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July 2014

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From:
Robert Kondner <[log in to unmask]>
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Date:
Thu, 31 Jul 2014 11:26:33 -0400
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Phil,

 By any chance is your in-house using an air knife or similar to remove
excess wash water and is another group (CM) allowing to dry?

 I did some playing with remaining residue and if folks wash with tap water,
and blow off excess water, the surface contamination will appear very low.
But a single tiny drop of water in the wrong place could be a problem. Worse
Yet even rinses with DI will not clean under a QFN.  And tiny drops of DI
get to tape water equivalent with even tiny tiny amounts of dissolved
material. If you think about flux getting cooked into nooks and crannies
what good is a DI wash/ You relly might need an additive that reduces
surface tensions of the wash water. But now that stuff is in there. Can't
Win.

Also, you mention opto devices and between the various grades (transfer
efficiency), vendors, user designs  and  ageing many of an opto isolation
circuits "Go Bad" because the designer did not account for all the operating
parameters. I know, been there and done that. They are not easy to use
devices.

Bob K.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil
Sent: Thursday, July 31, 2014 10:38 AM
To: [log in to unmask]
Subject: [TN] component failures due to post wave/reflow washing

Over the past years we have had relays, optos and pots fail due to corrosion
being found inside the parts after post soldering wash.  The quick solutions
have been to use other manufacturer's parts or install after wash with hand
solder "no-clean" cored flux.  One manufacturer said it was because of
halides in the flux.  Our primary CM's and our flux contains halides, but we
haven't seen as many failures with in-house processed PTH boards as with CM
built boards.  Recent pot failures have been explained  by component
manufacturers as too much wash pressure or bad flow direction from the wash
jets.  Opto failures were similarly diagnosed.

Our in-house process, all PTH, is foam flux with OA flux, wave solder with
63/37 or SC995e followed by a closed loop DI wash with no chemicals.  This
has been a great process since 2006 when we switched from a no-clean foam
flux, 63/37 wave and terpene/alcohol wash to our listed system above.

We do not dictate flux/wash process to our CMs, but we do specify
cleanliness to J-STD-001 C-22 using the 1.56 ėgm / cm2 NaCl equivalent
contamination value.  Forcing the CM to change fluxes for our product met
great resistance and added cost so no change in flux was made.

Our business is high voltage from 30 kV to 240 kV so clean with NO residue
is very important.  The DI wash has provided that in-house.

We are using different CMs going forward, but not necessarily for this
reason.

Is this a problem unique to us or do other suffer from similar problems?

Phil Nutting  |  HVP Development Engineer   |  Excelitas Technologies Corp

Lab: +1 978.224.4332   |  Office: +1 978.224.4152
35 Congress St, Salem, MA  01970 USA
[log in to unmask]<mailto:[log in to unmask]>
www.excelitas.com<http://www.excelitas.com/>


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