In most cases of soft soldering process, direct application of heat is avoided as it is difficult to control. It is to avoid damage to electrical connections, melting away of thin sheets and damage to other equipment where application of direct heat can cause distortion. It can also interfere with the heat treatment given to the job for a specific purpose. Hence a hand tool called soldering iron is used for indirect application of heat.
The soldering iron is used to melt the solder in the soldering process. The bit of soldering iron is made of copper which conducts heat into the metal to be soldered. Soldering irons are used for soldering metals having small areas of contact. Soldering iron is generally heated by means of a gas flame, blowlamp, coke fire or electricity. After obtaining the required temperature the soldering iron is used for soldering operation.
Types of Soldering Iron
The various types of soldering iron are described below:
(a) Common Soldering Iron
(b) Electric Soldering Iron
(c) Alumino Thermic Soldering Iron
Common Soldering Iron
This is a hand tool which consists of a copper bit, pointed at one end, riveted to a steel rod which is fitted to a wooden or plastic handle that acts as insulation. Soldering iron is generally heated by means of a gas flame, blowlamp, coke fire or electrical element. Before the soldering operation the bit is usually heated lightly and filed to remove the film of oxide formed on it. Then it is dipped in flux followed by applying the solder on the tip of the soldering iron. This process is known as tinning because a silvery appearance is provided by the solder sticking to the bit end.
The tool is available in various sizes according to the type of soldering iron required. The large iron will retain heat for longer period than a smaller one. The copper end of the tool is called soldering bit and is available in various shapes to deal with special work.
Care of Soldering Iron
Before using a copper bit of soldering iron, it is necessary to give a thin coating of tin or solder on the contact face. This is called a ‘Tinning’ of the iron and is carried out as follows:
(a) Clean the faces of the copper bit end with a file to remove all dirt and rough surfaces.
(b) Heat the copper bit sufficiently as to melt the solder.
(c) Apply flux to the hot bit and rub it against a stick of solder.
(d) The tin or solder will leave smooth thin coating over the faces forming the point of the bit.
Tinning of Soldering Iron
It is important not to overheat the copper bit as it will become oxidized or even melt on the surface. The iron should be heated to just above the melting point of soft solder (till the green flame appears). If overheated, the tinning will be burnt and re-tinning of the bit will be necessary. It is not possible to carry out soldering satisfactorily with an iron, which is not properly tinned. Dipping the iron into soldering flux every few second not only lowers the temperature of the soldering iron and slows up the speed of soldering, but also eats away the iron very quickly. When the iron is heated with the gas burner or blowlamp, the flame should not be directed towards the tip but towards the base of the copper bit.
While heating a soldering iron, due consideration must be given to a few factors to safeguard the soldering iron and obtain maximum efficiency. A common feature is the temperature of hearth and the duration of placing the iron on it. It goes without saying that a heavier soldering iron takes more time to be heated than a lighter one, also a brightly glowing furnace will take less time to heat an iron than a dull one. The portion to be heated should be the complete copper bit and not the tip of the bit only otherwise the tinning will go away. A green flame indicates correctly heated iron ready for use and any further heating would be harmful to the soldering iron, as it will burn the tinning.
Heating of Iron
When placing an iron on the open-hearth, care should be taken not to burn away the handle. If by chance a soldering iron is overheated to dull red colour, it must be immersed into water immediately. When source of heat is blowlamp, a clean blue non-smoking flame should be directed on the iron. The same thing holds good for any open fire. Similarly, when heating a soldering iron by a blowpipe the flame should be neutral one. In no case should an iron be subjected to a carburising flame.
NOTE. Always keep the soldering iron on the firebricks or any other non-combustible material such as asbestos sheets, etc.
Electric Soldering Iron
The electric soldering iron is very convenient for repetitive work and used in a workshop where other forms of heating are not available or desirable.
Earlier, this type of iron had a fixed bit which used to work at a fixed voltage. The latest electrical soldering irons are supplied in different ranges with replaceable bits of various shapes and sizes, which could be worked at varied voltages. These are generally classified by the voltage. A modification of the electrical soldering iron is the soldering gun. In shape, it resembles electric drilling machine with a pistol grip. On pressing the trigger, a steady flow of solder and flux is released through the hollow copper bit. An electric soldering iron maintains a temperature of 300ºC (approx).
Safety Precautions While Using an Electric Soldering Iron
Few precautions while using an electric soldering iron should be strictly adhered:
(a) The connection cable should not be worn out, displaying the bare wire.
(b) A 3-pin plug is to be used assuring good earthing.
(c) No part in the iron should be a loose fitting.
(d) The voltage for which it is to be used must be clearly stamped on it and must be used at that voltage only.
(e) When connected to a supply the business end shall not be left on a wooden surface.
(f) The bit should never be dipped in flux for cleaning it.
(g) Never use an electric iron for prolonged duration continuously. It will be detrimental to the heating elements.
(h) The correct grade of solder is to be used while working.
(i) It should be tested for sufficient heat by touching the solder wire to the tip of it.
(j) Cleaning of bit can be affected by wiping it off with waste cloth or scabbing with a blade.
(k) If it is giving shock, it must be disconnected immediately and sent for repair.
(l) Heating of an electric soldering iron should never be attempted by external means.
Alumino Thermic Soldering Iron
This is a special and handy type of soldering iron used during emergency and where other source of heat is not available.
The bit is hollow copper one with an extension at one end (business end). In the cavity a ‘Mox’ tablet is placed which is the heat energy of the iron. This tablet consists of aluminium oxide and magnesium oxide. These oxides are filled in a circular metallic casing with an opening in the centre to ignite it. It is set on fire with a special matchbox called ‘Fusee’. The opening is sealed with a paper covering, which must be removed prior to use. It remains hot for 5 to 6 minutes and should be made full use of this period. These irons must not be used near aircraft, MT, or any other inflammable store while the tablet is still burning.