3 Apr
2011
3 Apr
'11
6:41 p.m.
On 04/04/11 02:16, Steven Hirsch wrote:
Ended up removing the RTC clock latch, the two
adjacent LS166 chips
(mouse and game port interface), the Ricoh RTC chip, the F257 memory bus
driver and a couple of electrolytic caps. Things were chewed enough
around the clock latch that pads lifted despite my taking great care.
Took a full saturday afternoon to rebuild with 30g Kynar wire. Found two
more traces eaten - one under the Ricoh chip and one under the first LS166.
Yuck, and I thought my repair of the 386 motherboard was bad...
As I recall, that was two broken tracks, a few broken through-plated
holes, some broken solder joints on the resistors and diodes around the
battery, and a serious case of disintegrating PCB...
The HP16500B was worse. About a dozen broken tracks on the system board
which the previous seller neglected to mention, all on the address bus.
It "worked"... until I removed and reinstalled the Slot A card (16530A
digital scope timebase) while I was installing a CompactFlash card mod.
Net result of that was that the system software thought the LAN card was
hosed, and wiped the MAC address / LAN settings EEPROM.
That was NOT fun to fix. I think I went through about a dozen
surface-mount PLCC sockets before I managed to solder one down without
melting the baseplate. Those things are evil.
Still curious what folks' experiences are in using
a hot-air reflow tool
to soften up through-lead components from behind for removal.
Doesn't work that well in my experience. Although a hot-air preheater
combined with a soldering iron works wonders. Bring the board up to
about 150 Celsius, then use a soldering iron to provide the extra heat
required to melt the solder. A similar trick is used when soldering and
desoldering small SMD parts with a hot-air station...
I think your best bet would be a heated-vacuum tool. Basically a
solder-sucker mated to a soldering iron...
Some little cheats which might help out:
* If the surface of the solder is corroded, scratch into it with a
knife. This should reveal a bit of fairly fresh solder: scratch off
enough that your iron can make good thermal contact with the joint
(oxidised solder doesn't conduct heat particularly well).
* Melt some fresh solder into the joint. Rosin flux paste or gel
(usually used for SMD soldering) is also worth a try.
* Desolder wick will remove most of the solder, but it most likely
won't clear a through plated hole unless you make several attempts
(desolder, fill with solder, hold iron for a few seconds, repeat). Which
brings me onto my next trick...
* 24swg tinned copper wire is GREAT for clearing solder out of
through holes without damaging the plating. Enamelled copper might work too.
* If all else fails you may have to drill out the solder joint. Don't
do this on a four-layer board -- it'll ruin the plating. You can fix the
plating on a two-layer board using thin tinned copper wire (40SWG or
thinner). RoadRunner wire is good for this, especially the tinned-copper
stuff.
* If you repair tracks with wire, DO NOT use cyanoacrylate glue to
stick the wire down! Use a few small pieces of Magic Tape or dots of
epoxy. Cyanoacrylate degrades under heat (forming extremely hazardous
fumes which WILL burn your eyes, nose and throat quite effectively).
Epoxies can be removed with a small heat gun if necessary, or just plain
pulled off of the solder resist. Kapton (brown heat-resistant polyimide)
tape is good too (buy a few rolls of this stuff if you don't have some
already!).
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/