Soldering electronic circuits

Soldering is a useful skill to learn and it's not nearly as scary as some people think. It is a skill easily learned although like most skills improves with practice.

Soldering works by melting a metal alloy, called solder, to form a joint between components, wires and / or a circuit board. The solder melts at a much lower temperature than the components being soldered so that it can be melted with a soldering iron. The solder also includes a flux which helps the solder to flow onto the wire and pad.

This is achieved by using a hot soldering iron which heats up both parts of the solder joint and the solder. The following explains the process for soldering a component onto a board.

Place the tip of the hot soldering iron so it is in contact with both the pad on the PCB and the lead of the component.
Bring the solder in and allow sufficient solder to make the joint.
Remove the solder, but leave the soldering iron in place for a short time to allow it to flow around the pad and component.
Remove the soldering iron and allow the joint to cool.
Ensure that there is sufficient solder to make a good joint (if not repeat) and that it does not bridge to any other components or pads.
Snip off the excess wire from the component.

This is best explained by demonstration, so videos have been provided below showing this in use. After watching the video then give it a try on a simple circuit to gain some experience before moving a more complex circuit.

Safety tips

Some basic safety precautions do need to be taken when soldering, but as long as these are followed then soldering is relatively safe.

Hot soldering iron

It goes without saying that an iron that is hot enough to melt solder could easily burn skin. Accidentally touching your finger with a soldering iron can leave a small burn with no long lasting effects, but grabbing the end of the soldering iron could cause a more substantial burn. For this reason it's important to place the soldering iron into a holder and ensure that the wire is not going to get pulled. Also remember that a soldering iron will remain hot for a long time after it's unplugged.

If you do burn yourself then flood the area with water and leave under running cool water for at least 20 minutes. Any concerns then seek professional medical advise.

Solder fumes

Although most solder is now lead-free the fumes from soldering are still toxic. I normally do my soldering in a well vented room and avoid breathing in the fumes, but you can buy soldering extractor fans if you are concerned.

Cutting leads

When cutting leads from components then beware of any ends that may fly off to ensure that these don't go in your eye. This can be achieved by holding the leads in one hand or by wearing eye protection.

Supervise children

Soldering is not a difficult activity and can be undertaken by children of an appropriate age as long as it is under appropriate adult supervision.

Soldering for beginners - Electronics zoo badge

This is a demonstration of soldering of a zoo badge, which is a good way to try soldering for the first time. These are available from The Pi Hut (Zoo badge).

Soldering a GPIO to the Raspberry Pi - Pi Zero

The Pi Zero is the smallest of the Raspberry Pi computers. Unfortunately it does not have a header for the GPIO ports, but fortunately it's easy enough to solder these on yourself.

Hints and tips

Soldering iron tip

There are a variety of different soldering iron tips depending upon use and personal preference. I mostly use a 1.6mm chisel tip for soldering as I have found that is a good compromise between soldering smaller items and large soldering without needing to change the tip. Note the video examples above used a larger 2.5mm chisel tip. Larger tips may be useful when soldering onto large components (eg. large gauge model railway track).

Temperature and Power

Some soldering irons have adjustable temperature. If using lead-free solder then I generally use around 400°C, but for lead-based solder you can go lower say between 350°C and 380°C. The power setting does not normally dictate the temperature, but instead is how quickly the soldering iron can heat up and how well it can maintain the temperature. This is more important when soldering to larger objects that may otherwise absorb the heat (such as model railway track).

Soldering stations vs portable soldering irons

I generally prefer to use a fixed mains powered soldering iron at a dedicated bench. However portable soldering irons can be useful when soldering something in situ.

Gas powered soldering irons can be useful and are good at keeping their heat. Thse normally use lighter fluid. They may also have different fittings which can be useful to provide hot air for heat shrink sleeving.

USB powered soldering irons are becoming more popular, these can also be used with some power banks for a more portable solution. They tend to be lower power and may not be as useful for soldering larger items.

Flux

Flux is used to help prepare surfaces to be soldered. It is generally slightly acidic and removes gunk and oxidization. Most solder includes a flux based core which is sufficient, but if you are soldering to something with a larger surface area or are struggling with creating a good joint then you may want to consider using additional flux. This is available as a paste or a liquid which is applied just prior to soldering.

Additional flux may also help if you need to desolder a component.