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May 2009

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Subject:
From:
John Foster <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 14 May 2009 17:52:15 -0700
Content-Type:
text/plain
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text/plain (91 lines)
I was hoping I could get some input on a problem we are having with a
flex circuit.

 

I have a 2 layer flex circuit that is shaped like a L. one leg is about
5 inches long the other leg is about 4 inches long

At the corner of the L is a stiffener on both sides. The stiffener
extends into both L sections a very small amount about .3 inches.

Both ends of the flex have Omnetics connectors which are designed to be
soldered onto flex circuits. The A28300 series. Some of the traces are
17 mils wide and some are 8 mils wide. The traces are spaced fairly
evenly across the flex and are spaced like the omnetics connector which
means you can't see through the circuit as the traces overlap from side
to side.  One section of the L flexs the other section is stationary.
The bend radius for the flexible section is approximately .35 inches ( I
am not positive of this number and will find out tomorrow but I know it
is far more the 6X of the thickness). The flex is constructed as follows

 

The core is   AP-9121 with 1 oz copper on both sides.

The coverlays are LF-0110

The adhesive for the stiffener is LF-0200

And the stiffeners are .028 FR-4

The edges of the stiffeners are beaded with  emerson & cummings ecobond.

I have spoken with the fabricator and they assure me that the copper is
roll annealed and not electro deposited.

The flex circuit has failed after less than 1000 cycles. The cycles are
very slow and maybe 10 or 20 per day. 

The current going through the traces is very low on the order of 10 mA.

 

We have had two circuits fail and they both have failed on the 8 mil
traces. I have scraped the coverlay off of the flex and have concluded
that the open is about in the middle of the flexible section but I can't
see it. I would have expected to see an open at the end of the coverlay
as I have seen this before. We have been potting the connectors to move
the stress point from the omnetics connector pins to further down the
flex past the end of the coverlay. 

  I have a couple of questions.

1.       Is there a way to remove the coverlay without using mechanical
means I fear that I could screw up the sample by scraping it with a
razor.

2.       Does this surprise anyone that it would fail this soon with a
bend radius that I know is large and only a 2 mil core?

The flex has been removed many times from the system and reinstalled and
I am still having a hard time convincing people that flex circuits must
be treated with care. i.e. can't be laid on benches and smashed.

Tomorrow I will get to get into the system and see the mechanicals but
the mechys assure me that at no time is the flex stretched to the point
of pulling the bend radius down. I soldered a wire onto the connector
and ran a needle up and down the scraped flex and it sure seems as
though it opens about in the middle of the flexible section. 

 

Anyway any input will be appreciated 

Thank you

 

 


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