[SI-LIST] Re: Layout Cross Section

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: <dhoovy@xxxxxxxxxxxxx>, <cadpro2k@xxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
  • Date: Fri, 20 Feb 2004 12:47:27 -0800

I have to respectfully disagree with this method, a bit.  In some
instances it is a very good thing to get definitive fab geometries,
dielectric constants, and all the sometimes obscure data you must take
into account to build an accurate model to determine the properties of
your stackup.  But, in my personal experience, that's not a task to be
taken lightly.  Unless you have a specific need to delve into the
nitty-gritty, I would opt for relying on the fab vendor's empirical data
to determine stackup to achieve the right impedance, for instance.  Of
course, this depends on the vendor's data collection method and their
analysis, but they should have the data and experience.  If they don't,
and/or you opt to go through the exercise of precisely determining all
the relevant factors yourself, be prepared for a lengthy study.


Jeff Loyer


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of David Hoover
Sent: Friday, February 20, 2004 6:28 AM
To: cadpro2k@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Layout Cross Section


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 -----=20
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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