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August 2003

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 20 Aug 2003 10:56:35 +0300
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NEVER, EVER use 'softened' water for the final rinse. Tap water would be
safer. Let me explain:
Tap water contains (usually) a number of calcium salts, such as
carbonate (sparingly soluble), sulfate (ditto) and hydroxide (slaked
lime, somewhat more soluble). If used for a final rinse in hard water
areas, you will get white "drying marks", caused by these calcium
(and/or magnesium) compounds. These are ugly but only very moderately
dangerous as only the really soluble calcium salts, such as the halides,
ionise readily. The ones usually found in hard water do not ionise readily.

Softening is an ion-exchange process where the calcium in these salts
are substituted by sodium, and the anions remain the same, so that
softened water will contain sodium carbonate, sodium sulfate and sodium
hydroxide (caustic soda). These are all highly ionisable and somewhat
deliquescent substances, so that even trace amounts on a PC Assembly are
potentially dangerous and can cause a number of electrical malfunctions,
including dendrites. It is therefore VERY dangerous to use softened
water for the final rinses, although it may be used for the wash and
first rinse.

DI water is also produced by an ion exchange process and, here, the
calcium (and other) cations are replaced by hydrogen ions and the
carbonate (and other) anions are replaced by hydroxyl ions. The hydrogen
and hydroxyl ions combine to form - er - water. So DI water is the ideal
medium for at least the last two rinses. In practice, it is used, in
most cases, for the whole cleaning process.

Also sprach Brian!

Brian

Macko, Joe @ IEC wrote:
> Fellow Techies,
>
> Hopefully, someone can shed some light on my concern about rinsing
> boards (soldered with water soluble flux) with softened water vs. DI
> water.
>
> A board assembly house suggested that it is acceptable to do in-process
> rinsing using "softened" water (in an in-line cleaner) and save the "DI
> water" rinse for the FINAL in-line cleaning.  In-process cleaning would
> be for example after the 1st side is soldered on a double sided board.
> Seems like a lot of extra work to me switching from softened to DI water.
>
> Look forward to your comments.
>
> thanks
> joe
>
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