[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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