WARNING: This work refers to electrical soldering only.
Learning the correct technique means you can ensure trouble free soldered electrical joints which will be reliable over a long time, and progress to make cheap, easy repairs to your electrical equipment if they develop simple circuit joint faults.
- Make sure it's clean. Printed Circuit Boards (PCBs) may need washing – use a mild soap or detergent, but rinse thoroughly and blot on a paper towel, allowing the PCB to dry completely.
- Clean component leads by lightly scraping with a blade or use emery or wet and dry paper, used dry. Flux helps to clean the joint and may help if sparingly applied to a reworked joint.
- Ensure the join is mechanically stable. Solder is not really a glue; it provides electrical continuity for reliable circuitry. If components move during soldering, the joint may be unreliable or fail completely.
- Use a suitable-size soldering iron with a clean bit. Using too small a bit for larger solder joints is inefficient and may heat the joint unevenly. This may cause the 'solder' to set in different strata/layers and prove to be an electrically poor joint – known as a 'dry joint'. Conversely, using too large a bit is unwieldy and may overheat components which may fail completely.
- Check the tip temperature. You can do this by touching the tip with the end of the solder, which should melt immediately forming a thin film, tinning the end of the tip. This helps to conduct heat to the work evenly. If the tip is not up to working temperature, leave it a little longer to come up to temperature.
- Heat the joint – component leads and PCB pads – for a brief time (<5 seconds).
- Apply solder to the joint – ideally from the opposite side to the iron tip. Once the joint reaches working temperature the solder will melt and flow.
- Remove the solder and then the iron. If you remove the iron first the solder may stick to the work – simply apply the iron back and as allow the solder to melt off the work.
- Feeding just the right amount of solder at the right temperature is a skill that you can soon learn. If you apply too much, the excess can be removed using a de-solder pump or braid.
The same factors apply to plumbing soldering – making a soldered joint for a water pipe – but modern push fit and compression fittings may prove easier to use for a one-off repair.
A good solder joint will be smooth and shiny, and have ideally a 'volcano' shape – technically, a concave meniscus.