Hi Louis - there are a couple of things that come into play for your wave 
solder contaminates question. Contaminates that have densities greater 
than tin/lead are going to sink to the bottom of the solder pot and 
potentially stay there. A good example often cited in the industry 
literature is the iron/tin intermetallic compound FeSn2 that is the result 
of tin from the solder combining with a damaged area of a cast iron solder 
pot to form the IMC. Luckily, the act of stirring the wave solder pot 
provides a huge assist in not having density IMC accumulate and is the 
second reaction working in our favor. Tin likes to form IMCs with many 
things so many of the potential wave solder pot contaminates are removed 
during your solder pot de-drossing actions. Elements like copper, 
phosphorus, iron, gold, etc. will get removed as constituents of the 
dross. You can't totally rely on de-drossing to eliminate possible 
excessive contaminates as some elements (like nickel) do not readily 
"dross out". That is why the  IPC-JSTD-001 specification has element 
contaminate table limits and some level of periodic testing of you wave 
solder pot  should be completed. There is a good chapter on solder 
contaminates in the AWS Soldering Handbook (section 2.2) or RJ Kein 
Wassink's book Soldering in Electronics (section 4) that may help.

Dave Hillman
Rockwell Collins
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From:   Louis Hart <[log in to unmask]>
To:     <[log in to unmask]>
Date:   02/07/2013 07:07 PM
Subject:        [TN] solder skimming and contaminants
Sent by:        TechNet <[log in to unmask]>



Technetters, my apologies for asking this question again. It has been 2 
years since I asked, and I remember getting some good responses, but 
digging through the archives I can't find them. My memory of their content 
has gotten quite vague. I can only presume that the responses came 
directly to me, and I lost them.

What chemical species are removed from a 63/37 tin-lead molten bath when 
the film or coating on the top surface is skimmed off? I had the theory 
that, along with some oxide(s) of tin, oxide(s) of copper would be 
removed. I'm thinking of wave solder baths. The long-term result would be 
elemental copper would reach some stable concentration in the bath, which 
concentration based on my limited data appeared to be about  0.23% by 
weight.

And does gold, as an element, simply sink to the bottom of the bath?

Louis Hart
Compunetics
Monroeville, PA
USA
[log in to unmask]<mailto:[log in to unmask]>
412-858-1232


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