[SI-LIST] Re: Thick vs. thin diff. pairs

As always, it depends.  When we design a high layer count backplane, the
parasitic capacitance of the plated through holes needed for the press fit
connectors is a significant source of degradation and jitter.  Sometimes
this actually determines the upper limit on bandwidth irrespective of
losses.  As the backplane gets thicker this gets worse.  In such cases, we
have learned that setting the trace width as narrow as possible in order to
keep the dielectric needed to achieve the 100 ohm differential impedance
goal as thin as possible is the right trade off.  To compensate for the
added skin effect loss from the narrower trace, we use a lower loss
dielectric such as Nelco 4000-13SI or Isola IS620.  As always, there are
several variables that must be looked at and this requires a good
simulation model validated with good lab tests.

Lee Ritchey
Not a doctor, but an engineer with many, many scars!


> [Original Message]
> From: <Ravinder.Ajmani@xxxxxxxxxxxxxx>
> To: <Mark_Burford@xxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 11/20/2006 9:17:09 AM
> Subject: [SI-LIST] Re: Thick vs. thin diff. pairs
>
> Mark,
> Dr. Eric Bogatin had covered this subject in his monthly column "No Myths 
> Allowed" in the July 2006 issue of Printed Circuit Design & Manufacture. 
> The trace width selection is a compromise between higher dielectric
losses 
> for wide traces and higher conductor loss for narrow traces.  His 
> conclusion: 7 mils is the optimum trace width for GHz interconnects on 
> FR4.
>
> Regards, Ravinder
> Server PCB Development
> Hitachi Global Storage Technologies
>
>
> Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
>
>
>
> "Mark Burford" <Mark_Burford@xxxxxxxxxxx> 
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 11/20/2006 08:15 AM
> Please respond to
> Mark_Burford@xxxxxxxxxxx
>
>
> To
> <si-list@xxxxxxxxxxxxx>
> cc
>
> 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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