Practical Electronics/Soldering

Soldering is the use of a conductive substance with a low melting point (solder) to electrically connect components together. It is frequently used to join wires to leads of components such as switches or to join components of all kinds to a printed circuit board. The primary tool used for applying solder is a soldering iron, a device whose metal tip heats to temperatures well above the melting point of solder. This is used to melt the solder and allow it to flow into a joint.

Soldering is an acquired skill, and it takes practice to become adept. There are many tips, tricks and guidelines on how to produce good soldered joints, and this module aims to present them to you.

The first and most important rule of soldering is to choose your tools with care. Both the solder and the iron must be chosen to suit the application. The first two sections below deal with the different kinds of solder and iron and which to choose for what.

About solder

Solder is a metallic compound that has a low melting point, usually around 200°C. The composition of solder varies depending on the type, but usually contains lead or tin or both. The most common types are given below. It is available in wire, stick or pellet form. Sticks and pellets are for solder-pots; for normal soldering, you will need solder wire.

Solder wire is a very flexible, silvery coloured wire that is usually supplied on reels that are sized by weight. Common weights are 250 g, 500 g, 1 kg or 2 kg. It generally costs around £6 or $12 for a 500 g reel. It is very soft, and is easily cut by wire cutters and smaller diameters can be snapped by hand (though this is not recommended, as you will see below).

Gauge

Solder wire is available in widths given in "standard wire gauge" (SWG). The larger the SWG number, the thinner the wire. Common gauges are 18 and 22, although others are available.

18 gauge solder is suitable for soldering large components and thick wire, as a large quantity of solder can be delivered quickly. However, for smaller, more detailed work, 18 gauge is too thick. Soldering the pins of standard dual in-line packages (DIPs) is about as fine as 18 gauge solder can usefully go.

22 gauge solder is thinner than 18 gauge, and should be used for most electronics work, as it allows much greater control over the quantity of solder delivered, and the chances of accidentally bridging a gap due to over-application or the wire's width are greatly reduced.

Finer gauges such as 26 are available for very fine work with SMT (surface-mount) components.

60/40

A 500 g reel of 22 SWG 60/40 Solder.

60/40 solder is made of 60% tin and 40% lead. It has a melting point of around 190°C, depending on the exact composition. Iron tip temperatures of at least 300°C are recommended. It is also very soft, meaning that cracks do not form so readily if the joint moves during cooling.

63/37

63/37 solder is made of 63% tin and 37% lead. It has a melting point of 183°C, slightly lower than the more common 60/40 blend. The primary advantage of this solder is not the lower melting point, but its eutectic property. Non-eutectic solders, like the 60/40 solder, have a semi-solid state between solid and liquid. If a joint is moved during this stage, it can result in what is called a cold solder joint. Eutectic solders, like the 63/37, do not have this semi-solid state and are thus considered easier to work with as it produces fewer bad joints. However, these solders typically cost more than their non-eutectic counterparts.

50/50

This is made of a half and half mix of tin and lead. Never use 50/50 solder for electronics — it is meant for plumbing. (There are brands[Worthington 50/50 leaded #331887], which is used for electronics and specifically not intended for plumbing). Otherwise, you may end up with failed joints, as the 50/50 solder does not have the same properties as 60/40, having a higher melting point and lower ductility. Also, it is very unlikely to have flux included in it like rosin core solders.

Lead-free solder

Solder containing lead is slowly being phased out under new EU directives (especially the RoHS and WEEE directives) and being replaced with solders consisting of tin-antimony alloys. It will likely be many years before lead solder is supplanted for good, but even now many shops do not sell lead solder any more. Lead-free solder has a higher melting point than lead solder and uses more aggressive fluxes. This means that the soldering iron will have to be made for lead-free solder in order to supply the right temperature — it melts at around 230°C — and the iron bits need a different coating to withstand the flux. Using a soldering iron meant for lead solders may result in dry joints and shortened bit life.

Lead-free solder is generally about 20%-50% more expensive than lead solder.

There are many different alloys and their properties can be found in this Wikipedia article.

Flux and rosin core solder

A close up of rosin core solder, showing the channels for the flux.

A pen-type flux applicator.

Flux is a compound that is used to improve the quality of the soldered joint. It does this in three ways:

It chemically removes oxidation from the surfaces being soldered.
It prevents air from oxidising the surfaces once they have been cleaned.
It increases the "wetting" of the surfaces when the solder is applied.

