Kathy
I'm shocked to the core that anyone is still using softened water. On pp
204-205 of my big book, you can read:
"... Softened water has a similar conductivity to the raw water, so
softening cannot even be considered as a method of purification. ... By
itself, ***water softening finds no place in quality water cleaning of
electronics assemblies.*** As a pre-treatment for other purification
processes or purely for the initial wash, it may be useful." (the part
***...*** is in italics).
This book was published in 1986 and this chapter was probably written in
1984, so this data has been available for a long time, now.
Contrary to what has been stated by some others, water softening is a
cationic-only exchange process. The calcium and magnesium cations are
replaced by sodium but the anions stay put as carbonates or whatever.
There is relatively little leakage of chlorides from the zeolites, once
they have been rinsed after regeneration by the salt. However, sodium
carbonate (washing soda) is a deliquescent salt which ain't going to do
electronics much good.
At the very least, replace the final rinse with a reasonable DI water,
but you don't have to bend over backwards to get super-duper quality. As
a general rule, a conductivity of 1 uS-cm is perfectly adequate for
cleaning even the most difficult assemblies and I'd go as far as to say
that 10 uS-cm is OK for many. There is no standard for DI water, that I
know of. A definition is water that has been subjected to cationic and
ionic exchange for hydrogen and hydroxyl ions respectively, generally
using specific types of special plastic beads. It need not cost a lot.
When mixed bed regeneration stations do their work, they find that a
small proportion of the beads have fractured and they separate these out
and scrap them. These damaged beads are perfectly good for our purposes
and cost little or nothing.
I would guess that the person(s) responsible for the initial
installation (unfortunately, like many in our industry) simply did not
have a clue. I would hazard a guess that the same applies to 80% of
plants that operate without a person who has any specific knowledge of
chemistry for any of their chemical processes (and there are many such
processes in the electronics industry) and who can't be bothered to
research or purchase the required knowledge or who believe vendors who,
by definition, have a vested interest. A very sad statement when one
considers that products from such processes often go to drain, into the
air or soil with illicit quantities of all sorts of chemical substances,
including toxic metals such as tin, copper, lead and nickel.
FWIW
Brian
Kathy Kuhlow wrote:
>
> Help..... I have recently been assigned the task of justifing our water wash system.
> Here is what I know:
>
> 1) Spray over Spray Westkleen system
> 2) For some reason using softened water not DI. The softening is accomplished through sodium addidtives (got me going).
>
> What I am expected to do: Determine what is the effectiveness of the process. Justify why DI isn't used and why adding salt to the wash to remove salt is ok. Then determine the water surface tension and content (hardness, contaminates present, etc)
>
> TIA
> KK
>
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