The fab should be able to support your request, if not Trace Labs can help
you. All shops should have lab capabilities.

The defect you have describe is hard to review from words but here's a
couple of things to consider/review.

First the material you have selected is very good material.

How did they plate the PTH holes? There's only 2 methods panel/pattern plate
which will plate all the outside copper and gives you ED copper where you
don't want it or button plate where you only plate the PTH only leaving RA
copper for flexibility.

It's always good to know the bends/folds areas when creating your drawings
so it gives the fab house a idea what the flexes requirements are in the
product or identify the bend locations on the drawing.

Check the copper grain direction of the part against the panel. Grain
direction becomes very important when you are looking for many cycles.

Of course you have the obvious situations when processing a flex in
mechanical conveyors with etch bits, handling etc. It's not easy!

If they are plating up the entire circuit surface maybe you are starving the
adhesive when using 110 for coverlayers. Maybe going to 210 will help with
better adhesive encapsulation.

Without really seeing the problem it's hard to come up with root cause but
these are some ideas for you.

www.quickturnflex.com
Thank you, 
Jim Mahoney 
Quick Turn Flex Circuits, LLC 
Applications Project Manager 
P# 603-821-7071
M# 603-305-6250 
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Foster
Sent: Thursday, May 14, 2009 8:52 PM
To: [log in to unmask]
Subject: [TN] FLEX question

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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