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Solder used in electronics is made from approximately 60% tin and 40% lead with a flux core, and is designed to have a melting point of around 190 °C (374 °F). The flux is a special substance also known as a “wetting agent” that flows during the heating process to remove oxides from the metal surfaces giving a better contact between the metals. Solder is available in varying thickness - the thinner types used in more delicate electronics work.

In recent times there has been a lot of controversy surrounding the use of lead in solder and much interest has emerged in lead-free solder. Lead-free solders contain metals such as tin, copper and silver, and generally have higher melting points creating new challenges for component manufacturers.

Soldering irons

A soldering iron is a hand-held tool used to apply the heat that melts the solder when creating a solder joint. The iron consists of a metal tip heated by an internal electrical heating element and an insulated handle. There are two types of soldering irons generally used in electronics. The first type connects directly to the mains power and is relatively inexpensive. This type of iron is normally rated between about 15 and 60 watts - the lower the wattage the more suitable for delicate work. The second type, at about three times the price, is the temperature controlled soldering station more commonly used by professional technicians for prolonged periods of soldering. You can achieve a perfect solder joint with either type of iron.

An iron’s tip should match the type of soldering work you’re doing. Fine point (conical) tips are used for delicate work, such as when soldering integrated circuits, whereas wider (chisel) tips are more suited to soldering larger terminals.

Preparing the iron

Heat up the soldering iron and apply a little solder to the tip when it has reached its full temperature, and remember to clean the tip often when soldering. This done by gently rubbing a wet sponge, cloth, or paper towel along the surface of the tip. Special tip cleaning products are also available. 

Preparing the surfaces

The metal surfaces to be soldered must be cleaned of any foreign matter such as grease and dirty. This can be done using a cloth, soft brush, emery paper, or special steel-bristle brush, either dry or with a solvent such as isopropyl alcohol.

When soldering wires, it’s a good idea to “tin” their ends first. This is done by heating the stripped end of the wire with the soldering iron and applying a little solder to give it a shiny finish.

The components and terminals to be soldered should then be connected together to form a snug mechanical (and where possible) electrical joint. Don't rely on the solder alone to hold them together. Clip any lengthy component leads or wires prior to soldering. Where necessary, use tiny clamps or sticky tape to prevent components moving during the soldering process.

Doing the soldering

Make sure the soldering iron has reached the required level of heat and the tip is clean. Place the tip of the soldering iron directly against the connection to be soldered and apply the a little solder to the point where the iron touches the two metals. Allow the connection to heat a little further and apply more of the solder so that it quickly melts and spreads evenly over the entire connection. (Avoid the temptation to melt the solder on the hot tip and let it run onto the job, as this will result in a poor joint.)

Remove the solder and hold the iron in place just a little longer then remove it. The whole procedure should be over in a matter of seconds. Allow the joint to cool normally and reach its maximum strength. Resist the temptation to prematurely cool the joint (usually by blowing on it) and to prematurely test its mechanical strength by moving the newly soldered wires and components.

When soldering sensitive components, be careful not too apply too much heat that can damage the components or cause the copper tracks on the circuit board to peel. Small clamp-like heat-sinks can be connected to semiconductor component leads to prevent overheating and damaging the components.

Checking the joint

A good solder joint is one where the solder has flowed evenly over the surfaces to be connected, and appears smooth and shiny. Poor joints occur because of insufficient heat, overheating, insufficient solder, excess solder, premature cooling and premature movement. Poor joints may appear foggy, rough, be formed into little round blobs, or have sharp edges. Each joint should be visually inspected and redone if not correct. If there is too much solder on the joint already, remove the solder first (see below) before redoing it. Remember to also check for any solder bridges - that is, excess solder causing unwanted connections between two points.

De-soldering

De-soldering is normally done using one of three methods: de-solder wick; a solder sucker; or a de-soldering station. 

De-solder wick is a braid-like wire ribbon that’s placed over the joint to be unsoldered. The back of the wick is heated with a soldering iron so that solder from the joint is melted and drawn up into the wick. The wick is then removed and that piece discarded. The joint should then be free of solder.

A "solder sucker" is a handheld tube-like device with a spring-loaded plunger. The joint to be de-soldered is heated with a soldering iron to melt the solder and the tip of the cocked solder sucker quickly placed at the joint and “fired”. The rapid sucking motion draws the molten solder from the connection and into the solder sucker, which is subsequently cleaned.

A de-soldering station consists of a soldering iron-like device with a hollow tip that’s connected via soft tubing to a vacuum unit. The joint to be de-soldered is heated with the de-soldering iron’s hot tip and the vacuum unit is then engaged to suck air (and molten solder) through the tip.

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