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Friday, May 13, 2005 4:11 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Possible Lead

silver! and costly one...jk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of David Douthit
Sent: Friday, May 13, 2005 3:39 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Possible Lead


Charlie,

GOLD!!

David A. Douthit
Manager
LoCan LLC

Charlie McMahon wrote:

>Question:
>
>
>
>What is the optimal surface finish for PCBs working in the frequency
>band
>10-15 GHz?
>
>
>
>
>
>Charlie McMahon
>
>McMahon Sales Company
>
>P.O. Box 1024
>
>Windham, New Hampshire  03087
>
>Tel: 603-432-3111
>
>Fax: 603-432-6854
>
>Cell: 603-401-4646
>
>e-mail: [log in to unmask]
>
>

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