How To Make a Good Solder Joint

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.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Heat the joint – component leads and PCB pads – for a brief time (<5 seconds).
7. 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.
8. 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.
9. 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.