Paul,

Check if your part is considered aqueous compatible by the manufacturer.  If
it's not, you're off to a tough start.  We've seen the problem Brian
mentioned - cooling air inside the pots sucking in moisture past the 0-ring.
You may be vulnerable to this if you take hot boards directly out of wave /
reflow and toss them right into the cleaner.

A basic leak test and dissection of leaking parts will lead you to the
whereabouts of the water intrusion.  I've seen parts with poor or incomplete
ultrasonic welds that make them prone to leaking, primarily near the
corners.  We've also seen parts with an excessive percentage of "reground"
plastic in the case, which contributed to poor dimensional stability and
watertight integrity.

Sometimes, despite your best efforts, a few random parts may still get water
forced into them - especially along the edge of the board where they're
directly exposed to the high pressure spray nozzles.  In those situations
you can fabricate temporary spray shields to clip over the pots, prior to
aqueous.  Make sure that the shield lets the water slosh freely around the
areas you must clean.

Bruce Tostevin
Benchmark Electronics
Hudson, NH


> -----Original Message-----
> From: Paul Truit [SMTP:[log in to unmask]]
> Sent: Wednesday, January 16, 2002 2:33 PM
> To:   [log in to unmask]
> Subject:      [TN] Aqueous cleaning of surface mount potentiometers
>
> I wanted to hear how fellow Technet people are processing surface mount
> potentiometers.
>
> I have experienced some field failures recently in this area.  Water has
> leaked into a "Sealed" potentiometer.  When power is applied an
> alternate circuit grows inside the pot causing the part to fail.
>
> We are processing this part using an OA solder paste through a normal
> pick and place operation.  Reflow profile takes this through a normal
> reflow curve Max. Peak temperature of 195-205 degrees C with a Ramp rate
> of 2-2.5 Degrees C.  We then add on a few through hole parts through
> progressive assembly then  wave solder and clean the assembly.
>
> We are utilizing an Electrovert Aquastorm 100.  We process the boards
> through at 4 feet per minute.  Water temperature is 145 degrees F.
>
> We have been cleaning boards (and potentiometers) for over eight years
> with never a problem.  This one has sure surprised me.  Just wondering
> if anyone else has had any experience with this and what they have done
> to fix this other than hand soldering the pot on in final.
>
>
> --
> Paul Truit, Mfg. Eng.
> RBB Systems, Inc.
> 4265C E. Lincolnway
> Wooster, OH  44691
> Ph. (330) 567-2906 ext 514
> Fax (330) 263-5324
> Email: [log in to unmask]
>
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