Jim Adney wrote:
On Tue, 02 Aug 2005 05:35:33 GMT "Michael A. Terrell"
wrote:
Jim, I remove all the solder, one pin at a time. After the pin
cools, I put the side of the tip to the edge of the pin so the remaining
solder will let go without heating the PTH and pull the pin to the
center of the hole, where I let go an let it cool. Sometimes I have to
take a small pick and press gently against the pin on the top side to
free it from the wall of the PTH.
I find that the HP holes are so tight that I can't center the pin in
both the top and bottom of the hole. I've tried a wooden tool, and
often use toothpicks to clear the holes once the pins are out.
if I have trouble pulling all the
solder out of a hole, I add fresh 63/37 solder to the 80/20 that was
used for wave soldering. It now has a lower melting point, and will
come out easier.
I do this too, but I didn't realize that the wave solder was
non-eutectic. Why do they use the higher melting point solder?
80/20 solder is used because it goes to the solid state faster when
it cools, and it reduces cold solder joints form differential expansion
of the PC board, the solder and the component bodies and leads. The
board temperature is slightly below the melting temp of the solder, so
it sinks a little of the heat to help with the cooling as well. Sample
boards are sent through the oven with one or more thermocouples attached
to calibrate the temperature profile for that board.
One tip: Use liquid RMA flux with the solder wick to reduce heat
damage, and leave about 1/16" of the wick filled with solder when you
clip off the used portion. The solid part allows you to transfer the
heat to the pin, rather than the board so it flows into the braid a lot
quicker.
I've never had much luck with solder wick and PTHs, but I've never had
RMA flux. Where can one buy it?
Most large distributors carry it. "Rosin, Mildly Activated" which is
usually just called RMA. Ersin, Kester, and most other brands of solder
make liquid flux. Don't bother with the GC liquid flux. It is almost
useless unless for this application. I have used Kester 197 RMA and
1544 Fully Activate Flux for more difficult jobs were the old solder was
corroded, or the parts were old and hard to solder. The best way to buy
it is by the quart for a small shop, and you should buy a quart of the
thinner as well.
The RMA flux is a big help when you have to clean up old solder with
cracks. I use a small plastic bottle with a hypodermic needle to apply
the flux to a row of pins, apply a small fresh drop of solder to the tip
and slide the tip from pin to pin through the flux to resolder all the
leads on a chip or connector. This reflows all the leads and picks up
the oxidized solder from the joints at the same time. Then wipe the tip
on a barely damp sponge for the next row of pins. I have done boards
with thousands of pins this way. it works with through hole or SMD, but
both need the excess flux cleaned off when you finish. I found some
spray cans of brake cleaner at a "Dollar Store" that are a mix of
different alcohols that do a great job of removing the flux. Just do it
with plenty of ventilation, and tilt the board so it runs off, but is
there long enough to remove the flux.
Another thing, use a board holder to position the PTHs horizontally
so you can get the solder out easier.
That's another trick I never tried.
Thanks for the careful explanation.
No problem. I worked for four years in a manufacturing environment,
and learned a lot of tricks to do the job right, and with the least
chance of damaging the boards. It was a running joke when I took
repaired boards to QC and had to show them every solder joint because
they couldn't tell my hand soldering from the reflow oven.
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Jim Adney
Madison, WI 53711 USA
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--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.
Michael A. Terrell
Central Florida