: Soldering components onto the printed circuit board

Equipment and materials

Before buying soldering irons and solder, you should read the relevant safety documents for guidance. Further information on safety is given in the section on pedagogy.

There is a wide variety of tools and materials on the market, varying in price and quality. The following list is a general guide.

Soldering iron

24 volt operation
15 to 18W power
approximately 2mm diameter tip

Soldering iron stand with sponge  

Hand tools

Snipe nose pliers (these have fine points and are used for manipulating thin wire)





Side cutters (for cutting wire close to pcbs after soldering

Wire strippers

 

Desoldering tool (pump)

Or you could use ‘copper wick’ (This is fine copper wire which has been plaited. It ‘absorbs’ solder by capillary action.

Solder 

Solder used for electronic circuits is an alloy of tin (60%) and lead (40%) which melts at 188°C. It is bought as a wire: for most work the thinner gauge (22swg) will be more economical. (The thicker gauge tends to put too much on to a joint.)

The solder has one or more ‘cores’ of flux running along its length. The flux is a chemical which helps to remove thin films of oxide which easily form on copper surfaces, and so make a good joint where the solder bonds properly to the underlying metal.

Traditionally, the flux has been based on a resin compound, but this does produce some fumes which can affect a few people. Solder with non-resin based flux is available and is recommended for schools use.

Lead-free solder can be obtained, but the safety advice available at present indicates that there is no discernable risk associated with the use of lead-based solder. However, it is recommended that pupils should always wash their hands after soldering to reduce the chance of ingesting lead.

Safety

Soldering irons can cause nasty burns if used carelessly. Typically, soldering irons reach temperatures in excess of 300°C. You should hold components and wires with pliers if they need support or use the assembly aid for this. Refer to nationally published guidelines about soldering techniques, soldering irons, choice of solder and first aid procedures for dealing with burns. Further information on safety is given in the section on pedagogy.

If your LEA is a member of CLEAPSS you will have access to information about risk assessments and control measures to implement. A CLEAPSS information sheet related to soldering irons.

Soldering technique

The essential stages of this process are: 

• Clean components
• Heat joint
• Apply cored solder
• Allow to cool

Cleaning

The metals to be soldered must be clean i.e. free from any surface contamination, such as dirt or grease. Components as supplied generally do not need cleaning. If they are in poor condition, they can be cleaned with the green abrasive pads used for scouring kitchen pans (they are better when used damp).

Grease and photo resist can be removed from the pcb using methylated spirits, or rubber scrubbing blocks are available from suppliers. Do not use glass paper or ‘wet and dry’ because they are too fierce. Avoid wire wool because the fine steel wires can come loose and cause short circuits between the tracks on the pcb.

It is not always necessary to remove the photo resist layer. It is possible to solder through this directly, although it is often easier to do this using a resin-based flux.

Heating and applying solder

Wipe the soldering iron tip on a damp sponge to clean it - the tip should be covered with a shiny layer of solder.

PCBs should be designed so that the components form a neat layout. Generally the components should lie down flat on the plain side of the PCB, unless there is a specific reason for having a gap between the component body and the PCB.

Sockets should be used for integrated circuits, although ic's can be soldered directly into a PCB. Sockets allow for easy replacement (and later, recovery of the ICs for use in other projects) and reduce the chance of damage by overheating when soldering.

There is a tendency for components to fall away from the board during soldering, since the components are then underneath. It is possible to avoid this problem by bending the wires of the components outwards after inserting them so that they are held tightly in place before soldering. However this is not done commercially and can usually be avoided. It does make component replacement more difficult.

Apply the soldering iron tip to both parts of the joint.

Then apply the cored solder with the iron tip still in place.

Aim to get the solder to run smoothly over all surfaces of the joint. Remove the solder first and then the iron as soon as this is achieved. Don't move the parts until the solder has set and avoid over-heating. Excess wires should be cut off after soldering.

A well-soldered joint 

The soldered joint should have a shiny surface - over-heating will give a dull appearance, due to the solder becoming oxidised. Inspect the joint for possible faults.

A 'dry' soldered joint with the solder not properly 'wetting' the lead and pad

Note - bad joint is in the foreground of the picture

The solder should have flowed nicely on to both the wire and the PCB - both surfaces should have been 'wet' by the solder. If the solder appears to be sitting against either surface as a 'blob' - rather like water droplets on a greasy surface - the solder may not have wet the surface properly. This is known as a dry joint, i.e. the two surfaces (wire and PCB) are not electrically joined by the solder.  

Safety: If your LEA is a member of CLEAPSS you will have access to information about risk assessments and control measures to implement. CLEAPSS information sheets related to the types of solder used in pcb manufacture are available from here.

Soldering faults

Very often a critical visual inspection, possibly using a magnifying glass, will enable you to identify and correct problems at this stage. These are the common things to look out for.

A detailed guide to soldering and de-soldering is available on the web at: www.epemag.wimborne.co.uk/solderfaq.htm