10 Sep
2011
10 Sep
'11
3:52 p.m.
On 10 Sep 2011 at 22:59, Rob Jarratt wrote:
What I am not clear on here is if the two suggestions
go together
(temperature control and chip-quik or similar). It sounds like you are
suggesting either on their own would be good, but I assume the idea is
to use low temperatures to avoid damage, and because of this you need
to use low melting point solder to desolder. But surely you need
higher temperature to melt the original solder first? Sorry, but I am
still not experienced enough to understand this.
Ah, okay--let me be clearer.
You need a TC iron if you're going to be doing much repair work. The
danger of lifting traces is exacerbated by a non-TC iron.
With CQ, I remove SMT devices using the heat from a PAR38 lamp, You
don't actually melt the original solder; it dissolves in the fusible
alloy as the latter melts. You could probably use a soldering iron
with an adjustable temperature control turned down (say, 300F).
Cleanup is easy--I use an old toothbrush.
There are many very decent irons around selling for a fraction of
what my Weller WTCP cost me (on the other hand, it's put in more than
30 years of service and I can still get parts for it). Xytronic and
Hakko (as well as Weller) are some brands that are worth considering.
Others will have their own suggestions, but the important thing is
temperature control.
In some cases, this is done as an electronic closed-loop controller;
in the case of my Weller, each tip is stamped with a number
indicating its temperature. A magnet in the handle closes the heater
contacts by being attracted to the alloy slug at the base of the tip.
When the alloy reaches its Curie temperature, the magnet is no
longer attracted to the tip and the circuit opens. Very old school
and very reliable.
What others haven't mentioned is that in multilayer boards with power
and ground planes, it can be difficult to get enough heat to a joint
to melt solder, as the heat is conducted away from the iron tip as
fast as it's supplied. CQ comes in handy there, by lowering the melt
temperature of the joint.
For whatever it's worth,
Chuck