[SI-LIST] Re: Thick vs. thin diff. pairs
- From: "Christopher McGrath" <christopher.mcgrath@xxxxxxxxxxx>
- To: <Mark_Burford@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 20 Nov 2006 09:25:04 -0800
Of course, it depends on the speed of the signals that you're working
with, but from previous literature and discussions we have found that
the additional area at the corners is negligible except at really high
edge rates (20G+). In other words, it is a second or third order
effect. (I have an article somewhere that talks about how 90 and
greater degree turns are totally fine up despite "conventional wisdom"
that I grew up with stating the contrary.)
Bends can be handled through creating routing. For thicker lines, you
tend to have more spacing on the pairs to maintain the differential
impedance, so now we're talking about loosely coupled pairs where small
deviations from the pair coupling to overcome discontinuities due to
bends totally fine. Breakout regions are great places to make up for
length discontinuities.
I think that if you follow good grounding techniques (running the diff
pair over a ground plane, including a ground via near any plane jumps of
the pair, etc.), I would be surprised to hear of a major difference
between the radiating effect of the traces when using thicker traces.
EMI susceptibility can be controlled just as for thin traces and, if you
are concerned about it from antenna theory, then you can make your
spacing rules more conservative and make sure that your
box/board/whatever has a good EMI shielding scheme.
For me, reducing skin effect (and its effect on the impedance) was the
element that made thick traces the most compelling. I don't recall the
particular frequencies when it started to cause concern, but I think
that I started seeing some effects when simulating 4G Fibre Channel.
Provided that you route using common sense differential routing rules,
you actually get some freedom if you use loosely-coupled diff pairs
since this usually comes for free when using thick traces on dense PCBs
(thin dialectrics). (From a recent SI post, the tolerance impact of +/-
1mil is also a compelling reason for thick traces that slipped my mind.)
Summary- All of your concerns are valid, but you need to figure out
which one or two are the ones that are going to have the most impact and
focus on those.
-Chris
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Mark Burford
>Sent: Monday, November 20, 2006 9:15 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Thick vs. thin diff. pairs
>
>Dear all,
>I would like to ask opinions, on the merits and disadvantages of using
>wide or narrow microstrip lines.
>
>From where I am sitting it looks like narrow microstrip diff. pairs
have
>it won hands down.
>
>
>
>Thick lines:
>
>More area...which gives more dielectric loss as dielectric loss over
>takes skin effect losses at higher frequencies.
>
>More S11 because any corner on the line will give a greater area and
>more capacitance.
>
>Thicker substrates to keep the impedance at the right value.
>
>As thicker microstrip lines go round a bend, the inter-pair skew will
be
>more than for two narrow lines.
>
>There is also the possibility of increased EMI from thicker substrates
>(been reading antenna design books).
>
>
>
>So please someone tell me why should we use thicker lines? And also
>please could someone tell me the trip ups of HDI (high density
>interconnect) such as how small can one realistically go with back
plane
>and inter-chip routing?
>
>
>
>Thanks
>
>Mark
>
>
>
>
>
>
>
>
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