Hi all,
I have a small surface mount amplifer/filter chip on a packed PCB that
I need to replace. In order to solder in the new one I obviously need
to desolder the old one first being as careful as possible not to
damage anything else on the board. I've never desoldered before and
was wondering if someone could give me the basics or atleast point me
to a tutorial for desoldering in this type of application. Thanks so
much for any advice,
Kev
If you really have no experience of desoldering, then I would venture to
suggest that you are dicing with death to attempt successful removal of a
surface mount chip on a packed board, as a first project ...
You really will need to practice first on a scrap board. I usually start by
flooding the pins with new solder, then wicking off as much as possible.
This will remove much more solder than just going for it with the existing
solder, which will not melt and flow up into the solder wick very well. I
then slide a length of single strand rework wire under all the pins down one
side, and tack solder the end to any convenient solder pad. If you then
gently pull on the free end of the wire, away from the chip body, whilst
applying heat to each pin in turn, they will come up off the board cleanly,
as the wire passes below them.
When most of the pins are separated from the board, you may see the chip
body start to move. Be careful then that the removal of the last couple of
pins does not twist the IC and pull the last couple of pads off the board.
If you do not see the IC moving, it is likely glued to the board. The glue
bond is usually easily fractured by inserting a blunt scalpel blade under
the edge of the IC body, and twisting. When the IC is off the board, re-wick
the pads to make sure that they are very flat, and remove any traces of
glue.
Position the new IC very carefully, making sure that it is the right way
round, then tack two pins at opposite corners. Check the pin alignment again
with a strong magnifying glass. When you are satisfied that it is aligned
correctly, apply some liquid flux to one row of pins, then just go ahead and
solder with the smallest iron tip and finest gauge solder that you have.
Don't worry about solder-flooding bridging the pins at this point. If you
are using a good quality liquid flux, this should be minimal anyway, if you
are not going mad at applying solder. Repeat for the other one or three rows
of pins, depending on package type.
When you have finished, examine your work with your magnifying glass again -
a jeweller's loupe is ideal for this - and then go back to any pins that
have bridged, and apply your solder wick with your iron tip to the vertical
faces of the affected pins. This will remove the excess solder causing the
bridge, without compromising the joint. Clean down with flux remover and an
old toothbrush, and recheck the work with the magnifier. You should find
that the job you have done is nearly as good as the original flow soldered
product.
Arfa