[SI-LIST] vias in flex circuits

From: "Jan Vercammen" <jan.vercammen.jv1@xxxxxxxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Date: Mon, 13 Jan 2003 11:02:32 +0100

Hello si-list,


I have a question concerning vias in flex circuits. A colleague of mine
wants to
use twisted pair lines on a double sided flex circuit. In order to twist
the lines
he wants to use vias to actually twist the lines. This kind of twisted pair
lines is
described in the literature as TDL (twisted differential line) and various
configurations
exist (TDL, O-TDL, V-TDL and OV-TDL) - see e.g.11th EPEP Oct 2002.

However, the flex cable is flexed on a cyclic basis with following
parameters:
-1- number of cycles:          >200.000
-2- bend angle:                     180 degrees
-3- minimum bend radius:  20mm

Currently we are planning to use standard flex technology with a 25 or
50micron
thick PI substrate and 25micron adhesives and 25 micron coverlayers.

After a discussion with a flex manufacturer the idea of vias seems not a
good
idea because of reliability reasons - the vias are the week aspect in the
idea of
using TDL on the flex circuit. Actually the cable is about 20cm long and
each twisted
pair would need about 40 vias (if we use one twist/cm).

Of course are there alternatives, we could do without the vias and just use
a
pair of lines - one o top and one of the bottom layer.

So given the data above:  would you recommend the use of vias in our flex
design??


Kind regards,

Jan Vercammen
Agfa-Gevaert NV
Mortsel, Belgium



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[SI-LIST] vias in flex circuits

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