4 Apr
2011
4 Apr
'11
4:55 a.m.
On Mon, 4 Apr 2011, Philip Pemberton wrote:
For components like a right-angle DE9 connector, the top side of the board
is not accessible. Do you think using the hot-air handle for preheat
would do any good? If it was set at e.g. 150-200C with medium airflow
that might bring things up enough that I won't need a long application of
the iron. Or, maybe I should just break down and buy a preheater...
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...
Alas, I have two of them. An older OK Industries tool and a new BlackJack
Solderwerks thingie from Circuit Specialists. Neither of them is worth a
crap from my experience. Occasionally I'm able to clear the hole, but
more often it simply doesn't have enough suction to do the job and the
heat ends up damaging the via and/or adjacent traces.
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.
I do that as a matter of course - it does help a lot.
* 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.
This would be _after_ the component is removed, correct? If not, would
you mind elaborating a bit?
* 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.
I picked up a quantity of carbide-tipped PCB drills for just such a
purpose. Sort of a last resort, I'll grant you.
* 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!).
Is the Kapton tape available from electronic suppliers? I'll take a look
at the DigiKey catalog forthwith.
Steve
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