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"vintage electronics articles and other useful information"
Soldering (or How to Avoid the Molten Lead Blues)
by Jack A. Orman
The art of making a good solder joint is essential to anyone who
wants to make electronic equipment, and it is an art form that is
not too hard to master, once the soldering basics are learned. Without good
solder joints, your project will not perform up to its potential.
To start with, you need the proper equipment. This means, at the
minimum, a soldering iron and solder. For working with
components, IC chips, transistors, resistors, etc., you need a
low wattage iron. Typically a 20 to 35 watt iron is good for this
purpose. If you are working with connectors, cables and other
devices that have more mass to absorb heat, then a higher wattage
iron may be necessary.
Many soldering irons are available with interchangeable heating
elements which may be changed to suit the job at hand. Having a
33w element and one in the 45w to 50w range would be ideal. But
if that is too costly, the 33w unit is a good general purpose
element. Many electronics suppliers stock good quality irons and
an acceptable iron is stocked by Radio Shack.
Weller, Ungar, and many electronics suppliers also carry
soldering stations which have a control that allows one to vary
the temperature of the soldering iron tip. This is a good way to
control heat and allows an iron with one element to be used for
any soldering job by varying the heat to suit the size of the
components being joined.
Iron-clad tips are the best however solid copper ones will serve the
purpose but require a little more care. The iron tips do not
corrode as rapidly after being used and typically last longer.
Many soldering irons have tips that may be changd to suit the
type of part you are soldering. A fine point tip will be used on
IC leads and a larger chisel-shaped tip may be put on to transfer
more heat to heavier components like jacks and cables.
Any tip should be tinned the first time it is heated up. That is,
a light coat of solder is applied to the hot tip to protect it
from oxidation and corrosion. This coating may be removed and
reapplied as required. The thinnest layer that can be adhered to
the tip is the best. A damp sponge is often kept by the soldering
station to wipe the hot tip on between uses. This will remove
flux and excess solder. A plain sponge from the hardware store
will suffice. Be sure it is not the type that is saturated in
soap or cleaning products. Cut a few parallel slits in the
sponge, then when you wipe the tip in the slits, it will clean
the entire tip in one pass.
A copper tip that has become corroded or cover with oxidized
solder can be renewed with a metal file. The oxidation and solder
are removed and the tip reshaped to a fine point. Then the shiny
copper tip should be retinned to protect it. This technique
should not be used on an iron-clad tip since it will remove the
iron coating. Between treatments with the file, I will scrape the
tip with the edge of a knife to remove light corrosion or solder
build-up, again, re-tinning after cleaning.
In case you haven't noticed, there are more than one type of
solder. In fact, there are quite a few types. Basic electronics
solder is an alloy of tin and lead. A 60/40 solder has a
composition of 60% tin and 40% lead. This will work well for
almost any project. A 63/37 solder or "eutectic solder" is also
available which has a lower melting point and hardens to a more
Wire solder is sold in coils or on spools and comes in a variety
of diameters; 0.016", 0.020", 0.032", and larger or smaller
sizes. I tend to use a small diameter solder since it is easy to
control the amount being applied, but any size in that range is
Wire solder is usually hollow with the core filled with rosin
flux. This flux material removes metal oxides from the surface of
the work and allows the solder to adhere better. Always use a
solder containing flux. Never use acid-core solder, or paste
flux. Sal ammoniac blocks are sold for tip cleaning but they
should be avoided as well since the chloride salt residue from
the block will cause corrosion on pc boards and connectors.
Remember, rosin flux only.
It is a believed by many builders of high end audio equipment
that silver solder gives a better, more conductive joint and
therefore better sound. Typical silver solders contain 2% to 4%
silver along with the tin and lead, and are available readily
from the same sources as regular solder.
Lead-free solders that contain only tin and silver are also sold,
but they are more difficult to use since the melting point of the
alloy is higher and it is easier to make "cold joints" with it. I
stay away from it for electronics use.
The trick to making a good solder joint is that less is more.
Less heat; less solder. You want just enough heat for the solder
to wet the surface, but not enough to damage components or raise
the copper pc trace. It is easy to tell when this occurs. Solder
applied to a too cold surface will ball up or refuse to adhere.
As more heat is applied, the solder will flow over the heated
surfaces and form a smooth shiny coating. It is important to hold
the pieces of the connection completely still while the solder
cools. Any movement can cause a poor solder joint. It only takes
a few moments for the iron tip to heat the components so that the
solder flows evenly over the metal. Excess solder can cause
bridging of cicuit traces and malfunction of the project, or
drips of solder can fall on other parts of the circuit.
A thin shiny coating with a good mechanical connection is what
you are trying to achieve. Heat must be applied to both of the
surfaces being joined so that the solder can make that
The copper traces on the pc board should be very clean before you
mount any components. A steel wool pad (without soap impregnated
in it) or fine grade sandpaper (400 or 600 grit) may be used to
remove surface oxidation from the copper. Lightly buff the pc
board traces until shiny, then wash the board with acetone or
isopropyl alcohol on a clean cloth to remove any sanding residue.
Allow the board to dry thoroughly before soldering on it since
both of these solvents are flammable.
The leads that extend from many components also need cleaning to
make a good connection with the board. I have found that
capacitor leads are tinned to protect them but the solder coating
has oxidized after manufacture. Resistor leads are likewise
subject to oxidation. There is also a waxy residue found on
component leads from the tape rolls on which they are handled or
stored. A quick wipe with a piece of fine sandpaper or a light
scrape with the edge of a knife blade will remove the oxidized or
waxy coating and make soldering easier.
For the beginner, a good way to practice is with a piece of
perfboard and some surplus components. Use perfboard that has
copper pads etched on it and build up your technique by soldering
components to the pads. Apply heat to the pad and the component
lead simultaneously. After a few seconds apply solder to the pad
and lead, not to the iron. If it does not melt, wait a few
moments and try again. It will not take long before you are able
to make round shiny joints on the first try.
Soldering leaves behind some of the rosin flux after you have
made the joint. The flux residue should be removed before using
the circuit since a clean board will perform better and flux can
even act as a small-value capacitor when it is left between
adjacent traces or connectors. Circuit board cleaners and flux
removers are availble at most places that sell solder and irons.
Use it sparingly to remove flux or other residue from the board.
TF-113 was used for many years and was the ideal solvent for this
purpose, however, it is no longer available since it is a CFC and
has ozone-depleting potential. Unfortunatey, many of the
replacements are flammable and more irritating if inhaled so care
must be exercised in their use. As with any chemical, handle with
care, and wash your hands after using, especially before eating
Remember a cold solder joint is dull and often had a crusty or
crystalline appearance. A good solder joint is shiny, smooth and
mechanically strong. Practice makes perfect, but making good
solder joints is a task easily mastered.