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

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From: Peter Baxter (peter.baxter@tantau.com.au)
Date: Sat Feb 26 2000 - 13:54:43 PST


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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