Agree with one small caveat:

Just as Bob points out that proper grain orientation makes dynamic flex applications possible, proper grain orientation also makes it possible for the traces to survive low cycle high stress due to initial bending/testing/installation:  The " fatigue cycles to failure" curve gets raised everywhere, not just at the high cycles end!

If you care about grain direction (and why specify RA if you don't?) make sure the biggest feature on the fabrication dwg is the grain direction arrow.  As I've indicated, my experience is that it is often ignored, but at least there are no arguments when you demand a re-spin.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Sheldon
Sent: Tuesday, January 29, 2013 1:53 PM
To: [log in to unmask]
Subject: Re: [TN] Flex design guidelines / assembly guidelines

Ray & "the group"---
From our experience DuPont has sufficiently documented grain direction on their Class 7 RA foils. That being said- grain direction is most critical in dynamic (constantly flexing) applications; if grain direction is not specified on the drawing and we are working with the customer on the dynamic application development- we will certainly inquire. Most suppliers for flex-to-install applications will quote based the most efficient panel utilization unless grain direction is specified. 

IPC-2223 (Flex Design) has some guidelines for "Bend R" considerations which will be updated in the next revision. The "flex forming / install"
configuration is dependent on multiple factors including the length of the flex service loop, flex thickness and bend radius, type of flex design for multilayer ("loose leaf" or bonded), artwork layout to avoid stress risers, etc.  Thin copper in a bonded (read "thicker") flex with tight bends may not offer sufficient elongation values compared to one ounce copper (or more) and the associated trade off for the added thickness,  etc. Check with your supplier for what applications support they may offer. 
Consideration of grain direction is just one factor of many- and is often not the most important factor.
 

Bob Sheldon
Technolgy and Engineering Director
Pioneer Circuits, Inc.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
Sent: Tuesday, January 29, 2013 2:37 AM
To: [log in to unmask]
Subject: Re: [TN] Flex design guidelines / assembly guidelines

They learnt how it was to be brushed against the grains by Wayne Inge

On 28 January 2013 14:09, Wayne Thayer <[log in to unmask]> wrote:

> I don't have help for you in finding sources for design guides, but
after
> designing/procuring highly stressed (in terms of bend radius) flex
designs
> over the past 10 years, I can tell you that it would be a very rare 
> fabricator who knows how to tell the grain direction of the RA foil:
Out
> of 18 orders of flex, 9 have come in with the grain in the wrong
direction!
>  After I educated those fabricators, they MAY now be able to identify
it,
> but I never trust them.  If you need to push things at or beyond
standard
> limits, count on learning to read the grain and on the extra time
required
> to have a fabricator remake boards.  One fabricator I used last year
made
> the initial delivery in May.  By the time they agreed the grain was
wrong
> and they re-made the boards, it was October!  They had purchased the 
> material from DuPont.  DuPont also did not know how to read grain.
DuPont
> then went to their foil supplier.  I have my doubts on whether any of
the
> "education" stuck!
>
> Wayne Thayer
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Coburn
> Sent: Monday, January 28, 2013 7:45 AM
> To: [log in to unmask]
> Subject: [TN] Flex design guidelines / assembly guidelines
>
> T-netter's;
> Can anyone provide direction regarding design and assembly guidelines
for
> flexible PCB's / PCA's?
> Thanks in advance for your responses.
> Rob Coburn
>



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