Soldering is an essential skill in electronics, fortunately it is easy to get started. Just follow our 10 step guide and you'll be soldering in no time!
Step by Step guide to soldering
1. Start with the smallest components working up to the taller components, soldering any interconnecting wires last.
2. Place the component into the board, making sure it goes in the right way around and the part sits flush against the board.
3. Bend the legs slightly to secure the part. Place the board so you can access the pads with a soldering iron.
4. Make sure the soldering iron has warmed up. If necessary use a brass soldering iron cleaner or damp sponge to clean the tip.
5. Pick up the Soldering Iron in one hand, and the solder in the other hand.
6. Place soldering iron tip on the pad.
7. Feed a small amount of solder into the joint. The solder should melt on the pad and flow around the component leg.
8. Remove the solder, then remove the soldering iron.
9. Leave the joint to cool for a few seconds, then using a pair of cutters trim the excess component lead.
10. Some connections are made with stranded wire. It is usual to ‘tin’ wire to make it easier to place through the holes in the PCB, and to help it solder successfully. To tin wire firstly strip a small length of the insulation off. The twist the strands together to form a single neat core. With the soldering iron in one hand, and solder in the other place the soldering iron tip at the end of the twisted core. This will heat the wire. 'Wipe' the end of the solder down the twists. This will melt when the wire is hot enough and apply a small amount of solder.
A well tinned wire only has a small amount of solder on it, just enough to hold the twist together. It should be possible to see the twisted strands through the solder.
Examples of Solder Joints
The appearance of a solder joint tells a lot about how good it is. A good solder joint is shiny. This means that the solder has not been overheated and exhausted the flux. There is enough solder to form a nice fillet, without excessive solder bulging out. The component leg is ideally in the centre of the joint. This photo shows an ideal solder joint.
The following illustrates an acceptable solder joint. The solder is shiny, but there is slightly more solder than required, and the component leg is off centre.
It can be tricky to add just the correct amount of solder when learning. The following photo shows a typical joint with not enough solder. The component hole is not covered. It is possible this joint will work, but it will probably fail quickly.
Slightly too much solder causes a bulge, as illustrated in the following photo.
The PCB tracks are covered in a special paint called solder resist. This helps prevent the solder from flowing away from the joint. As more solder is added it builds into a blob, as shown in the next 2 photos.
Too much solder on a joint can cause bridging to adjacent components, leading to a short circuit. Solder contains flux to ensure the joints are sounds and not oxidised. The flux works by reacting with the oxides that are present, and converting them back to metals. Overheating the joint, or taking too long can use all the available flux. This leads to the solder oxidising and what is known as a dry joint. The following joints are also 'Dry'. Note that a dry joint does not necessarily mean that it does not have enough solder, but refers to the solder not 'wetting' the component and the pad correctly to flow.
Notice the 'Orange Peel; roughness of the solder. This is caused by the metal in the solder turning to oxides. Compare this dry joint with too little solder to the earlier one with too little solder – notice the comparatively dull appearance of the solder on the dry joint.This joint, with too much solder, is also showing the symptoms of dry joints – notice the dull grey colour, and orange peel texture.Overheating a solder pad on a PCB can also cause the copper layer to come away from the fibre glass that the board is made from. This can cause circuit failure when the track on the circuit board breaks, leading to an open circuit.
This tutorial was original published by, and used with permission from, Kitronik Ltd.