These notes were written before the RoHS regulations. Before this the standard solder was composed of tin and lead (SnPb in a 60/40 ratio). All commercial producers now have to use lead free solder but at the present home users can still use the lead solder. Modern lead free solders contain tin, silver and copper (Sn95.5 Ag3.8 Cu0.7). This lead free solder melts at a slightly higher temperature than the older lead solders. However most soldering irons can accommodate this without too much of a problem. Note however that these new solders don't cool to form a shinny joint as the lead solders did but tend to cool dull, a bit like a poor looking 'dry' solder joint of the lead solders.

Soldering requires more skill than it looks. As with a lot of practical skills one learns a lot by trail, error and experience. Some tips are discussed below which I hope will be useful.

* always make sure the circuit board, components and wires are clean at the start
(scrape connections with a sharp knife or use sandpaper, abrasive blocks are available which give a good clean surface without the need for water / solvents and are less rough on circuit boards than sand paper)

* make sure the soldering iron tip has heated up properly and is clean.
Use a soldering iron spong to periodicaly clean the tip (and keep the sponge damp).

* never add solder directly to the iron.
Always heat the component and board and then feed the solder into the joint (see below)

When soldering a component to a circuit board :
1) apply iron to board-component junction
2) leave in this position for about one second
3) apply the solder into the joint
4) keep flowing solder into the joint until it is evenly covered in molten solder.
5) remove solder
6) remove soldering iron from board-component junction
7) leave joint to cool for 2 seconds

NOTE 1: if the joint is moved to soon, or jogged, the solder will fracture - this is called a dry joint and will give a poor connection.

NOTE 2: Many electronic components can be damaged by over heating them. However, if you follow the guidlines listed above you should not do any harm. If you are soldering an integrated circuit (IC or chip) leave each soldered pin for a few seconds so that the device can cool down. You can also put a Bull-Dog clip over the IC's legs (the clip goes on the component side, not the track side of the board) to conduct the heat away from the sensitive device.

Solder used to be a mixture of tin and lead, which melts at a fairly low temperature (for metals anyway) and gives good electrical conductivity and mechanical strength to joints. Nowerdays lead is not allowed and other mixtures of metals are used.

NOTE 3: Lead is highly toxic, as there is a great deal of old electrical equipment 'out there' you should be aware that you might be exposed to lead based solder if you do work on old equipment.

The smoke that comes off when the solder is melted is due to flux - an additional chemical that is cleverly contained within thin channels along the length of the solder wire. Flux helps the solder to flow, producing a better overall joint. The flux, which can be for example ammonium chloride, completely dissociates when heated, producing hydrogen chloride e.g.:

NH4Cl -> NH3 + HCl

The flux will convert any metal oxides that are formed on the components (and circuit board) during the soldering process (or because they are old or not clean) into volatile chlorides by the following reaction :

CuO + 2HCl -> CuCl2 + H2O (both are volatile and move away from the joint as fumes leaving a clean joit behind)

this reaction (brought about by the flux) maintains a clean metal surface really helping to create good clean surfaces on which the joint can form (wires and circuit board for example).

NOTE 4: the flux soon burns away which is why the solder should always be applied directly to the joint and not to the iron first.

NOTE 5: as solder produces various toxic fumes during the soldering process its advisable to always use a fume extracter system when soldering. Even when soldering outside you should be aware that you can still breath in fumes.

for a short mini-film on soldering go to the Vega Trust: mini-film on soldering



Dr Jonathan Hare, Physics, The University of Sussex
Brighton, East Sussex. BN1 9QH.

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