Wetting is the degree to which the solder flows across the surfaces being joined. Without flux, a dry joint may be formed, making a poor connection. See below for a description of dry joints.

Rosin core solder has channels inside filled with rosin flux. Because flux melts before the solder, the embedded flux automatically cleans the joint before the solder flows into it. Rosin core solder should always be used for electronics. Both lead and lead-free solder can be rosin core. Never use acid core solder for electronics — it is intended for plumbing and sheet metal work only, and the acid residue will corrode and ruin electrical joints. Acid flux remains chemically active at room temperature, while rosin flux becomes inert and harmless once the joint cools.

After soldering, a layer of flux may be seen around the joint. This is usually a burnt-looking brown layer (usually found when the joint has been overheated), or a shiny clear layer. This is not usually a problem, but you may want to remove it with a cotton bud soaked in methylated spirits or alcohol to produce a more professional appearance.

Flux is also available in "pen" applicators. This allows it to be applied, before soldering with rosin core solder, to surfaces that need to be very well fluxed such as SMT pads and unfluxed solder braid in order to be reliable.

About soldering irons

Soldering irons come in all shapes, sizes and power ratings. Most irons are made of a heating element attached to a handle, with interchangeable tips used to work on the joint.

Power rating and temperature control

A programmable soldering iron.

The power rating governs how hot the iron gets and how quickly it heats up. A high power rating means that if a joint with a large thermal mass is being soldered, the bit will not cool down much. However, a high power rating used for fine work may overheat the joint, damaging the component or circuit board.

Generally, a rating of around 18 W is good for fine work, while a 25 W to 50 W rating is good for heavier electrical soldering, such as on mains lighting.

Inexpensive soldering irons have a fixed power consumption — they rely on the dissipation of heat into the surroundings to achieve thermal equilibrium at the desired temperature. These are sufficient for most hobby electronics work. A simple iron like this costs around £20 to £30 (US$30 to US$50).

For more advanced work, especially work involving very small components that overheat easily, a temperature-regulated or variable temperature soldering iron is needed. These usually are sold as a soldering station, comprising a soldering iron and a separate power supply. The station monitors the temperature at the soldering tip and adjusts the power output accordingly. Some stations use a magnetic switch that cycles the power on and off, using the Curie Effect; the soldering tips are designed to lose their ferromagnetic properties at a defined temperature, usually 370°C (700°F), 425°C (800°F) or 480°C (900°F), shutting off heater power until the tip cools and regains its magnetic properties. Tip temperature in such irons may fluctuate in a ~4°C (~7°F) range. Such stations are frequently used in manufacturing environments. For laboratory use, stations are available with a thermistor at the tip to monitor temperature and the temperature can be continuously varied by a rotary control. Tip temperature may also be displayed by an LED readout on the base unit. More expensive stations have an hot air pencil for desoldering purposes, as well. You can expect to pay up to £500 (US$800) for an industrial-quality soldering station.

Electric soldering gun

A soldering gun, running on AC mains power, has the advantage of quickly heating up and cooling down, and can deliver significant heat to a joint. They are excellent for general electrical work, but not particularly useful for fine electronics work, as they are bulky, inhibiting accurate positioning of the tip, and supporting the weight of the transformer will cause rapid operator fatigue. Tip selection for these tools is limited to a few styles, generally blunt, for larger joints. Moreover, the tip generates an intense electromagnetic field when operating, which can damage sensitive semiconductors.

A soldering pencil, heated by a battery or refillable butane cartridge, can be used for small repair jobs in the field where AC mains power is not available, but they are not practical for continuous use.

Choosing a soldering tip

The most vital part of choosing the soldering iron is having a suitable tip on the iron. Larger tips have a higher thermal mass and hold their temperature better when working on large joints, but they are physically too big for many applications, as the tip touches more than one joint at a time. Small tips are much easier to use for most work, but they will tend to cool down when presented to a large joint.

Some tips are shaped as an obliquely cut cone, with many different sizes available. They are often called bevel or sloped tips. Typical sizes may range from 1 mm up to around 5 mm across. The advantage of this versatile tip shape is that the cut face can be used to transfer a large amount of heat to the joint, but when the edge of the face is used, it works like a fine tip for small joints or sensitive components, greatly reducing the heat transfer rate.

