RE: [SI-LIST] : Different board vendors different impedances

tomda (tom_dagostino@mentorg.com)
Wed, 24 Mar 1999 14:50:13 -0800

If you have a high speed scope and a fast pulse generator you can check run
impedance by looking at the source impedance and Zo voltage divider. If
the generator's Zout is 50 Ohms and your are testing a 50 Ohm run the
voltage waveform at the junction of the coax and board will show a step to
half the final value for a time duration of 2X the run's prop delay. Vstep
will change with Zo. Use a high impedance (FET) probe to measure this.

Tom Dagostino

-----Original Message-----
From: Laurence Michaels [SMTP:lmichael@techmail.gdc.com]
Sent: Wednesday, March 24, 1999 10:52 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Different board vendors different impedances

Katie Rothstein wrote:
>
> Laurence,
> Just a quick thought about your question. Different board vendors
have
> different processing capabilities (line width, total board thickness,
board
> size, etc.) This could be why some board shops can meet your required
> impedances and some cannot. I'm not sure exactly what you are asking
about the
> different calculations, but many board houses have impedance calculation
tools
> that take empiricle data to help make corrections for their own
processes.
> -Katie

It sometimes seems that, even with 'equivalent' processes, allowing the
same board thickness, size, line width, and layer stackup, board vendors
will come up with different values for trace impedance, generally
requiring them to tweak the board stackup. Since we sometimes like to
use different board vendors, even on the same board, it would be nice to
be able to specify FR4 and copper, and use only one layer stackup,
instead of having to keep track of which vendors require which different
dielectric heights. It would be even better to come up with a
theoretical calculation that works in practice, since some people seem
to think that anything should be able to be calculated without having to
add tweak factors for different vendors. I'm beginning to realize that
this is not the case.

What I'd really like is to come up with a set of layer stackups, perhaps
including different stack heights for different vendors, for a few
different controlled impedances and numbers of layers. Doesn't seem too
likely, as even including the true dielectric constant for the specific
vendor's material in the calculation doesn't always come out within 10%
of the measured impedance. Plus I'd rather only have one layer stackup
(including diel. heights) per board, regardless of vendor. Wishful
thinking...

I'd also like a less expensive way to measure trace impedance than a
TDR, so that we can check the board vendor's claims ourselves. Any
ideas here?

-- Laurence

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