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August 2007

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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:
Thu, 9 Aug 2007 09:17:47 -0400
Content-Type:
text/plain
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text/plain (161 lines)
Steve-

Then is it electroless gold?  Even if it is, I'm not seeing how you'd
make gold slivers with it, but it can plate out on top of dielectrics if
not controlled properly--so I could see independent particles which
could drastically reduce breakdown voltage, but not slivers which could
bridge a significant gap.

Wayne 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly
Sent: Thursday, August 09, 2007 8:41 AM
To: [log in to unmask]
Subject: Re: [TN] Failed Flex Circuit Assembly

Hi Wayne,
The circuit is not pattern plated with gold - the gold plating is done
after the kapton covercoats are laminated on. Regards Steve Kelly

Steve Kelly
PFC Flexible Circuits Limited
PH: (416) 750-8433
Fax: (416) 750-0016
Cell: (416) 577-8433


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Thursday, August 09, 2007 12:02 AM
To: [log in to unmask]
Subject: Re: [TN] Failed Flex Circuit Assembly

Hi Steve!
 
Sounds like your circuit may be pattern plated with the electroplated
gold.
This is a common way to get the slivers:  The copper etchant undercuts
the gold--actually, I've seen it eat the nickel a whole lot faster than
eating the copper!  If these are flaking off, you've got a fairly high
voltage stress, so imagining a short isn't difficult.
 
If the circuit is indeed pattern plated, then it's usually possible to
either re-design with shorting features to plate the nickel and gold
after solder masking or switch to electroless processes.
 
But that, of course, is only speculation.  I fully appreciate your
frustration at not being able to replicate the problem.  
 
Wayne Thayer 

________________________________

From: TechNet on behalf of Steve Kelly
Sent: Wed 8/8/2007 5:05 PM
To: [log in to unmask]
Subject: [TN] Failed Flex Circuit Assembly



Fellow Technetters,

We have a problem with a field failure on a flex circuit we manufacture.
This explanation is going to take a while so please bear with me.

The flex problem area is between two wire bondable gold pads. The wire
bond pads are approx. .008 wide with .006 between the pads. The stack
from the top down is 50-75 microinches of soft gold, 150 microinches
electroplated nickel, .0007 RA copper. .002 kapton, .0005 adhesive,
.0005 kapton. After electroplating,  the edge of the flex circuit where
the wire bond pads are is die cut. We have noticed some slivers coming
off from the die cutting (look like gold). The flex is electrically
tested twice before shipment (test voltage is 250V). The flex then goes
for die attach. After die attach another electrical test is performed.
It then goes to the end customer who assembles it into the final box and
does another electrical test. The box is then purged with nitrogen then
sealed. After 5 months in the field (benign office environment) some
channels on the die failed. Maximum voltage in the field is 190 volts.

Our theory is that one of these slivers somehow propagated over between
the wire bond pads and over time caused the short. And to make matters
more complex when the flex cable assembly was removed from the box it
worked. We have done SEM and EDS and do have some evidence of copper
between the pads but the questions are a) can we grow some kind of
whisker in this environment to cause this short b) why are only able to
detect copper (no nickel or gold) c) anyone ever seen this before.



Thanks in advance.

Regards Steve Kelly



Steve Kelly

(416) 750-8433 (work)

(416) 750-0016 (fax)

(416) 577-8433 (cell)




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