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

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From: John Ellis (jjre@ix.netcom.com)
Date: Mon Feb 28 2000 - 08:10:34 PST


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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