flex circuit specs

[roseann]

Can anyone refer me to specifications for fabrication drawings for flex circuits?
I did not find any board thickness tolerancing for thin flexible circuits IPC-D-300G (using 1/2 mil construction).
Also, is there a 'standard' guideline for flex fab drawings?
Thanks

[Tom]

Roseann,

There are two flex circuit articles, Flex Board Design.pdf & Flex Circuit Layout.pdf at the pcbstandards.com "Site Index".

At this URL:
http://www.pcbstandards.com/downloads/Metric%20Environment/General%20Documentation/Articles/

Flex circuits are on the rise. I can give you some basic tips on how to design a good Flex Circuit Board. If you ever find a good set of "Flex Notes", please send them to me.

1. Never use acute angles or 90 degree angles. Try to radius corners 1mm (40 mil).
2. Always use teardrops
3. Stagger (offset) traces on both sides. I.E.: Do not run traces directly on top of each other on both sides.
4. The trace spacing should be 2 times the trace width - minimum.
5. To Minimize weight and Maximize flexibility copper should be kept thin - 18um max (1/2 OZ).
6. All through holes should have a 0.25mm (10 mil) annular ring - minimum. Preferred annular ring is 0.38mm (15 mil).
7. Get familiar with different through hole pad tie down supports or Hold-down Ears.
8. Avoid "T" routing and if you have to use it, add additional copper at junction.
9. Conductors should run perpendicular to fold lines.
10. Minimum trace width 0.2mm, but ask your fab shop.
11. Minimum bend radius equals 3-6X the material thickness for single sided, 6-10X for double sided, 10-15X for multilayer.
12. Tolerances should be +/-20% to avoid expensive tooling.
13. Tear stops must be incorporated where sharp internal corners are utilized.
a. Use glass cloth or Polyimide film reinforcement.
b. Add copper trace or "dam".
c. If a slit is required, add a round hole to keep slit from tearing.
14. Cross Hatch ground planes improves flexibility.
15. Use stiffeners as an assembly aid.
16. After routing, beef up trace widths wherever possible.
17. Factors that affect flexibility are:
Type of material used
Thickness of material used
Type of copper
Bend radius
Proper balance in construction
Placement of metal in neutral axis

Hope this helps a little.

[Mark Larson]

in addition to Tom's list you should check out a few MFG websites, here are a couple documents you should read

http://www.minco.com/pdf/aa24.pdf
http://www.minco.com/pdf/aa31.pdf
some good tips in both including explanation of the process

I still have my Sheldahl design manual from 1979 that helped me design my first flex circuit, not much difference between then and now for your basic design.

[J Raymond]

Roseann,
Here is another web site to check out.
M-flex has a really nice CD that has lots of info also.
http://www.mflex.com/design.html

Joyce

[robert Tarzwell]

I know the problem of trying to get good design information on flex circuits, and an presently writing a book on flex design and manufacturing. it will be self published and I try and keep the cost down probibly 49 .99 It will be available on my web page megadawn.com
I have 2 other pcb books now heavy copper power supply and high voltage.
Ill post a noyte when its ready, hopfully end of May2003
Robert Tarzwell