Learn How to Solder: A Complete Guide for Beginners

A Basic Skill When Building Electronic Circuits

Soldering is a fundamental skill essential to learn when building electronic circuits or fixing circuit boards. In this tutorial, we'll cover:

• An explanation of soldering and solder
• Tools you'll need
• Description and preparation of a soldering iron before use
• Soldering wires to tags
• Through-hole soldering of components onto a circuit board
• How to desolder

What Is Soldering?

Soft soldering is a technique used to bond metal components together. The components could be electrical wires, terminals or electronic components. Printed circuit boards (PCBs) used in electronic devices are populated with lots of electronic components, and these components must be attached securely. Solder is like a glue that fixes the components to the PCB. Unlike glue, which physically bonds materials together, solder also ensures that there's a good electrical connection between the component and the PCB.

Soldering is also used in plumbing to join capillary fittings to pipes, in metalwork and in jewelry making. The term soft soldering originates from the fact that soft metal alloys are used, and lower temperatures are required to melt them. Hard soldering or brazing is another soldering technique that uses harder metals such as silver and brass for joining metal items, again typically jewelry and bicycle frames.

What Is Solder?

Solder is a fusible metal alloy used for making solder joints. It comes in rolls of varying gauge wire, as a paste and in bars. Originally, solder was an alloy of tin and lead; however, lead is toxic and in the EU, the WEEE and RoHS directive came into effect in 2009, restricting the use of lead in consumer products. Lead-free solder now contains tin, copper, silver and other alloys.

Solder typically has a melting point between 50° C and over 200° C (122° F and 392° F). Solder wire used for electrical/electronic joints is normally flux-cored. Flux, made from a resin that melts when heated, flows over the joint when soldering. It aids flow of solder and also shields the solder and components from oxygen in the air. This prevents an oxidation film from forming, making it more difficult for solder to stick and also potentially cause a "dry joint" or bad electrical connection.

What Tools Are Needed for Soldering?

Solder joints are made with a tool called a soldering iron. This is powered by electricity or gas, but older irons used for metalwork were heated with a blow torch. It consists of a handle with a copper bit at the end, usually coated with a metal such as nickel or iron, to minimise oxidation of the bit and improve thermal conductivity when soldering. An electric element or catalytic converter heated by burning gas heats the bit to about 180° C (356° F) or higher.

Other tools used for soldering are:

• Side cutters (wire snips) for cutting wire and leads of components.
• Desoldering pump ("solder sucker"). This sucks up molten solder from a joint, allowing the removal of wires or components.
• Tip cleaner. During normal use, a moistened sponge is used. However, abrasive tip cleaners that use curled wire are capable of removing harder deposits on the iron's tip.
• Soldering iron stand for holding the hot iron in between uses. Irons aren't always supplied with stands, but they can be bought separately.

Optional

Recommended

Emily Dickinson's "There is a morn by men unseen"

• Vice. A small vice is useful for holding connectors or circuit boards while soldering. Usually, these vices have rubber coverings for their jaws to avoid crushing components and sometimes a suction base for sticking to a bench. Some vices clamp onto the edge of a worksurface.
• Solder wick. This is braided copper wire, used to "soak" up solder when de-soldering.
• Flux is used when soldering with non-flux cored wire. This is mostly used when soldering plumbing fittings or metalwork joints. Flux is available as a paste or liquid.

Choosing a Snips

Side cutters are available for cutting varying gauges of cable. I have used an Xcelite side cutters for over twenty years. The high mechanical advantage of long handles and short jaws means that they can easily snip through light to medium gauge wire used in electronics. Jaws are closely spaced when closed, and this is important for cleanly cutting very fine wire.

Preparing an Iron for Soldering

If you have a new iron, you need to tin the bit. Then wipe/tin it regularly as it gets covered in burned flux deposits.

Tinning is necessary to improve thermal conductivity between the bit and joint. Basically, it ensures that heat flows quickly and easily to the parts being joined so that the solder wire melts fast and flows over the joint.

1. Wet the cleaning sponge and squeeze out excess water. It shouldn't be saturated, just moist, or it'll cool the bit during cleaning.
2. Plug in the iron and wait for a few minutes for it to heat up. Some irons have temperature control, so adjust to about 180° C (356° F) or whatever is recommended for the solder wire. Lead-free solders require a higher temperature, typically over 200° C (392° F).
3. Clean the tip on the sponge, giving it a brief wipe.
4. Hold the solder wire in one hand and the iron in the other. Touch the end of the wire off both sides of the tip and allow it to flow. If a blob forms on the end, wipe it off on the sponge.
5. Solder the joint immediately as described below.
6. Periodically clean the tip on the sponge while soldering further joints, giving it a brief wipe. Repeat tinning when the solder coating on the tip becomes dull.

Steps to Soldering Joints and Components

First, we'll make a joint from some stranded wire to a tag on a loudspeaker. Remember to tin the iron before soldering.

How to Solder Wires to a Tag

1. Strip about 6 mm (1/4") from the end of the wire.
2. If the wire is multi-stranded, twist the strands together
3. Feed the wire through the hole in the tag
4. It helps to double over the end of a wire after pushing it through the tag or wrap it around the tag to stop it from detaching during soldering. However, this makes it more difficult to remove if the joint needs to be desoldered in the future.
5. Press the iron's tip against the tag and touch the solder wire against the tag, not the iron. Allow the solder to melt and flow all over the tag and through the strands of wire.
6. If you have several more connections to make, wipe the iron after soldering every few connections to remove burned flux.
7. Tin the iron's tip every so often with solder and wipe. The solder coating on the iron helps to improve thermal conductivity with the components being soldered so that they heat up quicker and solder melts.

How to Solder Components Into Circuit Boards

Many components will have pins or leads short enough so that they can be soldered directly to a circuit board; however, other discrete components need to have their leads shortened. This is called through-hole soldering.

1. Trim the leads of the component so that they project about 3 mm (1/8") from the underside of the circuit board. It helps to bend the leads slightly so that the component doesn't fall out when soldering.
2. Leads are normally made of copper and coated with tin to stop them from tarnishing. A tarnished lead can be difficult to solder, and the joint may be poor. Bare copper wire or component leads can be cleaned with wire wool.
3. Hold the tinned tip of the soldering against the lead and board.
4. Again it's better to touch the wire against the lead and track on the board, rather than the iron's tip. This is so that flux melts and flows over these parts, shielding them from oxygen rather than burning on the tip.
5. After a couple of seconds, the solder will melt and flow. Allow it to form a mound around the lead. Remove the iron and wire.
6. Take care not to get solder on adjacent components or tracks (like I did in the video!). Usually, tracks on PCBs are covered with a solder mask layer (this is what makes the board green). However, it's easy to bridge the gap between solder pads on a densely packed PCB.
7. If leads project excessively from joints after soldering, nip them off with side cutters.

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