It's a long time since I thought about wave soldering. Here is a summary of
what I recall.
What we are talking about here is inputs and outputs.
Inputs is top up solder, and whatever gets washed off the work.
Outputs is what gets taken away as joints on the work and what is removed as
dross. 
Some of these are related to throughput and some to "on time". 
In other words if the machine is on and pumping its making dross whether or
not boards are running, the amount of dross generated is a big variable and
is loosely related to the amount of dross removed which is also (operator)
variable. Most dross is actually solder.
In low through put machines dross removal can be significant portion of out
put. In high through puts it is not. Thus the effect of nitrogen/dross
removal on impurity levels will be different according to utilisation.
.
By and large solder input will depress contamination levels, Work processed
will increase them
If you plot pot contamination against throughput (and time) you will see a
gradual increase which will eventually achieve steady state. If you are
lucky this steady state will be below the level at which faults reach an
unacceptable level. Of course you are also plotting faults, so you can now
use the pot analysis data to predict pot changes.
Starting with purest solder will depress the contamination level start point
and purest solder for top up will similarly flatten the rise curve and give
a lower steady state.
Tin will deplete faster than lead so your analysis will also predict likely
tin additions required.

In pumped wave soldering machines pot contents tend to be reasonably
homogeneous.
In emergencies or if you  are pressed for down time, you can avoid a full
pot empty by lowering the pot temperature with wave off. This will allow the
pot to stratify and reduce the solubility of some intermetallics. Heaver
ones will sink and lighter ones float as a pasty sludge. Ladle them out and
then top up again. Return pot to operating temperature and pump back to well
mixed state.


Regards 
 
Mike Fenner 
Bonding Services & Products
M: +44 [0] 7810 526 317 
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Louis Hart
Sent: Friday, February 08, 2013 7:14 PM
To: [log in to unmask]
Subject: Re: [TN] solder skimming and contaminants

Thanks for this tidbit, Greg. ~10 years ago I tracked contamination of
copper and gold in a wave solder pot, after fresh solder was put in, for
about 1.5 years, at intervals of 1 month. I remember copper rose in the
first 6 months or so to about 0.22%, then increased only a few hundredths
over the next year. Right now I'm looking at our solderability test pot and
the HASL machine itself. In view of what you saw, I can believe the top
layer does protect the solder.  Louis

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gregory Munie
Sent: Friday, February 08, 2013 9:17 AM
To: [log in to unmask]
Subject: Re: [TN] solder skimming and contaminants

Over ten years ago as Lucent was phasing out its on-shore manufacturing I
worked with some wave solder engineers monitoring solder pots.

They had recently added N2 inert to several of them and were concerned about
some "changes" in the solder joints.

Over the space of several months we sampled the pots and found that
generally ALL trace elements steadily increased.

We never had a chance to finish the study and the data is long gone but it
was the general belief that the dross was actually helping keep the solder
"clean".

I don't remember any numbers but I believe copper was one element slowly
increasing.

Greg Munie PhD
IPC Technical Conference Director
630-209-1683
[log in to unmask]
 

 
 
http://www.ipcapexexpo.org/
http://www.ipc.org


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