Flex circuits - Rigid flex question

[Carl_at_xrite]

Does anybody have any real experience in this?
The last flex circuits I did were 10 years ago and were basically
straight through flex cables with through holes.
I have a project right now that incorporates an 8 pin CSP which I dont feel we can pick and place and solder through the IR reliably.

Unfortunately I opened my big mouth about a flex "carrier" and now it's exploded into a full flex board.
Any hints, design guidelines etc. would be appreciated..
Thanks in Advance,
Carl

[Tom]

Carl,

Flex circuits are on the rise. I can give you some basic tips on how to design a good Flex Circuit Board.

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.

[Carl_at_xrite]

Tom,
Thanks for some of the hints, some were common knowledge but there were definately a few there I didn't even consider
e.g. stagger the traces.
I've been spending quite a bit of time researching this subject
and looking into materials and requesting quotes.

We develop instruments that measure color and light and one issue I have run into recently is "Black" material or covercoat,
I even looked into flexible soldermask, they make it the same color as the KAPTON, not a show stopper but light leakage at super low levels creates havoc in repeatability in measurements.

Anyway, Thanks again.

[Mark Larson]

I designed my first flex circuit in 1979, I did quite a few over the next couple years then didn't do any till the early 90's. The design rules hadn't changed at all, then I had to do another one a couple years ago, still no change! I have various design guides from the last 20+ years, check out the web site for Minco circuits or Sheldahl, I'm sure others have the info you want there as well. They have good online info and even a manual you can download.

[randychase]

I am in the middle of a large flex right now... ack. It has a over 1000 pins on it, mostly surface mount including 23 TSSOPs. I need to autoroute this to get it done on schedule (today!).

Any hints for Blaze?

Previously, my flex designs have been not as large as far as components and I did a lot of hand arcs and reliefs.

[Ray from NJ]

I'm working on my first flex circuit design to replace a bunch of supply and digital signal cables. It's a static design (no flexing once installed) other than RMA repair. I plan on using SMT ZIF locking connectors between modules for repair.

Some questions:

1. Can I fold the strips of flex circuit onto itself to make the appropriate bends/directions or do I have to design everything on one plane?
2. Do I use stiffener material under the SMT ZIF connectors?
What type?
3. What is the typical spacing between conductor and edge of
flex circuit strips?
4. What fab house do you recommend?

Thanks,

Ray

[robert Tarzwell]

Sorry for the blatent advertismant but I have just finnished a book on designing and manufacturing Flex flex/rigid circuits, its at the printers right now and should be on the web page in a few days. I priced it to cover printing costs and web page. $49.99
see megadawn.com to purchase,
robert Tarzwell

answers to some of the last posts questions,

best tolerance is what your board shop requires but 20 to 30 mils to the edge from a trace is about right allowing for die cutting tolerance.
You can design the flex to fold and bend as you like, make a cardboard cut out to better design the flex as well as make the board fit into a pannel better. Only do not ever crease the flex It will reduce its life, all bends need a radius.
as far as a board shop goes I'll leave that up to others as I am connected to the industry.
stiffners come in many forms, fr4, ployimide and kapton, they can be sticked on or glued on witha press. you may have to research this one more.. toms suggestions are right on and should be followed. other then the copper weight, he is correct on very flexable circuits but you need 1 oz on regular flex to bend type circuits such as rigid /flex.

[cadpro2k]

Hi Carl,

I'm not sure if it'll help, but check out http://www.allflexinc.com . They have a design guide you can download. I might have something else in .pdf form. I'll see if I can get it to you.

Mitch

[cadpro2k]

Carl,

Also try:

http://www.minco.com/pdf/aa24.pdf

Another good source.

Good day.