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January 2013

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Subject:
From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Mon, 28 Jan 2013 15:18:22 +0000
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text/plain (72 lines)
To tell flex grain direction:

Strip a little of the RA copper off one side.  Look at it under about 60X magnification with an off-axis light source.  There will be scratches on the dielectric.  These are in the grain direction.

Etching and polishing also can be used, but are more difficult to do and interpret, and foil from different manufacturers looks very different.

The scratches in the dielectric can be nearly obliterated by pumice scrubs or other abrasion, but if you look at 200X with off-axis (darkfield) illumination, then you can still see the scratches at the edge of metal where the abrasive couldn't reach.

Flex manufacturers often stamp the grain direction on the panels, but fabricators usually clean the stamp off.  Then the panels get subdivided into whatever the manufacturer likes to deal with and the employee who does the cutting either doesn't understand the problem or doesn't care.  QC down the line is never trained in grain direction, so it comes down to the customer.

Other flex truisms:

ANY (copper, nickel, etc) plating over a few microinches thick totally destroys the flex capability which the RA copper gives you.  All plating cycles have to be pattern plated, which destroys resolution where vias are because the photoresist has to climb the hump up to the plated via.

Resolution across the grain of 0.5oz RA copper is limited to about 100micron lines and spaces because the grain boundaries etch faster than the copper.  Resolution with the grain can be done at 75 micron line and space.

It is almost always that the failures of flex traces occur on the compression side of a bend due to delamination causing crinkles.  Tension side traces only fail very rarely.

Solder pads on flex need to have large teardrops going underneath the soldermask/coverlay since the solder joint forms a hard spot.

Laser ablating dielectric off of thin flex in an attempt to get fingers you can solder to never ends well.  The additional heat stress caused by the ablation re-forms the copper grains in a way which makes them extremely brittle.

Wayne

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Monday, January 28, 2013 8:49 AM
To: [log in to unmask]; Wayne Thayer
Cc: [log in to unmask]
Subject: RE: [TN] Flex design guidelines / assembly guidelines

Wayne,

   You make a statement of "GRAIN DIRRECTION and how critical it is to functionl assembly but you don't explain how to accomplish that task".   Are you at liberty to share your valued experience?

Victor,


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, January 28, 2013 7:09 AM
To: [log in to unmask]
Subject: Re: [TN] Flex design guidelines / assembly guidelines

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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