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August 2000

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Subject:
From:
Jerry Cupples <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 21 Aug 2000 14:00:31 -0500
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At 12:36 PM 8/21/00 -0400, you wrote:
>TechNetters, is tin preferentially depleted from the solder pot in wave soldering?  If so, what is the mechanism?  My recollection is that I learned some months back that, left unadjusted, tin content of a solder pot will slowly drop.  Thanks for any comments.
>
>Lou Hart

Tin has a much higher oxidation potential than does lead. Dross formation is predominantly tin metal being removed in oxide form, so as your pump operates and dross is formed (and probably scooped out) you will deplete the pot's tin content somewhat.

My experience is that in perhaps 6 months to a year of typical operation, you drop a few percent. If you are adding bars daily and replenishing the pot, the process is quite slow. Wave solder pots run daily which see lots of removal/replenishment by soldering large quantities of boards deplete more slowly than those which are used intermittently.

Suggestion: keep a few pure tin bars around, and if you have your analysis done semi or annually, calculate back to the amount of pure tin to get back to 63% w/w. I have typically seen this as one or two kg bars over a year's period, if necessary at all.

My own somewhat mystical approach is that about every year or two, you are wise to just drain that pot into some pie pans, sell it for 80% of metals value, and replace it all with "fresh" metal. The result will be amazingly bright, shiny joints for a few weeks, then a slow change back to the old look. Various people have told me they agree, others roll their eyes and call me funny names.


cheers,


Jerry Cupples
Interphase Corporation
Dallas, TX USA

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