The dross does not (generally) sink, it's a mixture of air, metallic
oxides, contaminants and solder that floats to the top. Some dross like
normal will form in the impellers due to entrapped air.
What is happening here is that the pot slowly builds up impurities from
your component leads, and (mostly) PCB finishes into the solder, and
these impurities tend to sink to the bottom, (copper from OSP, nickel,
silver, gold, etc). This is not DROSS, but solder impurities. If you
leave any SS components in the pot after your LF retrofit, it will eat
away and show up in your measurements (Fe) over time.
Suggest changing sampling to pull your slug from the nozzle, instead of
the pot. That way when you have the solder impurities tested you can be
confident that the measurement is of the solder your PCB actually sees
in the process. If you are not sure about the solder integrity, or you
are allowing Pb parts in the mix, you should move up the frequency of
your testing to monthly form 6 mos.
Also, it costs about 20,000.00 US to fill a 1300 lb wave solder machine
with pure tin for cleaning, and re-fill with LF solder, so you should be
real careful after the retro to make sure and KEEP the lead out!
asta
Rick Smith
Senior Product/ Component Engineer
ClearCube Technology
8834 Capital of Texas Highway North
Austin, TX 78759
Phone: (512)652-3544
Fax: (512)652-3545
[log in to unmask]
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Tom Braswell
Sent: Monday, March 07, 2005 2:11 PM
To: [log in to unmask]
Subject: Re: [LF] Lead-Free Dross in Wave Solder systems
Mark,
It is possible, as the density of the dross (SnCu) is greater than the
molten solder. However, I have heard that lowering the pot temp and
letting
it sit idle, can cause the "dross" to float, so you can skim.
Tom Braswell
Manufacturing Engineer
Fawn Electronics
6441 East NC 97
Elm City, NC 27822
Ph: (252) 977-8745
Fx: (252) 977-3477
Email: [log in to unmask]
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Boyd, Mark
Sent: Monday, March 07, 2005 10:58 AM
To: [log in to unmask]
Subject: [LF] Lead-Free Dross in Wave Solder systems
During the IPC conference in 2004, a couple class instructors mentioned
that
dross formation in some types of lead-free wave solder system could form
dross that had a density greater than the molten solder - and would
therefore sink to the bottom of the pot, making dross skimming
unmanageable.
My solder reps assure me that this is not the case in SAC solder, but
that
there may be some intermetallics that would sink.
Has anyone encountered this?
Thanks,
Mark
M Boyd
Pelco Engineering
www.pelco.com
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