[sdiy] SMT rework?

[Simon Gatrall]

>In a message dated 4/6/01 9:19:23 PM Pacific Daylight Time,
>patchell at silcom.com writes:
>
><< > I'm using surface mount for a commercial project that I'm working on,
>  > and I find that with the exception of 100 pin SQFP IC's with 0.6mm
>  > pin spacing, I actually like soldering SMT parts. >>
>
>coincidentally, that's exactly what I'd like to figure out how to
>rework..I've got an industrial PC with an IDE controller chip that's 100 pin
>SQFP, and I'd love to know how to get it off the board and put another one on
>(without a SMT rework station...) The idea of a heat gun crossed my mind, but
>I'm not sure it's hot enough.
>
>Anybody got ideas?
>
>Chris

Whatever you do, you have to be really careful of the pads on the PCB 
with such a fine pitch.  It is too easy to tear them off, especially 
when the PCB gets hot, and the pads aren't connected to anything.  I 
know from experience.

If you want the part to work afterwards it's hard, but... the heat 
gun will work.  If it gets too hot, the part may be pretty fried 
afterwards and any plastic parts nearby may get funky.  I've removed 
smaller ICs this way without too much trouble.  100 pins is a bit 
more of a pain...  Heat carefully while pushing the part to one side. 
I think that if you pull straight up you're more liable to damage the 
pads.  When all of the solder is molten, the part will just slip off 
the pads, but it may re-attach itself in an awkward position.  Just 
reheat and push some more.

If you are trying to be more delicate, you can try adding some solder 
and then solder wicking as much as possible away.  (The "extra" 
solder makes it easier to wick the old solder away)  Then gently pick 
up a side of the chip up with a small pry tool while running the iron 
on the pins.  This works better with chips that only have pins on two 
sides.

Another thing that I have read - but never tried - is to thread a 
thin piece of wire behind the pins on one side of the chip and then 
heat-gun them or heat them with a soldering iron while gently pulling 
on the wire.  The wire acts as a wedge to pry the pins off the pads.

-s!mon