I did a little more work today and lifted yet another pad out of about
7 or 8 solder joints. I am a beginner, but is this a very common
occurance in soldering work? I am using braid on the joints (they
don't have alot of solder, usually single componenet lead), 450F (using
Weller WESD51), and am placing the tip onto the braid for a second or
less at a time, letting cool for 10 seconds or so before going back and
wicking more solder.
The unit I am working on is one of the brown boards mentioned, in an
old HK 330A receiver ('60s). I notice that throughout the board, the
solderer not only soldered at the pads, but spread it out onto the
trace for a bit, did it quite often. Why is this? It also seems that
it is quite easy to add solder and have it follow the trace, I tacked
down a lead onto the trace of the pad that lifted--worked well,
actually--and there wasn't any spillover. What is at work here, some
kind of wicking action? I am also brushing on rosin paste onto the
areas to be soldered and am using rosin core solder. I'm finding that
the solder is hard to place at first when there is rosin on the pad,
perhaps I am using too much? I'm finding that doing a good solder job
is a bit harder than one would imagine...
In my humble opinion, based on over 40 years of soldering, 450 deg is too
hot for standard leaded solder. A typical workshop workhorse iron, the
Weller PCT magnastat controlled station, comes with a " 7 " series tip as
standard. This develops approx 370 deg at the tip, and that is plenty for
standard solder. It is even just about good enough for the new-fangled
lead-free solder on small joints. Even the next tip up, the " 8 " series,
only develops 430 deg. Apart from the track to substrate bonding failing,
which is very typical of old boards anyway, even without overheating,
another downside of a too-hot iron is that the internal flux cores on the
solder wire, burn off before they have had time to do their work. You never
need to use external flux ( enter Smitty at this point !! ) for ' normal '
soldering work, although I would absolutely recommend the use of liquid flux
for surface mount rework. Until you become MUCH more skilled with an iron
though, I wouldn't recommend that you have a go at SM ...
There is nothing wrong with desoldering braid for a beginner, but there is a
technique to using it. Firstly, you MUST use good quality braid, which is
expensive. The cheap stuff that you get at radio rallies and the such, is
not good at wicking up solder. It should also be kept in its air-tight
container, otherwise its ability to wick solder will deteriorate. You must
cut off any length that has been used, or even heated without taking up much
solder. Don't use a guage which is too big or small for the job, Typical
professional workshop guage is 2mm. Before using the braid apply your iron
and some FRESH solder to the joint. New solder flows MUCH easier than old,
plus the joint will be pre-heated when you come back a few seconds later
with your braid. Don't under-heat the joint when wicking. Place the wick
against the side of the joint, then get the iron tip flat against the braid.
The joint should melt within about a second, and then another second or so
should see the solder come off cleanly. Slide the wick and iron off
together, to the side.
Finally, I wonder what shape tip you are using on your iron ? For
desoldering work using braid, you need one with a flat end - a small chisel
or screwdriver type is ideal. A pointed tip is good for soldering, and OK
for use with a desolder pump, but does not have enough thermal inertia for
work with braid. Oh yes, nearly forgot, before everyone starts screaming
that you can make your own braid. Yes you can, but it's a messy business,
and unless you've got lots of time on your hands, not really cost effective
for a busy repair shop. Probably OK for the cash-strapped amateur though.
Hope this helps. Stick at it. Like lots of things in life, soldering is a
skill that has to be practiced and developed, and you will get better and
better over the years. Keep it simple in the early days, and experiment on
scrap boards.
Arfa
