-----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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