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January 1999

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Subject:
From:
Jerry Cupples <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 25 Jan 1999 12:14:32 -0600
Content-Type:
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Paul Wareham asked:

>     We are using a Fox type HC49U crystal on one of our boards and  after
>wave solder and washing, the body the crystal seems to be tarnished or
>discolored with a yellowish tint.  It does not appear to be a residue and
>the rest of the board is clean.    Any idea what could caue this kind of
>stain  anyone?   Paul Wareham DynaGen Systems Inc.

I have seen such "tarnish" many times. Even so-called stainless steels will
in fact "rust" and/or discolor when exposed to detergent, water, and heat.

When the SS is welded, this can cause carbide precipitation at the weld
seams and makes the metal more prone to rust.

Also, bear in mind that a "stain" on a shiny piece of metal may only be a
monolayer thick. If you are NOT talking about a penetrating erosion of
metal at the seam, then you may be dealing with a purely cosmetic effect.
It may be possible to deal with this cosmetic issue by changing chemistry,
water temperature, drying speed, etc.

My opinion about water cleaning is that it is best done with simple hot
water; and that additions of detergents, wetting agents, descalers,
saponifiers, etc. cause problems in a certain proportion to those solved.
This depends upon using a flux/paste chemistry which permits complete
removal without added solvency.

Changing your process to avoid this type of "corrosion" is a matter which
should not be done without due consideration, and I would not assume that
there is a real problem without knowing more than you have related in this
question.


regards,



Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com

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