Hi Brian - we have not seen an increase in dross with our use of recovered
solder but the phenomenon you described definitely needs to be a
consideration when evaluating recovery equipment. One additional note, the
payback of the recovery system we realized was 4 months so I would
recommend to Phil to double check the value stream cost numbers comparing
using recovered solder versus sending dross out.
Dave
Brian Ellis <[log in to unmask]>
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09/17/2012 10:36 AM
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Re: [TN] Recycled solder?
A couple of Eurocents worth and, here, I'm talking Sn/Pb as I have no
experience of other alloys.
Alpha went to great lengths with Vaculoy, Multicore with Extrusol etc.
What were these bars all about? Both processes were to reduce the dross
in the bars to near-zero. When you first filled a pot with either, there
was a minimal quantity of dross floating on the top. The wave was clean
and stayed that way for quite some time. If you used either injected or
surface water-soluble oil, you could stay almost dross-free for months
(OK you had to skim the polymerised oil/dross residues, but very little
solder was lost). Then, quite suddenly, dross quantities increased and
the Sn:Pb ratio dropped. If you used a poor quality cast solder bar,
with metallic impurities well within specs, you would get much larger
dross quantities on both the first melt and in daily service.
So, what am I saying? It is my belief that dross begets dross or, to put
it less biblically, microscopic dross particles in suspension in the
solder, when they see oxygen, will cause the dross formation to
accelerate. I speculate that there is a threshold effect, possibly
dependent on the size of the suspended dross particles. This is not a
scientific theory but an observational hypothesis.
Dross is a mechanical mixture of tin oxide and solder. Metal reduced
from dross is tin-rich, which is why the pot is tin-poor after much
removal of dross. IMO, metal recovered from dross should ideally be
refined to virgin elements and re-alloyed before re-use.
The point I wish to make is that re-using the metal recovered from dross
may contain more suspended dross than the pot and thus may be the cause
for an acceleration of dross formation. It will probably be cheaper to
send dross for true refining, rather than to reuse metal extracted from
it.
That's 4 eurocents, not 2.
Brian
On 14/09/2012 16:56, Pete wrote:
> My understanding is that they will do the recycling themselves, on site.
They said the recycling would be mechanical, not chemical, so maybe it is
just squeezing out the droww. They also said it was an effort to be
environmentally conscious, but they are in China, so I'd think it's really
cost.
>
> We are among their smaller customers, so I can't make a lot of demands.
They asked if we would accept boards made with recycled solder. I suppose
the best I can do is reply with a list of concerns and ask for test
results.
>
> Thanks!
>
> Pete
>
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