rgb-man said:
I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.
I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.
Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.
Should I give up before I even start?
If I decide to give it a go, will I need to buy much specialised
equipment?
Are there maybe any web sites which people here would recommend that
are particularly useful?
Thanks
I have done this successfully with a $20 heat gun. It is best if you can
come up with some kind of mount to hold it over the chip so you have two
hands free to remove it.
Heat guns, on the low setting, put out about 650 degrees F. That temperature
will damage the chip internals, and if you have a moisture sensitive chip
you might generate steam bubbles inside and damage the die with internal
stresses. So the plan is to heat up all the pins as quickly as possible, tap
with a dental pick until you see the entire chip wiggle, and then quickly
flip it over with the dental pick and turn off the heat gun. This gets the
chip off the board before the inside of the chip has the chance to heat up
to dangerous levels. The ideal distance seems to be about 1.5 inches above
the device. This is really no different from the way a professional hot air
surface mount rework station operates, except they have metal airflow
concentrators that direct most of the hot air directly to the outer edges.
You could find a piece of ceramic tile that fits the top of the device, and
use that to help keep air flow off the top of the chip. Perhaps even take
some clay (real, not plastic) and form little pyramids of different sizes
for different chips. My heat gun happens to blow most of the air around the
edges of the barrel, so it works fine for me the way it is. I have used this
technique on devices up to PQFP-208.
For soldering, you want to keep in mind from the very beginning to use as
little solder as you can get away with. Go ahead and buy the thinnest solder
you can find, or a syringe of solder paste if you can get it. Use the barest
amount of solder and a fine-tipped iron to tack down the chip in alignment
on the board, then go around with a small screwdriveror razor blade and
ensure every pin is in contact with the pad below it. Run a flux pen along
the length of the pads. Then take a length of fine solder (.016" or smaller)
and lay it across the pins, and use a LARGER soldering iron, about 40W, with
a bigger tip to smoothly drag along the pins and solder. If you do it right,
you should have nearly every pin soldered. If you used too much solder, it
will wick up in between the pins and lodge itself right up against the chip,
where it is difficult to remove even with solder wick. That's why you need
the thin solder, because if you at any point have too much in one area it
will wick up. You need the large soldering iron because it stores more
energy and won't cool down before you get to the end of the row. ONLY use it
for dragging though, and don't keep it in contact with any single pin for
more than a second. Use the fine tipped iron for touchups.
Alternately you could just run a thin bead of solder paste along the pads,
position the chip, and heat up again with the heat gun. Solder paste is
expensive so I don't do this with large chips that are easily done with the
above method.