wrote in message
ups.com...
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


Hint for de-soldering surface mount ICs.
Use a hot-air paint-stripper,1400W,500 degree centigrade,with 2 level heat
control to prolong element life.
Form a ring of silicone covered wire around the IC{to isolate the remaing
components on the pcb.Push a thin piece of wire
under one side of the IC and form a loop around the IC,repeat on the other
side;this is to remove the IC when the solder melts, tug on these wires
while heating up to ensure minimum heating contact time.
Place a slab of PTFE with right size hole cut into and
clip pcb and slab together with clothes pegs/Bulldog clips etc.
If the IC is for re-use then cover body of IC with heat insulating material
or blast IC with freezer spray.Allow the hot-air gun to get up to heat{say 1
minute}before applying to IC.
For more crowded boards make "conical" shrouds to surround the IC. I used
some PTFE strip that i had but thin paxolin or similar but drilled and wired
together would probably do. Cut 4 small trapezoids from the PTFE strip.
Fixed together with all long edges one side and short edges adjascent on the
other side. Fixed together with paper staples but for the smallest shroud
for 8 pin SM had to wire together the final join. Forms a sort of truncated
cone, frustrum, in shape. Tie to the PTFE cone (to stop the blower blowing
it off) with
copper wire or temporarily solder to distant points.
When practised the heated contact time should be less than
2 seconds - no board distortion or collateral damage surprisingly.
If you can't get the tugging wires under the IC then pass
under a few pins at each corner.
Because this tugging frees the IC at the earliest moment, the solder
on the board is not fully melted and leaves a profile for localising
the new IC in place and then solder pin by pin.
SAFETY NOTE:- ensure good ventilation, use safety goggles,and beware of very
slight risk of combustion.

other hints and tips on

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/