RE: [SI-LIST] : Searching For Equations Governing Serpentine Trac

Hall, Stephen H (stephen.h.hall@intel.com)
Mon, 18 Jan 1999 09:31:07 -0800

I typically use a rule of thumb that says the edge-to-edge spacing between
the parallel sections of the serpentine must be at least 3x to 5x the height
above the ground plane. This will ensure that the trace is heavily coupled
to the ground plane and minimally coupled to the adjacent serpentine
sections of the trace. On a recent design, the 3x rule of thumb produced a
timing skew impact due to increased velocity of approximately 40ps on a 5
inch long heavily serpentined line. This was a 55 ohm trace on typical FR4.
These rule of thumbs produce minimum timing problems, although, it sometimes
causes layout issues.

I hope that this helps.

-- Steve Hall
Intel

-----Original Message-----
From: Abe Riazi [mailto:ariazi@anigma.com]
Sent: Sunday, January 17, 1999 9:03 PM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : Searching For Equations Governing Serpentine Traces

Dear Si-List Members,

Serpentine traces are frequently employed in design of high speed
digital PCBs to increase propagation time of certain signal lines.
These wavy shaped tracks provide an effective means for matching trace
lengths and flight times of various signals.

I am searching for equations and rules of thumb which allow
determination of magnitude of crosstalk and changes in signal
propagation delay (or velocity) as a function of serpentine geometry
(dimensions and separation of adjacent trace segments).

Thank you in advance for your suggestions regarding laws or formulas
which govern serpentine traces..

Best Regards,

Abe Riazi
email: ariazi@anigma.com

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