Soldering technique

Good soldering is all about technique and practice. It is vital that all parts are prepared properly, otherwise a defective joint will result.

Preparation

First, the solder must be prepared. As noted before, the solder you use should have a flux core. When pulling solder off the roll, you may stretch the soft tin/lead alloy past the flux, leaving a pure metal tip with no flux. This will not stick to anything very well, and may cause problems. To avoid this, always cut a few millimetres from the end of your solder with side cutters, or melt off a small piece with a hot soldering iron.

Next prepare your iron. You need a soldering iron stand, both to stop the iron from setting anything on fire and to protect the iron from being knocked to the floor by accident. Every good stand has a tray for a piece of cellulose sponge. This is kept moist and used to clean the tip of your iron. Always wet your sponge before using the iron — having it can improve your soldering immensely. Distilled water is best, but you may use tap water.

First, wait until your iron is up to temperature. Using your iron before it is hot enough will just take longer as you will have to fix the damage you did by forcing it into the joint. Once it has heated up, clean the end. Usually when you pick up an iron, its end is blackened and covered in old flux that has burnt onto the tip. This will not work as a fluxing agent, and will impair the ability of the iron to conduct heat. Add some solder to the tip (it will probably form a sphere due to the burnt coating repelling it) and wipe it onto the wet sponge. Do this until you have a shiny, silver tip to your iron. Every time you remove the iron from its stand, and also every now and again while holding the iron, clean the tip like this. The second and later times in a session you do this it will take usually only one wipe to clean the end. Always ensure that there is no excess solder on the tip before you start to solder.

Through-hole components

A through-hole mounted in place, ready to be soldered. The legs have been bent outwards to prevent the device falling out of the PCB during soldering.

Through-hole mounted (THM) components are components with leads that protrude through a PCB and are soldered in place on the other side to the components.

When the PCB has copper tracks on only one side of the circuit board, the two sides of the board are usually called "solder side" and "component side".

When placing THM components on the PCB, insert the leads through the hole, and press the device down so it sits flat against the top surface of the PCB. It is often helpful to bend the leads outwards on the other side of the board to prevent the device falling out when the board is turned over for soldering. This is normally only needed for devices with two or three leads, such as axial-lead resistors or transistors.

Often, when a device is being soldered without having the leads bent to secure it mechanically, it may "sag" slightly and fall out of the hole by a millimetre or two. To prevent the components being crooked after soldering, you can tack-solder only one or two of the leads to temporarily secure it. Then, while pushing the component against the circuit board from the other side, reheat the joint(s) and allow the solder to solidify. Finish soldering the remaining leads, and, if needed, return to the original joint(s) to add solder as needed. This technique is particularly useful with DIP packages and similar multi-lead packages.

SMT components

Surface-mount technology (SMT) has been gradually replacing older components with wire leads. Some special tools and techniques are used to solder and unsolder SMT components.

Safety

If there is an accident, you can buy more safety glasses. You don't have that luxury with eyes.

Always wear safety glasses or goggles with plastic or tempered glass lenses while soldering, to protect your eyes from flying droplets of hot, liquid solder.

The tip of the soldering iron is, by design, very hot, exceeding 370°C (700°F). Avoid letting the tip touch skin, clothing, or anything that might melt, scorch, or burn. Use a proper soldering iron holder when the iron is powered on. These offer a convenient place to rest the iron and keep anything from touching the hot tip. The small plastic-and-wire stands that come with inexpensive soldering irons can be dangerous, as they leave the tip exposed and the iron can easily roll off the stand.

The flux in solder (as well as additional flux applied to the joint) will boil and burn when exposed to the heat of the soldering iron. Although smoke from rosin flux isn't particularly toxic, it is an irritant to the lungs and breathing passages, and extended exposure can lead to health problems. You can avoid breathing the smoke from flux by not "hovering" over your work, setting up a fan to keep your face "upwind" of the work, or by using a fume extractor specifically designed for soldering stations. Always solder in an area with adequate ventilation to keep the fumes from accumulating. A room with windows that open is preferred.

The lead in solder will not typically get hot enough to vaporize and become airborne. However, you will end up with lead on your hands after handling solder, so it is necessary to wash your hands with soap and water afterwards. Lead is toxic and can cause a variety of health problems. Children and pregnant women should avoid all contact with lead due to the severe developmental problems it causes.