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Electricity > Soldering Guide

Soldering is the art of joining metals with a filling material (solder). It allows electronic components to be joined together to make up an electrical circuit. In school sciences, electrical equipment regularly gets broken or damaged. Connecting wires may have their plugs pulled off or components may need replacing. Some of these repair jobs can be undertaken using a soldering iron. It takes practice but practice makes perfect and novices are quickly competent.

Firstly we would like to say NEVER try to undertake any electrical repairs that involve mains electricity unless you are a qualified electrician. There may be someone in the school with relevant experience such as D&T staff that you could ask to repair the item in question, if in doubt, ask the manufacturer or supplier of the equipment which is the best way to remedy the problem encountered.

Likewise, never touch capacitors that are sometimes inside electrical devices whether mains or battery powered. Even long after they have been disconnected they can give a potentially lethal shock. Again, seek an electrician if in any doubt.

Soldering plugs onto connecting wires that are used in schools is a simple job that anyone can do, however many modern plugs can be connected with screw fit plugs which eliminates the need for soldering. These are simple and quick to fit. Schools that make up their own connecting leads (for labpacks etc) may want to look at companies such as Rapid who sell ready made leads cheaper than their individual parts.
  • Soldering requires several pieces of equipment, firstly a soldering iron. These are rated in watts. The higher the wattage, the faster they heat up. An iron between 15 and 30W is advisable. Most electrical irons consist of a heating element which transfers heat to the tip. Gas soldering irons burn a compressed gas and 'cold heat' irons have the added advantage of heating up and cooling down very rapidly between uses.
  • Solder is the 'filling material' which melts at high temperatures, joining the two metals together. Most solder is a mixture of lead and tin although lead free solders are becoming increasingly available. Most solders also have rosin 'flux' mixed into the metals in order to remove impurities and therefore make a better contact.
  • A 'soldering station' is essential to hold the iron when not in use and most come with a sponge at the base which can be used to remove unwanted solder from the tip of the iron. This sponge should be soaked in cold water before use.
Safety Precautions

Remember the tip of the iron is at around 400°C which can give a nasty burn. Keep the iron in its stand when not in use. It takes a long time for the iron to cool down after use.

Work in a well ventilated area. The rosin flux can be harmful if breathed in. Make sure a window is open near by producing a good air flow.

Do not touch the mains flex with the tip of the iron. The flex should be heat resistant but damage may occur causing a dead short if the protective coating is melted.

Ensure no combustible materials are near to the hot iron.

Always wash you hands after handling solder. It may contain lead and other poisons.

The soldering iron should be held like a pen, making sure your hand is only in contact with the plastic casing and not a metal part which will obviously get hot.

Holding a length of solder, melt a little onto the tip of the iron, just enough to cover the tip. This is called tinning and it allows heat transfer from the iron to the metal part you are soldering.

Touch the tip of the iron onto the joint, hold for a few seconds and then feed a small amount of solder onto the joint. You should see it melt and spread. Take the iron away keeping the joint held together for a few seconds. If you heat it for too long you may damage or melt anything connected to the wires. A good joint should be shiny and clean.

If soldering components onto a board, make sure the joint is a 'volcano shape' see below and not a dry joint where the two metals being joined may come apart easily.

Fig 1: Good and bad joints
Fig 1:
Good and bad joints

If the joint is not a volcano shape all is not yet lost, just apply another small amount of solder to the joint. If you end up with too much solder on the joint, 'spoon' some off with the tip of the iron, dabbing any excess onto the damp sponge in the soldering station.

Heat sinks

Heat sinks are required if the component you are soldering can be damaged by heat. Crocodile clips can be clipped between the component and the joint you are soldering to dissipate some of the heat that travels along the metal wire. It is good practice to use heat sinks when soldering all components, in any case you should not over heat any component or joint, touching the tip of the iron with the joint for as little time as possible.

Desoldering

Desoldering pumps can be purchased which act like a syringe, removing excess molten solder from boards or components. Most are spring loaded devices that need pressing down to lock. When required a button is pressed which sucks the solder into the device. This solder will need removing occasionally.

Solder wick continues to be used and is sometimes called desoldering braid. It consists of a roll of 'knitted' copper wires in a wick shape. These can be used by applying the end of the wick and the tip of the soldering iron to the unwanted solder. The wick should absorb the molten solder. Simply cut off the end of the wick which has absorbed the solder once it has cooled.

If soldering regularly you WILL burn yourself, everyone has at one time or another. Splashes of molten solder can also cause burns. Always use slow movements when soldering as to reduce potential splashes. If you do burn yourself, hold the affected area under a running cold tap for as long as possible and get first aid attention.

Remember that the soldering iron will remain very hot long after it has been turned off so make sure it has been replaced into its soldering station and is not in contact with any combustible materials.

Links

For tips on how to prolong the life of your soldering tips, please see Soldering Tips (from Rapid).