In a message dated 04/23/99 12:31:22, [log in to unmask] writes:
>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?
>Eric Christison

Hi Eric,
Are you sure of your thickness values—"polyimide dielectric - 0.005mm thick 
plus Kapton cover top and bottom bringing total thickness to 0.15mm"? What 
are the thicknesses of each layer in your flex?
You say "Two layer 1/2oz copper - with electro deposited copper on top"—does 
this mean you start with 1/2 oz electrodeposited copper onto which you plate 
?? mils of copper? Are you aware that commercial foil only has to give you 2% 
elongation to meet IPC-MF-150? For your kind of bend I would specify rolled 
annealed copper foil.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]  

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