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Subject:	Your First Flex Circuit
From:	Andy Magee <[log in to unmask]>
Reply To:	[log in to unmask]
Date:	Mon, 24 Aug 1998 15:23:43 -0400
Content-Type:	text/plain
Parts/Attachments:	text/plain (108 lines)

David,

1. Today's flex traces can go down to <2 mils with thin copper, and your
8/8 mil. concept is pretty standard stuff on 1 oz copper. You can also
go to copper alloys like 7025 to increase the strength and resistivity.

2. The border limits are based the tooling tolerance you can maintain to
preclude trace breakout along the edge. Hard tools will out perform
soft tooling, but both should be able to easily maintain 1/16".

Is this a polyimide or polyester flex? It sounds a lot like an
automatic, in seat, air bag disarming circuit we worked on a while ago.
Between the tendency for rapid catastrophic damage to the circuit in use
and the extreme low cost target it was a non-starter. Foam is a terrible
thing to attach a flex circuit to and all of the normal flex dynamics go
out the window.

You can probably take advantage of the various flex analysis programs
that I wrote for Rogers. If you don't already have them, go to
http://www.rogers-corp.com/cmu/software.htm where they're available for
download. If you have trouble getting them let me know and I'll e-mail
you copies.

Andy Magee
Flex Guru
[log in to unmask]

David wrote,

I'm having to critique a board that is a very large flex circuit. It's
kind
of funky in that it isn't a conventional board, but rather an array of
22
resistive sensors spread over a (roughly) 18x24 inch area. The board
looks
like like six 1/2 inch fingers extending, vertically, 24 inches from a
2
inch, horizontal, section. The connector comes off the 2 inch part and
each
sensor is mounted on the 1/2 inch stripes (either 3 or 4 sensors per
strip.)

The current version of the board appears to have fairly wide traces
(eye-balling, they appear to be 10-15 mils), This sensor array will be
in a
constant state of movement (it will sit on a dense foam pad which will
be in
a regular state of "flex" as heavy items are laid on the pad), there
fore,
the flex circuit will be flexed, a lot, over time.

Here are my questions:

1) How narrow can I make the traces on flex circuit?

Currently, the 2 inch horizontal part is full of 48 traces (2 per
sensor)
and I'd like to re-spec that section to be narrower.

2) How much flex board material is needed to support the traces?

The current design has about 1/4 (maybe 3/16) inch of flex material on
the
board edge. That is, the routing outline is 1/4 inch inside the board
edge.
Can this be reduced to 1/8 or 1/16 inch, or less? If I can go to 8 mil
traces (with 8 mil space) and use 1/16 inch between the board edge and
the
routing outline, then the portion of the board that has only 2 traces
would
be .149 inches wide ((.0625 X 2) + (.008 x 3)).

Thanks,
David R

-----------------------------------
David M. Ransier
Senior Consultant
Intercept Technology Inc.
[log in to unmask]
503-692-0111 www.intercept.com
503-691-9535 - Fax
404-352-0111 - ITI Headquarters

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