[SI-LIST] Re: Routing Parallel vs. Paralell

  • From: "Jack Olson" <pcbjack@xxxxxxxxx>
  • To: "Xilei Liu" <xileil@xxxxxxxxxxx>
  • Date: Fri, 15 Sep 2006 08:54:40 -0500

That's okay.
Maybe I shouldn't have asked such a question to this group.
It seems like there should be a ratio between the field strength
of two random traces routed broadside and the same two traces
routed parallel (same distance away).

Say I have a long skinny board with nothing special on it,
8MHz
6 layers
the engineer wants all the routing on layers 3-4
between planes,
no impedance control, no pairs, no nothin'
and there's one area where the traces overlap for a short
distance (not on purpose, just no room).

So the question was, how much worse are the overlaps
than the parallels?

I would assume it would be worse, but the only answer I got
was a ratio of .60 to .65, which I never would have imagined.

Anyway, my example is far below the kinds of problems this
group is usually thinking about, maybe its just too ridiculous
to consider

thanks all the same, though
Jack (aka "the new guy")



-=-=-=-



On 9/15/06, Xilei Liu <xileil@xxxxxxxxxxx> wrote:
>
> Jack,
> Sorry I didn't realize that you were serious. I don't have field solvers
> in
> hand either, though I am curious about your question. APLAC can analyze
> some
> simple multilayer structures but can not be used for the whole PCB package
> simulation. Even from this simple model, I can't draw the conclusion that
> routing broadside is "worse" than routing side by side. It depends on
> where
> the grounds are. But I can't see much difference between them in terms of
> coupling factor from APLAC simulation. Actually, what do you mean by
> "worse"?  This coupling factor gives idea about how strong the coupling
> signals can be between the two traces, but not how much interference
> between
> them. The rule " if you must run tandem, run the lines offset on the two
> layers" Steve points out gives minimised inteference from the two traces
> configuration to others. Anyway, I'd better run away now.
>
> Regards,
> Celine
>
>
>
>
> >From: steve weir <weirsi@xxxxxxxxxx>
> >Reply-To: weirsi@xxxxxxxxxx
> >To: "Jack Olson" <pcbjack@xxxxxxxxx>,"Xilei Liu" <xileil@xxxxxxxxxxx>
> >CC: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Routing Parallel vs. Paralell
> >Date: Thu, 14 Sep 2006 18:35:38 -0700
> >
> >Jack, no that is not a good generalization.  To get an accurate
> >answer for a specific situation you need to use a field
> >solver.  However, if it is a rule that you are looking for, if you
> >must run tandem, run the lines offset on the two layers.  You will
> >get less total noise and the noise will be less coherent.
> >Steve.
> >At 11:27 AM 9/14/2006, Jack Olson wrote:
> > >I apologize if I didn't phrase the question very well.
> > >When I said MATH, I just meant I don't know how to get the answer.
> > >When I used the 1inch vs .2 inch, I just chose random numbers,
> > >they don't mean anything.
> > >
> > >Sorry for not knowing how to ask.
> > >
> > >If I have a trace already routed that is 8 inches long, and I have
> >another
> > >net not related to it but going to the same general area, and I have a
> > >choice whether to route it under the first one or next to the first
> one,
> > >how much worse is it to route it underneath?
> > >(even though everyone says "don't do it, put a plane between")
> > >
> > >If I am reading your analysis correctly, running traces broadside is
> > >only slightly worse than running them together on the same layer.
> > >The difference between .60 and .65 seems a lot closer than I would
> > >have predicted, but I'm not arguing...
> > >
> > >thanks for taking the time to respond,
> > >Jack
> > >
> > >
> > >On 9/14/06, Xilei Liu <<mailto:xileil@xxxxxxxxxxx>xileil@xxxxxxxxxxx>
> >wrote:
> > >Jack,
> > >Wondering whether I should give your MATH question my PHYSICAL answer
> or
> > >not.
> > >
> > >Firstly, I assume that the coupling you mentioned refers to the
> coupling
> > >between these two traces of different configurations.
> > >
> > >Secondly, assuming that you have 1.6mm FR4(er=4.31), double sided, size
> > >30*30mm^2; by APLAC simulation, the coupling factor(Ze-Zo)/(Ze+Zo) for
> >6mil
> > >traces (1/2oz) routed 6mil broadside is about 0.65; and the coupling
> >factor
> > >for 6mil traces routed 6mil(edge to edge) side by side is about 0.60;
> >1oz
> > >copper doesn't change the results with 1/2oz copper.
> > >
> > >So, it sounds that the statement"traces routed 1 inch in parallel will
> > >couple the same amount as traces routed .2 inches broadside (layer to
> > >layer)" is incredible, unless there is air or other material between
> >"layer
> > >to layer"?
> > >
> > >Well, should leave this MATH question to experts. Please give me $3
> >whenever
> > >possible ;)
> > >
> > >Regards,
> > >Celine
> > >
> > >
> > >
> > > >From: steve weir <<mailto:weirsi@xxxxxxxxxx>weirsi@xxxxxxxxxx>
> > > >Reply-To: <mailto:weirsi@xxxxxxxxxx>weirsi@xxxxxxxxxx
> > > >To:
> > >
> ><mailto:pcbjack@xxxxxxxxx>pcbjack@xxxxxxxxx,<mailto:si-list@xxxxxxxxxxxxx
> >si-list@xxxxxxxxxxxxx
> > > >Subject: [SI-LIST] Re: Routing Parallel vs. Paralell
> > > >Date: Thu, 14 Sep 2006 07:34:38 -0700
> > > >
> > > >Jack that is what we have field solvers for. You used to be able to
> > > >rent time on Polar 8000 for about $20/hr.  Otherwise find someone
> > > >with Polar or Ansoft and run the model.
> > > >
> > > >Steve.
> > > >At 07:29 AM 9/14/2006, Jack Olson wrote:
> > > > >I got a MATH question I don't know how to solve...
> > > > >I want to know the difference between running traces
> > > > >parallel on the same layer (side by side) compared
> > > > >to running them on different layers right on top of
> > > > >each other (broadside).
> > > > >
> > > > >I know won't be a simple answer,
> > > > >so I will provide a simple example:
> > > > >
> > > > >Say I am using 6mil traces with 6mil clearances on
> > > > >layers that are 6mil apart using half ounce copper
> > > > >(which is nearly .6mils) and both layers are internal.
> > > > >
> > > > >What distance can I route side by side and get the
> > > > >same coupling as if I had routed them broadside?
> > > > >
> > > > >So I want a simple thing to remember that says
> > > > >something like, "traces routed 1 inch in parallel will
> > > > >couple the same amount as traces routed .2 inches
> > > > >broadside (layer to layer)
> > > > >
> > > > >Does anyone know how to make my rule-of-thumb?
> > > > >and if I change to 1oz copper, will the rule double?
> > > > >
> > > > >onward thru the fog,
> > > > >Jack
>
>
>
>


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