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September 2008

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Subject:
From:
Phil Nutting <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Phil Nutting <[log in to unmask]>
Date:
Fri, 19 Sep 2008 17:25:44 -0400
Content-Type:
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text/plain (98 lines)
From what I've learned from the TechNet experts, no-clean flux can, with the combination of heat, time, moisture and voltage cause leakage.  So why use no-clean?  That is something to be wrestled with.  If I may quote Doug Pauls... "It depends".  IMHO no-clean works fine on large pitch or through hole, but where real fine pitch is used or you need high reliability I'd build with a washable flux and wash the board.

Best regards,

Phil

-----Original Message-----
From: James Verrette [mailto:[log in to unmask]]
Sent: Friday, September 19, 2008 5:18 PM
To: Phil Nutting; TechNet E-Mail Forum
Subject: RE: Can desoldering and soldering fix a leaky MLCC

The board passed ICT testing and final assembled product test.  It is
believed that something is happening at some point during a 24hour heat
soak in a 55 deg C oven.

-Jim

-----Original Message-----
From: Phil Nutting [mailto:[log in to unmask]]
Sent: Friday, September 19, 2008 4:44 PM
To: TechNet E-Mail Forum; James Verrette
Subject: RE: Can desoldering and soldering fix a leaky MLCC

Jim,

I would suspect resistance from flux or some other foreign matter.  We
had a board that had all sorts of instabilities.  It was an R&D board
that had many hand reworks.  After many long late hours the engineer was
telling me his tail of woe and I suggested we wash the board.  Board
washed, problem solved.  I converted another non-believer that removing
flux was a good thing.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette
Sent: Friday, September 19, 2008 4:23 PM
To: [log in to unmask]
Subject: [TN] Can desoldering and soldering fix a leaky MLCC

I had a case where I was measuring about 100k accross two nets on a PCB
Assembly with an Ohm meter.  I removed a .1uF X7R MLCC SMT 0402 size
from the board and the 100k short went away.  I measured the cap and the
ohm meter and the resistance was OL.  I put the capacitor back on the
board and remeasured the resistance and there was no short.  I looked
for obvious damage on the cap under a microscope, but didn't see
anything.

Is it possible that desoldering and resoldering by hand could cause a
leaky MLCC to stop leaking?  I am trying to find an explaination for
what made the 100k short circuit go away.  The only other expaination is
that there was something under the part that caused leakaget.  The
boards are assembled with no-clean flux, SnPb solder paste.

Based on what I have read flex cracks could cause the leakage we saw
before the modification, but could desoldering and soldering cause it to
go away?

Jim Verrette
Senior Electrical Engineer




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