Re: [SI-LIST] : Max Zo of Flat Flexible Cable

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From: Ron Miller (rmiller@Brocade.COM)
Date: Mon Feb 28 2000 - 11:27:28 PST


> Hi John

Good Idea.

If Peter used Goretek or foamed teflon he could get the er down to
1.3 or 1.4, using the technique you describe.

Ron Miller

> Hi Peter,
> I worked on a similar construction many years ago. The cable was a Teflon
> ribbon, but the same solution would apply. What we did was place a 10 mil
> layer of nonwoven fiberglass mats on either side of the ribbon beneath the
> adhesive shield layer. This pushes the shield farther from the cable raising
> the Zo. The fiberglass mat captures a lot of air in its structure so it has
> a pretty low dielectric constant (<2.0) further raising the Zo. The
> material is available in several thicknesses.This is a reasonably
> inexpensive solution to your problem .
>
> John Ellis
>
> Vice President
> Signal Integrity and
> Interconnect Business
>
> TriCN Associates, LLC
>
> 1431 Garalyn Road
> Harrisburg, Pa. 17110
>
> Phone:717-213-9566
> Fax: 717-238-0302
> email:jellis@tricn.com
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Peter Baxter
> > Sent: Saturday, February 26, 2000 4:55 PM
> > To: si-list@silab.eng.sun.com
> > Subject: [SI-LIST] : Max Zo of Flat Flexible Cable
> >
> >
> > Hi,
> >
> > I've been looking at shielding a 1 metre long (39 inches) "Laminated
> > Flat Flexible Cable", and I am finding it difficult to get anything but
> > a very low impedance.
> >
> > It is stripline with (w) 0.7mm (28mil) by (t) 0.1mm (4mil) conductors on
> > a 1.25mm (50mil) pitch. The shield to shield distance is (b) 0.08mm
> > (3mil) + 0.08mm (3mil)= 0.016mm PET, however, I don't currently know its
> > di-electric constant (er - 2~6).
> >
> > I'm trying out Silver Conductive Ink as the shielding material over the
> > polyester insulator/substrate.
> >
> > Insulator
> > Shield Silver Conductive Ink
> > Insulator Pet 0.08mm
> > Conductor Cu 0.1mm width 0.7mm
> > Insulator Pet 0.08mm
> > Shield Silver Conductive Ink
> > Insulator
> >
> > The conductors go ground/signal/ground and are being driven by 3V3 TI
> > 74LVT16501 driver (18-bit 74xx245). I want to pass 50MHz digital
> > signals. Parallel termination is used at the destination end, single
> > direction signals only.
> >
> > Trying samples supplied, which were never designed for transmission line
> > work, I get impedances from signal to ground (actually ground conductors
> > and shield connected together) of between 18 ohms and 33 ohms. This is
> > measured using an LCR meter at 100kHz with shorted "L" and open "C".
> >
> > Adhesive "Wrap Over" shields can give 41 ohms. More because the "wrap
> > over" material sits further away from the conductors. The "Wrap Over"
> > shields can bubble/ripple when the cable is flexed tightly.
> >
> > I'd like to get 100 ohms (very optimistic) but above 75 ohms would be
> > workable.
> >
> > Shielded "thin" flexible polyimide PCBs have been made for many years.
> > It is a similar concept. I am not aware of what Zo's they tend to deal
> > with. Polyimide is very expensive though.
> >
> > The avenues open to me include:
> >
> > Reducing the conductor thickness from 0.1mm down to ....? (0.032mm is
> > limit)
> >
> > Reducing the conductor width from 0.7mm down to .....? (0.035mm is
> > limit)
> >
> > Increasing the insulation thickness from 0.08mm up to....?
> >
> > Changing from PET to a lower di-electric material.
> >
> > Another approach...?
> >
> > My question is:
> >
> > What is the upper practical limit for Zo that I should expect to get?
> >
> > What Zo's do they get from shielded polyimide PCBs? Is cross hatching
> > the way they achieve higher Zo with less radiation?
> >
> > Does anyone have a suggestion as to what make-up I should consider?
> >
> > The idea is for the cable to be, "Not Too bulky". Increasing thickness
> > of the insulation is the easiest way to get a higher impedance. But I
> > can't imagine a 2mm (80mil) thick cable, not being bulky.
> >
> > Thanks
> >
> > Peter Baxter
> >
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>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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