We had thought about using that combination at one time, but the plater
told us that it was difficult to do because of the AI difference between
the two alloys, even plated. The anodic index for the silver is around
.03, and aluminum, if I recall, was around 1.4 or something like that,
so if the plating was not done perfectly the dissimilar metals literally
eat away at each other and any other hardware (brackets, stainless steel
screws, chromium-plated SBA connectors, etc.). Did you have any problems
with that?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks,Bill
Sent: Monday, May 16, 2005 10:35 AM
To: [log in to unmask]
Subject: [TN] Best plating for 10-15 GHz (Ku band?)
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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