--- Andreas Freiherr <Andreas.Freiherr_at_Vishay.com> wrote:
> On the risk of getting OT, but desoldering ICs is certainly on-topic...
>
> > > Good luck! Meanwhile, I'll try to get that blown 74123 out of my
> > > RX211
> > Best method for replacing chips is to cut the pins of the broken
> > part and then take out the single pins with a desoldering pump.
> > Works good even on multilayers with a good soldering iron.
>
> Yes, I have not only heard this hint, but also tried to follow it. At
> least with my tools, I always have trouble getting to the pins to cut.
> There is not much room between a pin, the board, and the IC body.
I have a set of very sharp-nosed diagonal cutters - Xcelite, ISTR. They
are not available at the hardware store or Radio Shack. You can get them
at places like Greybar and such that sell electrical components.
The IC-to-board clearance shouldn't matter. The best way to cut the pins
is to hold the dikes so they point straight-up-and-down and cut right
where the pin goes into the package (not possible for side-bonded ceramic,
but those are rare and frequently socketed anyway). From the description
of your problem, it sounds like you are trying to hold the dikes parallel
to the board.
The goal is not to eliminate an semblance of a pin above the board, but
to free it from its neighbors so it can be removed on its own. I've
found that the severed pin can frequently be lifted out by heating at
the base, then lifting up when the solder flows - the severed shoulder
snags on the tip of the iron and lifts right out.
On multi-layer boards, it can be difficult to desolder the power and
ground pins because the power and ground plane acts as a heat-sink. The
best technique I've found is to apply the right temperature (too hot can
blister off the traces) and flow in a lot of solder (1-2cm) Having a
set of heat-resistant nonmetalic tweezers can be a help in pulling on
the severed pin. It can take a while to get things flowing. Cleaning
out the hole can be difficult, but it's much easier with the pin out
of the way. I've found that mopping up can be aided with solder wick,
but the actual hole needs to be suctioned out, sometimes from the other
side (while heating the first side).
> I think if I use not too high a temperature, I can save the copper leads
> from getting off the epoxy, and it wouldn't matter if desoldering takes
> a bit longer: the bug is dead anyway.
Right. The point here is that the chip is known dead (or you don't
care because you have a replacement) and the most important thing
is to preserve the circuit board.
-ethan
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Received on Thu Mar 07 2002 - 08:47:13 GMT