When I repair old equipment, indeed I often find myself removing old
solder from a joint as completely as I can, and soldering it with new
solder.
I can come with several plausible reasons why the old solder works
terribly if I just try to reflow it applying some flux:
- Antique solder is often 30/70 tin/lead! That solder goes through a
very long paste phase when hardening, causing a great risk of
cristallizing and creating bad joints. 40/60 solder was common in the
70's, and is only slightly better. Today the most common (unless RoHS
compliance is needed) is 60/40, which works fine, and the eutectic (no
paste phase) 63/37 solder is easily available too.
- Old solder can be heavily oxydized or sulfated, and when reflowed,
these oxides and sulfates mix with it.
- Whenever solder is heated, some of its tin evaporates. Even if it
originally was 60/40, after melting it a few times it can be 50/50 or
even worse.
- Soldering consists in creating a surface alloy between the solder and
the base material. The longer the solder is liquid, the hotter it gets,
and the more times it is heated, the more base metal it dissolves. So
the solder can end up containing enough of the base metal to give it
poor characteristics.
A very common cause of failure in modern equipment is that a
surface-mount device develops an intermittent connection to the board,
because the solder under one or several of its pins cracked. Typically
the microscopic arcing and the localized heating inside the crack will
cause oxidation. If such a damaged joint is simply fluxed and heated,
the solder will beautifully re-arrange itself, one half on the pin, the
other half on the board pad. And between them there will still be the
oxide layer! The joint will look nice and shiny, but the intermittent
connection is still there! Even close inspection with a strong magnifier
often doesn't reveal the problem, but a simple ohmmeter does. Another
trick is to push down the suspect pins with a rubber stick while the
circuit is powered on, and see if it starts working.
In these cases, I do the radical solution: Completely unsolder and
remove the part, flush the pads with fresh fluxed solder, wick it off,
check with the magnifier to make sure the solder properly wetted ALL of
the pads, then do the same to the component, then reinstall.
So, yes, I agree, old solder should be removed and replaced, not simply
reflowed, whenever a joint is remade. Specially if it looks even
slightly dull.
Manfred
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