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

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From: Ronald Miller (rmiller@Brocade.COM)
Date: Sat Feb 26 2000 - 18:55:00 PST


Hi Peter

Most plastics have a dielectric constant between 2.9 and 6 or so. Some up to
about 9.

For a 50 ohm line on 4.5(fr4) you need approximately a Width /Height ratio of
somewhere around 2 for microstrip.

So, your problem is with your present dielectric height to ground plane you have
much too wide a line.

Good luck

Ron Miller

Peter Baxter wrote:

> 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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