[SI-LIST] Re: Layout Cross Section
- From: "David Hoover" <dhoovy@xxxxxxxxxxxxx>
- To: <cadpro2k@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 20 Feb 2004 06:27:45 -0800
The best thing to do is make sure you understand your fabricators true
finished product
parameters. The trapezoidal shape of the etched copper conductor makes a
huge difference
in the calculated impedance results. (Hopefully your calculator has that as
an entry)
Next, you need to use the effective relative permittivity (Er') of the
material at the frequency
that the TDR sees (based on the risetime of the TDR. Usually around 1 to
1.75 GHz
nominally for FR4) You can't just use the bulk material values (around 4.5
at 1 MHz).
Those are fine for 2 sided because there is so many sheets of material
stacked up.
But multilayers have higher resin to glass ratios and end up with a much
lower Er'
value.
Each type of material is different, so ask your fabricator for a list of Dk
values
at different frequencies to help you better understand the variables.
Removing parameters
and adjusting the variables to get the calculated result to match the
measured is not the best
thing to do. Trust me I KNOW! The fabricators have done it for years and
some may still
be doing this. Ever hear the words "Empirical Data Adjustment"? (It's like
a mental slide rule)
The 2D and 3D Field Solvers today have the ability to accurately calculate
and solve the
mystery. That coupled with entering the real values will produce results so
close that there
is no need to adjust parameters to "tweak" the results.
Of course if you only have a static equation calculator (like the MECL
stuff) then I
guess your stuck fidgeting with the numbers. : /
David Hoover
Sr. Field Application Engineer
Multilayer Technology, Inc.
----- Original Message -----
From: <Aubrey_Sparkman@xxxxxxxx>
To: <cadpro2k@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Thursday, February 19, 2004 10:36 AM
Subject: [SI-LIST] Re: Layout Cross Section
> Uri, Mitch,
> You don't need to mess with your material.dat file; adding a layer of FR4
> works. Before you do your testing, delete your interconn.iml file; you
want
> to make sure Allegro re-computes the parasitics. I opened a test board I
> have, selected "DISPLAY/PARASITICS", and clicked on a trace on the top
layer
> and got:
> 57.113 ohms
> 36.207 nH
> 11.1 pF
> 0.633954 nS
>
> I then removed the FR4 layer and did the "DISPLAY/PARASITICS" thing again:
> 64.334 ohms
> 36.2065 nH
> 8.74793 pF
> 0.562789 nS
>
> To me this says everything changed about the right amount in the right
> direction.
>
> Aubrey Sparkman
> Enterprise Engineering Signal Integrity Team
> Dell, Inc.
> Aubrey_Sparkman@xxxxxxxx
> (512) 723-3592
>
>
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mitch S. Morey
> > Sent: Thursday, February 19, 2004 10:48 AM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Layout Cross Section
> >
> >
> > Hi Uri,
> >
> > Before you get into material.dat file, and the parameters and
> > what-not, I
> > experimented with changing some variables in my stackup and
> > the impedance
> > DIDN'T change. I tried FR4 over top; conformal coat; etc; changed
> > thicknesses; Dk and when I do DISPLAY/PARASITICS on my Clines nothing
> > changes. Might be worth your time to experiment first. :)
> >
> > Mitch
> > >
> > > Hi !
> > > Thanks for all answer me Kim, Mitch ( nice joke :) and Jeff.
> > > Do you know what parameters of soldermask ?
> > > Now I know that possible edit material.dat and now I need
> > to do this.
> > > Uri
> > >
> > > -----Original Message-----
> > > From: Jeff Seeger [mailto:jseeger@xxxxxxxxxxxxxx]
> > > Sent: Wednesday, February 18, 2004 8:38 PM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Layout Cross Section
> > >
> > >
> > > Uri Chaplianka wrote:
> > >>
> > >> snip <
> >
> >
> >
> > -----------------------------------------
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