From: Chad Fernandez <[EMAIL PROTECTED]>
> Jerry,
> Do you have the tools to solder stuff that small, or to unsolder the
> original chip? I can solder pretty well, but haven't ever dared
> soldering chips. I would end up soldering all the pins together.
In the hope of helping those less adept at unsoldering, I will now
pontificate at great length... :-)
I would suggest some pretty minimal tools:
A "pencil" type soldering iron rated 25 to 30 W., with a small "chisel"
tip (like a small screwdriver) about the width of a leg - or space
between centers of the legs on a chip. (NOT a 140 W. soldering gun!)
A "solder-sucker" manual desoldering tool. This tool has a spring-loaded
piston that you load and "cock". Press the trigger and the piston
snaps back, sucking the molten solder into the cylinder.
A "heavy-duty" pair of tweezers or a small, fine-tipped pair of
needle-nosed pliers.
Each of the above probably cost less than $10.00 US.
Wipe the tip of the hot soldering iron on a damp, cellulose (stiff when dry)
sponge to clean it frequently. The tip should be shiny, without any black
deposits.
Keep the desoldering tool clean and well-lubricated. They come apart for
cleaning. (Watch out for the spring - it will still be somewhat compressed
- like a Jack-in-the-box.) A small brush and a quick shot of oil-spray
(like WD-40) into the cylinder work wonders. If you save enough solder, you
can sell it back to the refiner - and keep the lead out of the environment.
Remember that electronic solder is 37 to 40 % lead, which is a cumulative
(stored in the body) poison, and more easily absorbed when it is finely
divided.
Other things that could come in handy:
Something to hold the circuit board in a convenient position. (An old
apron would help you hold it between your knees without ruining your
pants or burning your legs with drips of hot solder.) A small vice with
padded jaws (a couple of layers of vinyl tape) should do. Just don't
pinch any components when you grip the edge of the board.
Electronics supply stores often sell fine copper braid ribbon (with a trace
of flux in it) for soaking up solder. This is particularly useful for
wicking the last of the solder out from under surface mount components.
[In a pinch, you can even use fine, multi-stranded copper wire, with a
little rosin flux - NEVER, ever, use ACID flux on electronics. You
can use acid flux on plumbing, but keep them separate.]
If you are getting old enough that your eyes are losing their
accommodation (Symptom: You have to hold the classified ads. at
arms length to read them, without glasses), buy a pair of cheap,
plastic, "drug-store" glasses. Try say, +2.00 diopters in the
store. Don't go overboard. You should be able to read comfortably
with them, for at least 1/2 hour at a time. Take them off before
walking around, or falling down stairs... :-(
Good light is important, too.
You could use one of those TRIAC lamp dimmers, or a VARIAC variable
transformer to adjust the tip temperature. Just be sure it can
handle the current. This improves tip life and is more convenient
than always plugging/unplugging the iron. There are commercial
soldering stations that control the soldering iron temperature.
There are commercial desoldering stations, too - but they cost about
$500.00 US. You just might be able to borrow one, however.
Technique:
Being right-handed, I set up the board edge-on with the component
side facing to my right. With the soldering pencil in my right hand
I melt the solder around one of the legs on the top or near side of
the chip. (A little extra flux on a clean, not-too-hot tip helps.)
Pushing the leg inwards should cause it to wiggle slightly. I place the
tip of the cocked desoldering tool over the end of the leg using my
left hand, remove the soldering pencil and immediately trigger the
desoldering tool. Repeat along the whole side. Rotate the board
180 degrees and do the other side.
This should remove almost all the solder. Using the pliers/tweezers,
wiggle each leg to prove that it is free. You will probably have to
repeat several legs, but eventually, with a little patience, you will be
able to lift the chip from the board. You can use a small screwdriver
to wiggle the legs from the component side. Once all the legs are free,
you can use it to gently pry the chip free. Use as little heat as
possible, and minimal force lifting the chip. You don't want to rip
up the traces on the component side. It is possible to repair them
with copper tape or fine wire and solder - but it is a REAL pain.
Some people prefer to work from the solder side only, but they have
to be very quick to remove the soldering iron, place the desoldering
tool, and trigger it before the solder cools. That is where the
commercial units excel. Just place the temperature-controlled hollow tip
over the leg, wiggle it, and trigger the vacuum pump.
I have seen commercial desoldering tongs that grip all the legs on each
side of the chip to remove it. I have also seen tools shaped to
unsolder all the legs from one side of the board and vacuum tools to
remove the chip. These tend to be high-wattage devices, so that it is
important to remove the chip quickly, before the solder pads separate
from the board.
Others use small cutters to cut away the defective chip. Then they
can remove one leg at a time. Done carefully, this can save wear and
tear on the board, and is good at relieving your frustrations, but the
chip is not salvageable... :-(
Don't forget good anti-static practices:
Avoid the obvious sources of static, such as brushed acrylic sweaters and
nylon pantyhose...
Room relative humidity over 35 %, over 50 % if possible. Ground yourself
FIRST, before touching the board, but if you wear a grounding strap,
make sure that there is a 10 MegOhm resistor (or higher) between you
and ground. Sometimes defective soldering irons are "HOT" in more
ways than expected... =:-0
Boyd Ramsay
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