On Wed, 30 Jan 2008 00:53:03 -0600, Ignoramus1782
wrote:

: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
r 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


Your desoldering tool (like most) is probably fitted with a small diameter tip
(approx 0.8mm - 1.00mm inside diameter) designed for sucking solder from around
small gauge component leads and IC pins where the tip make full peripheral
contact with the board. When the solder melts around a pin all the air passes
solely around the lead and maximum effect is achieved inside the through hole.

When you try to suck solder from around pcb mount switch terminals you have to
do it in stages because the tip doesn't have the closest contact with the pcb
for most of its tip periphery. There is usually a fair gap between the switch
terminal and the side of the mounting hole and when the solder melts and the
suction starts it will pull solder from only a small section of the hole before
the rest of the solder cools due to the air passing. There will be much more
solder around a switch terminal as well so it may take several goes working your
way around the pin.

My Hakko desoldering station has good suction but when it comes to desoldering
switches and other large terminal items I usually prefer to use my old manual
Soldapult. These only take one or two goes to remove all solder.
http://www.stanleysupplyservices.com...spx?pn=114-202