Desoldering question (Miller XMT welder repair)

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I just finished a mini project. I fixed a "broken" Miller XMT 300
CC/CV welding machine, which had the switch for the display
broken. That switch was switching the display between showing voltage
or current on a mini LED screen.

The broken switch had to be desoldered and a new one had to be
soldered in.

The issue that I ran into was desoldering. I have a "Pace SMD 2000
desoldering station" from my younger military surplus days. This
station has a tool that is like a soldering iron, but has a axial hole
in the tip and an adapter for a vacuum, and the built in vacuum. When
I push on a pedal, the vacuum starts sucking through the tip. So I
would melt the solder with the hot tip, push the pedal and...

My problem was that it barely sucked. Not enough to vacuum in the solder
from the circuit board.

As a stopgap measure, I used my 1/3 HP vacuum pump by connecting it to
the desoldering tool and turning on at the proper moment.

In the end, it all worked, the old switch was removed, a new one
installed, and the welder has a working selector of V vs. A display.

All this leads me to the conclusion that something is wrong with the
vacuum pump on this station. Would you say that it should provide very
strong suction?

Any experience here?

i

Re: Desoldering question (Miller XMT welder repair)


Two words.

Solder wick.




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Re: Desoldering question (Miller XMT welder repair)
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You don't have a proper desolder tool, then? ;-)

--
*A snooze button is a poor substitute for no alarm clock at all *

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: Desoldering question (Miller XMT welder repair)
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I have several solder suckers.  In close quarters sometimes solder wick does
the trick, and it always works.  I've got one of those big blue nasty
things, and a couple of smaller aluminum solder suckers.  In close quarters
I still find myself reach for the solder wick.






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Re: Desoldering question (Miller XMT welder repair)

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Any desoldering method sometimes needs a bit of fresh solder to
"loosen things up" so the solder can be sucked out of the hole.



Re: Desoldering question (Miller XMT welder repair)

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The solder in a joint can wind up having a much higher melting point and
much less wetting ability than it did when it was initially applied. That's
because it amalgamates to the material being soldered to some degree. If the
joint was the least bit overheated initially, or upon re-heating for
removal, it can be a fairly high degree.

You notice it more trying to unsweat copper plumbing. Sometimes you have to
wet the joint with some fresh solder to get it to come apart. But I've run
into the same thing taking components off of an old board.

--
Ed Huntress



Re: Desoldering question (Miller XMT welder repair)

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I've also noticed that I often need to add solder to make a second try
at unsoldering a component lead if the solder sucker didn't get
everything on the first try.  I theorize (but can't prove) that the
extra solder is sometimes necessary to carry heat all the way down into
the plated-through hole, to melt the solder all the way through the
hole.

If the component that's being removed has nice fat copper leads, those
can carry heat pretty well (assuming you can make good enough contact
between the iron tip and the lead).  But IC leads don't seem to be
terribly good heat conductors.  A nice bit of liquid solder does a
better job of conducting heat from the iron tip down into the PCB hole.

    Dave

Re: Desoldering question (Miller XMT welder repair)

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That's probably true, but there's also a lot of amalgamation going on.
That's why copper soldering iron tips get pitted from solder; it amalgamates
with the copper. Notice that the pasty glop on a soldering iron that isn't
continuously re-tinned (or that has an iron-plated tip) has a higher melting
temp. When you wipe the glop off and re-tin, it runs a lot easier. The same
is true in a soldered joint.

--
Ed Huntress



Re: Desoldering question (Miller XMT welder repair)
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What a shame lead-free doesn't do the same ... d;~}

Seriously though, most soldering iron tips have been made from nickel plated
iron rather than copper, for many years now, and they get just as pitted as
the copper ones did, but at least you could file the copper ones down. I
never used to put a new bit in my old Adcola, until it was a 'stump'. I am
more of the opinion that tip erosion, both back then and now, is caused
mainly by the mildly corrosive action of the flux in the solder core, and
that the nasty gob of metal that you find on the tip when the iron hasn't
been used for a while, is oxidised components of the original solder
almalgam. Smitty, what thinks you ?

Arfa



Re: Desoldering question (Miller XMT welder repair)
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Are all desoldering tips (such as 1/8" tips) compatible? I need to buy
a few and am a little confused about brand compatibility.

i

Re: Desoldering question (Miller XMT welder repair)
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Heh heh - even Pace tips ain't compatible across model ranges. Think they
use Microsoft for design work.

--
*A conclusion is the place where you got tired of thinking *

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: Desoldering question (Miller XMT welder repair)
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A solder sucker ain't the same as a desolder station. Of course solder
wick is useful but the desolder station works for pretty well everything
here. But probably too large an investment for occasional use.

--
*24 hours in a day ... 24 beers in a case ... coincidence? *

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: Desoldering question (Miller XMT welder repair)

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A good compromise are those suction bulb desoldering irons, I've used one
for probably 15 years now. I never had any luck at all with the separate
solder suckers, it's too hard to apply heat and suction at the same time. I
keep wick around too, neither is a one size fits all solution.



Re: Desoldering question (Miller XMT welder repair)
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Snag with both of those is two hands needed - which usually means rigidly
mounting the work piece.

--
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    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: Desoldering question (Miller XMT welder repair)

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Not with the suction bulb desoldering iron, I'm not sure how you'd use it
two handed if you wanted to..



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I've not used one but I'd have thought trying to hold it steady on the
joint while operating the bulb with one hand might be a tad tricky?

--
*A journey of a thousand sites begins with a single click *

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: Desoldering question (Miller XMT welder repair)

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Not at all, you hold it in your hand with your thumb on the bulb, it's
really easy to use. I'd rather have a vacuum operated desoldering station
but this fits in my toolbox. Only real trick is to keep a fresh tip on it,
when they wear out they don't get a good seal.

http://projects.jheiv.com/images/blog/2006-03-20/03-18%20Desoldering%20Iron.jpg



Re: Desoldering question (Miller XMT welder repair)
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None of those little solder suckers compare to a real properly
functioning desoldering station. When I spent a summer working at a
fiends stereo repair shop we had the Hako deoldering stations and I
could desolder blown output modules faster than you could even uncoil
your solder wick.

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I don't think anyone who's used one can argue against it being the superior
choice, but wick and other methods have their place, they fit comfortably in
a small portable toolbox.



Re: Desoldering question (Miller XMT welder repair)
Just a little update. I found a certain adjustment  that I adjusted
and the vacuum increased very substantially. I think that this station
is perfectly usable now as it is.

i

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