There are two different types of flex, Pat, rigid-flex-rigid and flex. The design rules are somewhat different when dealing with flex only. For one, "flex only" houses use steel rule die cutting and hard tooling (a machined steel die used to achieve a close toleranced outline typically used for ZIF connector terminations) to achieve the flex outline and a rigid-flex-rigid house uses the conventional routing process typical of an FR-4 pwb manufacturing process. This comes into play when considering the outline (and its tolerances) of the flex as well as any stiffeners required. The availability of design guides is impressive among the many vendors producing flex circuitry. One of my mentors on the subject has been Paul Kopnick of All Flex. I recommend All Flex as a "flex only" house.
The finest textbook on the subject I have ever found is a brand new publication (1997) by Electrochemical Publication entitled "An Engineer's Guide to Flexible Circuit Technology" authored by Joseph Fjelstad and available through SMT PLUS, INC. 5403-F Scotts Valley, CA 96055. Their e-mail address is [log in to unmask] and website is http.//www.smtplus.com. I recommend that you request a catalog of their publications and get on their mailing list. They are a rich source for materials.
I also recommend that you obtain the following standards: IPC-FC-231, IPC-FC-232, IPC-FC-241 (flex materials and adhesives) as well as MIL-STD 2118 and MIL-P-50884. I trust that you already have IPC-D-275. The areas that differ in flex design are the current and voltage charts as well as classifications defining the manufacturing limits of traces, etc. It would be a good idea to contact the flex material manufacturers as well and obtain data on the materials and adhesives to be used on your flex. In the rigid PWB design world a designer could skate by with a minimal knowledge of materials but a flex designer's effectively is largely dependent upon their knowledge of the properties price and combination of materials. I recommend that the entire cross section of your flex (including the stiffener and its bonding agent options) be detailed and referenced by their corresponding IPC specifications.
The following companies will gladly drop a design guide in the mail:
Teledyne Electronic Circuits (603) 889-6191
Advanced Circuit Technology (603) 880-6000
MINCO (612) 571-3121
Tech-Etch (508) 747-0300

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From:  Pierce Pat[SMTP:[log in to unmask]]
Sent:  Wednesday, April 23, 1997 12:01 PM
To:  [log in to unmask]
Subject:  des: Flex Circuits


I am a PCB design/drafter in an Avionics/Commercial keyboard and
inverter   
design/ manufacturing company.  We normally design keyboards with the   
switch patterns directly on the ridgid PCB. I am currently working on a
 
design using a flex circuit for the switch matrix which plugs into a   
connector on the logic board. We also backlight the keys with our   
Luminescent lamp technology therefore the flex circuit substrate must be
  
clear. Therefore, I am looking for companies which are in the flex   
circuit manufacturing business. If you are in the buisness or know of   
companies which are, please let me know. Thank you in advance.

Pat Pierce
[log in to unmask]
Luminescent Systems Inc.
East Aurora, New York  

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