On Mon, 30 May 2005 13:58:34 -0700 DaveC wrote:

On Sun, 29 May 2005 19:44:19 -0700, Jim Adney wrote
(in article ):

I've done things like this a few times, and the technique that I found
that seemed to work best was to position the board just above (1-2") a
hot plate, and set the hot plate to just below the solder melting
point. Then I could go into the board with a heat gun or soldering
iron, depending on the task, and apply the small amount of additional
local heat that the job required.

Thanks, Jim. This sounds like a good approach.

It brings up a couple of questions: how can I be sure that I don't exceed the
pre-heating temperature (with the hot plate)? I don't want components
dropping off of the underside of the board. And once I start to heat the
topside of the board with the gun, how can I be sure I don't exceed the
melting point of the solder? I understand that getting things too hot will
melt the solder connections within an IC (they use a higher melting point
solder to avoid this problem during manufacture of the PCB, or so I hear.)

You may have to work up on the hot plate temp. The trick is to use it
to heat the under side of the board to just below the melting point,
so that the extra heat from your iron or heat gun will be enough to
melt the parts you're interested in.

I don't think there is any solder in IC connections. The connections
there are spot welded. The temp concern with ICs is that high temps
will cause the doping ions to migrate and this migration may ruin the
careful construction of the IC. This will be a time*temp thing, so you
need to work as fast as you can.

ICs and transistors may have the silicon die soldered to a metal base,
but I'll bet that this solder will be somewhat higher temp than where
you need to work, but that's why you need to be careful not to heat
any hotter than necessary.

Yes, it sure seems touchy, but I've managed to succeed a couple of
times, with no failures so far.

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Jim Adney
Madison, WI 53711 USA
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