Overview
Soldering is a process used to make effective electrical connections between certain metals by linking them with a soft solder.  The solder is an alloy of which lead (Pb) and tin (Sn) are principal components.  The alloy melts at a fairly low temperature (~190 ^oC); consequently, the metal joint can be heated by means of hand-held devices called soldering irons.  At one end of the soldering iron is a handle while the other end contains a heating element.  A "bit" connects to the end of the heating element and is the physical part that heats the metal joint to be soldered.  Always be careful with soldering irons, their temperatures reach over 250 ^oC!

A variety of soldering irons are available, each with ratings from 15 W to >100 W.  A high wattage iron heats a joint quickly so the electrical connection can be rapidly made.  This type of iron is best used when a large volume of metal must be heated.  Smaller wattage irons do not heat joints as fast, but may damage insulation during the time needed to properly heat the joint.  A small iron is best used for small electronic components that may be damaged by considerable heat.

The easiest type of solder to use is either a 18 swg or 22 swg multicore; use a standard 60% Sn, 40% Pb alloy solder with cores of non-corrosive flux.  The flux is a chemical designed to clean deposited oxides from surfaces to be joined and to exclude air during the soldering process.  You will never be able to complete a joint by simply applying an iron tip loaded with molten solder - flux is a must.  Conversely, there is another process called "tinning" where the metals to be connected are first coated with a fresh layer of solder prior to joining by a hot iron.  The novice will also soon realize the utility of tinning the soldering iron.

Make the connection
Soldering, especially good work, is a skill learned through lots of practice.  Maintain your patience if this is your first experience.  The most important point to keep in mind when soldering is that both parts of the joint to be connected must be at the same high temperature.  If this occurs, the solder will flow evenly and establish a good electrical connection.  If the parts of the joint are at different temperatures a surface oxide film can develop that insulates the metals from one another.  For that reason, do your best to heat all parts of the joint.

After the iron has reached the appropriate temperature (usually a few minutes after turning it on), apply some solder to the flat work area of the bit and wipe it on a piece of damp cloth or sponge.  This action forms a thin film of solder on the bit - called "tinning."

Melt a little more solder on the bit and orient the tinned tip of the iron in a location that contacts both parts of the joint.  The molten solder permits the heat to flow quickly from the iron into both components of the joint.  If positioned appropriately, both parts of the joint will reach the solder's molten temperature within seconds.  Carefully apply additional solder to the point where both parts of the joint and soldering iron meet.  The solder will immediately melt and flow are all the parts that are at, or over the melting temperature.  After a few seconds remove the iron from the joint.  Make sure that no parts of the joint move until the solder hardens.

The right amount of solder is also important.  Too little solder yields poor heat transfer and too much causes solder strings to form as the iron is removed.  Strings cause solder splashes or other contact bridges to be established.  An excellent soldered electrical connection has a smooth, shiny appearance and fixes the wires in place with no solder on other parts of your project.  While soldering is similar to brazing, in that a molecular bond is formed between the various surfaces of the joint, it is not to be considered a mechanical connection.  A soldered joint should always have mechanical support near it.

Removing a connection
Tin your iron bit just as if you were to make a new connection.  Melt a little more solder on the bit and orient the tinned tip of the iron in a location that contacts the soldered joint to be removed.  The molten solder permits the heat to flow quickly from the iron into the joint and the solder turns fluid within seconds.  Carefully apply the point of a solder extraction device near the iron and fluid joint then release the plunger to apply a partial vacuum.  The solder should be removed in one or two applications.

It will be more difficult to correct an improper joint, but with time and perseverance anyone can learn to solder!

Last Updated on 04 February 2000
Paul T. Jackson

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