[SI-LIST] Re: The return of nickel!

Boris,

I assume you measured the two boards in a full system, with other
boards/components, where the other boards/components were the same for the
comparison tests.  I also assume that you measured fully populated boards,
with the same nominal components.

There are several possible reasons for the big difference you see in
radiation:
- Differences in silicon strength and edge rate can account for 10dB plus
differences in radiated emissions.  Unless you have very tightlly
controllede edges, silicons from different lots or from different
manufacturers may vary in a 3:1 range or more.
- The same applies to the core of big chips, where you may have no direct
access to the core in forms of signals (only through IO cells), but a faster
silicon will have faster internal edges inside the core, producing more
leakage to the PCB through the package or into the air through direct
radiation
- boards made at different times (as it was in your case) or made by
different manufacturers may be within the specifications, but: they may have
slightly different effective dielectric constant, creating different
resonance frequencies.  It may happen that the silicons are very similar on
the two boards, but the resonance peaks of the quiter board are not excited
at their peak value.  In case the dielectric constant of PCB material stayed
the same, you can still have a difference due to the different construction
of surface finishes on the two boards.  If you have a network analyzer on
hand, you can quickly compare the resonance structures of the two boards.

Best regards

Istvan Novak
SUN Microsystems


----- Original Message -----
From: "Boris Yost" <yost@xxxxxxxxxxxxxxxxxxx>
To: <Si-list@xxxxxxxxxxxxx>
Sent: Friday, April 11, 2003 2:23 PM
Subject: [SI-LIST] The return of nickel!


>
> Dear SI-ers:
>
> I have two boards.  One of them is louder than the other w.r.t. EMI at
high
> frequencies.  They were made at different times from different lots from
the
> same artwork at the same factory.  I'm trying to track down the
differences
> between them.  One real obvious one (a marketing guy pointed it out) is
that
> they are a different shade of green....
> The reason that they are different colors is that one board has
> copper->immersion gold->solder mask, and the other has copper->solder
> mask->immersion gold.  So therefore on one board all the length of all the
> traces on the outer layers has nickel on the surface; the other one has Ni
> just on the pads.  The one on which they laid the gold (and associated
> nickel) down first is the noisier one.
> Is this significant, or should I forget this one.
>
> Boris
>
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