"Ed Huntress" writes:

The solder in a joint can wind up having a much higher melting point and
much less wetting ability than it did when it was initially applied. That's
because it amalgamates to the material being soldered to some degree. If the
joint was the least bit overheated initially, or upon re-heating for
removal, it can be a fairly high degree.

You notice it more trying to unsweat copper plumbing. Sometimes you have to
wet the joint with some fresh solder to get it to come apart. But I've run
into the same thing taking components off of an old board.

I've also noticed that I often need to add solder to make a second try
at unsoldering a component lead if the solder sucker didn't get
everything on the first try. I theorize (but can't prove) that the
extra solder is sometimes necessary to carry heat all the way down into
the plated-through hole, to melt the solder all the way through the
hole.

If the component that's being removed has nice fat copper leads, those
can carry heat pretty well (assuming you can make good enough contact
between the iron tip and the lead). But IC leads don't seem to be
terribly good heat conductors. A nice bit of liquid solder does a
better job of conducting heat from the iron tip down into the PCB hole.

Dave