[SI-LIST] Re: Thick vs. thin diff. pairs
- From: "bratfest" <bratfest@xxxxxxxxxxxxxx>
- To: <scott@xxxxxxxxxxxxx>, <dave.instone@xxxxxxxxxx>
- Date: Tue, 21 Nov 2006 20:26:15 -0600
Of possibly further interest... from the other side of the elephant...
* It is the application... i.e. the number of lines that an engineer wants
to connect..
* and the area that is allowed for this connection... (Chassis spacing,
number of allowed layers, etc)
* and the desired process for placing the connectors on the board (PTH,
press fit, SMT, etc)
* and probably a couple other odds and ends
... that drive the pitch of the connector
BR
Gus
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Monday, November 20, 2006 12:01 PM
To: dave.instone@xxxxxxxxxx
Cc: Mark_Burford@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Thick vs. thin diff. pairs
Interestingly enough, it's usually the backplane connectors that dictate the
total width that a differential pair can occupy between the pins. I like to
adjust the outside edges of the two tracks to be within minimum spacing to
the PTH antipads, and then adjust the width toward the center of the pair
for 100 ohm impedance. This minimizes loss, maximizes process impedance
control, and makes best use of the board area.
Connector channel pitch then dictates pair-to-pair spacing, and that will
almost always meet Tx to Rx pair crosstalk requirements.
On line-card and supervisor cards, I tend to go for narrow pair designs that
will route through 1mm pitch BGA fields. I'm a bit different than most
others. I like narrow traces, because of the increased trace resistance. I
find that resistive losses help to de-Q impedance discontinuities, which is
a good thing, and having higher trace resistance at each end of the line
helps to buffer those low loss backplane connectors, traces, and
dielectrics, that can ring like a bell. There are so many discontinuities
that you can't get rid of them all, and I tend to want a bit of resistive
buffer to absorb the worst resonances. I like to keep the equalizer's job
simple, and as a result, most of my designs meet margins with only
pre-emphasis or de-emphasis.
All those other stages in Tx and Rx filters are there to deal with channel
non-linearity due to resonance.
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed? is the registered service mark of Teraspeed Consulting Group LLC
David Instone wrote:
> Mark,
> One plus point for wide traces is that the etching tolerance, +/- 1
> mil on a 10 mil trace has less effect than +/- 1 mil on a 4 mil trace,
> same goes for the spacing between the pairs.
>
> Dave Instone
> Oxford Semiconductor Ltd
> 25 Milton Park
> Abingdon
> Oxon ox14 4sh
> UK
> www.oxsemi.com
> +44 (0)1235 824963
>
>
>
> Mark Burford wrote:
>
>> 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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