Perhaps someone can help me diagnose a problem I'm having with a small flexi
we've just received.

We've been trying to put a fairly sharp bend in the flexi -0.40mm to be
precise. It's a one time bend the PCB doesn't get flexed in operation. Every
time we try, the tracks that cross the bend crack. Now I did my sums before we
ordered the board and the Rogers flexi program tells me that the maximum strain
is ~4.8%. This should be well within the ductility limits of copper as I
understand it - I believe strain to failure is typically 25-30%.

Just to see what I could get out of the PCB I tried increasing the bend radius
until the cracking stopped.  I had to increase the bend radius to 1.2mm before
I got no failures.

To give a few more details about the board:

Two layer 1/2oz copper - with electro deposited copper on top.
Layers separated by a polyimide dielectric - 0.005mm thick
Kapton cover top and bottom bringing total thickness to 0.15mm.

It seems to me that the copper has become embrittled would anyone
agree/disagree with me?

Regards,



Eric Christison
Mechanical Engineer
VLSI Vision Ltd
Aviation House
31 Pinkhill
Edinburgh
EH12 7BF

Tel:    +44 (0) 131 539 8165
Fax:    +44 (0) 131 539 7141
A company of the STMicroelectronics Group

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