Getting Somewhat More Serious 107Experiment 12: Joining Two Wires Togethertools
Eight most common soldering errors
- Not enough heat.
The joint looks OK, but because you didn’t apply quite
enough heat, the solder didn’t melt sufficiently to
realign its internal molecular structure. It remained
granular instead of becoming a solid, uniform blob,
and you end up with a “dry joint,” also known as a “cold
joint,” which will come apart when you pull the wires
away from each other. Reheat the joint thoroughly and
apply new solder.
A leading cause of underheated solder is the tempta-
tion to use the soldering iron to carry solder to the
joint. This results in the cold wires reducing the tem-
perature of the solder. What you should do is touch the
soldering iron to heat the wires first, and then apply
the solder. This way, the wires are hot and help to melt
the solder, which wants to stick to them.
Because this is such a universal problem, I’ll repeat
myself: Never melt solder on the tip of the iron and then
use it to carry the solder to the joint.
You don’t want to put hot solder on cold wires. You
want to put cold solder on hot wires. - Too much heat.
This may not hurt the joint, but can damage everything
around it. Vinyl insulation will melt, exposing the wire
and raising the risk of short circuits. You can easily dam-
age semiconductors, and may even melt the internal
plastic components of switches and connectors.
Damaged components must be desoldered and re-
placed, which will take time and tends to be a big hassle
(see “Tools: Desoldering” on page 109 for advice). - Not enough solder.
A thin connection between two conductors may not be
strong enough. When joining two wires, always check
the underside of the joint to see whether the solder
penetrated completely. - Moving the joint before the solder solidifies.
You may create a fracture that you won’t necessarily
see. It may not stop your circuit from working, but at
some point in the future, as a result of vibration or
thermal stresses, the fracture can separate just enough
to break electrical contact. Tracking it down will then
be a chore. If you clamp components before you join
them, or use perforated board to hold the components
steady, you can avoid this problem.
5. Dirt or grease.
Electrical solder contains rosin that cleans the metal
that you’re working with, but contaminants can still
prevent solder from sticking. If any component looks
dirty, clean it with fine sandpaper before joining it.
6. Carbon on the tip of your soldering iron.
The iron gradually accumulates flecks of black car-
bon during use, and they can act as a barrier to heat
transfer. Wipe the tip of the iron on the little sponge
mounted in the base of your soldering iron stand or
your helping hand.
7. Inappropriate materials.
Electronic solder is designed for electronic compo-
nents. It will not work with aluminum, stainless steel, or
various other metals. You may be able to make it stick
to chrome-plated items, but only with difficulty.
8. Failure to test the joint.
Don’t just assume that it’s OK. Always test it, by apply-
ing manual force if you can (see Figures 3-38 and 3-39
for the ideal protocol) or, if you can’t get a grip on the
joint, slip a screwdriver blade under it and flex it just
a little, or use small pliers to try to pull it apart. Don’t
be concerned about ruining your work. If your joint
doesn’t survive rough treatment, it wasn’t a good joint.
Of the eight errors, dry/cold joints are by far the worst,
because they are easy to make and can look OK.
Figure 3-38. Test result of
a bad solder joint.Figure 3-39. Test result of a good
solder joint.