Soldering

Introduction¶

Soldering is the process of using molten metal, usually tin, to make an electrical connection between two conductors with a low resistance. Being able to solder is important for electrical engineers because it allows you to build and test your ideas. Unsurprisingly, you will use this skill in many future labs and courses.

In the industry, where circuits are often mass-produced, the soldering process is largely automated. Having your circuit be assembled in a factory, however, is often too expensive or time inefficient to be worth it. For these cases manual soldering is the way to go. While there exist many soldering techniques, in this chapter we stick to basic manual soldering with a soldering iron and solder.

Equipment¶

Solder is a metal alloy which usually consists of 60% tin (Sn) and 40% lead (Pb), called leaded solder, or 96% tin and 4% silver (Ag), called lead-free solder. In the lab we use lead-free solder.

A soldering iron, such as the one shown in Figure 1 is a hot metal rod with a tip. It lets you melt the solder and heat up the metal that you want the solder to attach to. When soldering electronics, a 30 W soldering iron is sufficient. For large components or when soldering on a large surface, a soldering iron with more power is needed. Temperature-controlled soldering irons are useful but not essential. If a soldering iron delivers too much power or becomes too hot, it can destroy the component or the circuit.

How to Solder¶

Before you begin with soldering, the tip of the soldering iron must be at the correct temperature and cleaned. For a soldering iron without temperature display, an indication of the correct temperature is when the solder immediately melts if you hold it against the tip. If black crusts is present on the tip, first clean it with a wet sponge.

Before you can make a solder connection, both components should be cleaned well, otherwise the solder will not properly flow, or will create a bad connection. If necessary, clean the components by sanding them lightly with fine sandpaper or scratch them with a knife. You actually never have to do this when soldering new electronic components; when using older components, it may be needed.

Tinning and soldering stranded wires¶

Before soldering a wire, it must first be cut to length and stripped. Use wire cutters or sharp scissors to cut the wire cleanly. To remove the insulation, use a wire stripper set to the correct gauge — strip about 5–8 mm from the end. If a wire stripper is not available, the wire cutters can be used by scoring around the insulation at the desired point without cutting through to the strands, then pulling the cut piece off. Alternatively, a sharp knife can be used. In both cases, take care not to nick the copper strands, as this weakens the wire and causes a poor joint.

Stranded wires (wires made up of multiple fine copper conductors twisted together, as opposed to a single solid conductor) must be tinned before soldering, because the individual fine strands will otherwise not bond reliably. To tin a stranded wire, first twist the fine copper wires together. Then hold the wire against the tip of the soldering iron, and touch the solder wire to the wire itself — not to the iron tip. When the wire is hot enough, the solder will melt and flow between the strands. Add enough solder so that the wire is completely filled. Once fully tinned, remove the solder wire and soldering iron. Figure 2 shows the correct result.

To join two tinned wires, bring them together and keep them steady, then press the soldering iron against both. Once hot enough, add a small amount of solder at the joint. If the solder neatly flows between the parts, remove the iron. As with any joint, the parts must not move while the solder cools.

When joining solid wires together, or soldering a wire to a surface, pre-tinning is not strictly necessary — both parts can be heated and joined in a single step. However, tinning each part first can make soldering easier, particularly when one surface is difficult to heat evenly or when a surface is not perfectly clean.

Soldering components to a PCB¶

New electronic components are usually already tinned, and PCBs are delivered with a solder lacquer that promotes wetting (the ability of molten solder to flow and adhere evenly to a surface); you therefore do not need to pre-tin component leads.

To make a solder joint, place the lead through the hole and press the tip of the soldering iron firmly against both the lead and the copper pad at the same time. Remember that a small amount of solder already on the tip can help with heath transfer. Touch the solder wire to the joint — not to the iron tip — and let a small amount of solder flow in; it will start to flow when both parts of the joint are hot enough. With a good joint, you will see that the solder has ‘crept up’ along the lead due to the cohesive force (see Figure 3). Smoke indicates that the flux in the solder is active and evaporating; once the joint is made, remove the iron and allow it to cool without disturbing the parts. If the joint does not look right, re-heat it with the iron, add a small amount of new solder if needed, and allow it to cool again.

With old components, it is often advisable to tin the leads first, because the solder already on them may have become dirty and greasy. If the solder after heating does not flow properly, this is a sign that the surface is not clean; re-tinning or first cleaning as described above is the only solution. A solder joint with dirty connecting wires results in a poor, unreliable connection that may fail at any point in time.

Other sources for soldering instructions¶

On learn.adafruit.com/adafruit-guide-excellent-soldering/common-problems, some pictures of problems that can occur during soldering are shown. Some useful soldering instructions (in Dutch) can also be found at www.popschoolmaastricht.nl/college_solderen.php. Below are some of those tips translated into English.