Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.
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
- Soldering components onto circuit boards
- 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 glue that fixes the components to the PCB. Unlike glue, however, which physically bonds materials together, solder also ensures that there's a good electrical connection between component and 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 gage 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 has a melting point of typically between 50° C to 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 which would make 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 metal work 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 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 which 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
- 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 gages 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 gage 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.
- Wet the cleaning sponge and squeeze out excess water. It shouldn't be saturated, just moist or it'll cool the bit during cleaning
- 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 or whatever is recommended for the solder wire. Lead free solders require a higher temperature, typically over 200° C
- Clean the tip on the sponge, giving it a brief wipe.
- 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.
- Solder the joint immediately.
- 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
- Strip about 6 mm (1/4") from the end of the wire.
- If the wire is multi-stranded, twist the strands together
- Feed the wire through the hole in the tag
- 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 detaching during soldering. However this makes it more difficult to remove if the joint needs to be desoldered in the future.
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.
- 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.
- Leads are normally made of copper and coated with tin to stop them 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.
- Hold the tinned tip of the soldering against the lead and board.
- 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.
- 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.
- 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.
- If leads project excessively from joints after soldering, nip them off with a side cutters.
How to Desolder
Desoldering is the reverse process to soldering, melting the solder on the joint so that wires or components can be removed. Melted solder is sucked up using a desoldering pump or "solder sucker". The pump is primed by pushing down a spring-loaded plunger which has a piston attached. When the trigger button is pressed, the plunger and piston spring back up, sucking molten solder through the nozzle.
- Wipe and tin the soldering iron
- Push down the plunger of the desoldering pump until it locks into place
- Hold the tip of the iron in contact with the joint as well as the nozzle of the pump
- Once the solder melts, press the release button on the desoldering pump
- Press down the plunger off the pump to expel solder from the nozzle
- Remove the component
You may have to repeat this process several times if solder remains and still holds the component. On larger joints with a lot of solder this is usually the case.
Eventually solder can accumulate in the piston of the pump, just above the screw on nozzle. Unscrew the nozzle to remove this. Although nozzles are made from plastic that is heat resistant, they do deform and wear over time, but can be replaced.
Lead or Lead-Free Solders?
Lead solder is an alloy of lead and tin, typically with a 60/40 lead/tin composition. It is definitely easier to work with because of its lower melting point and tendency of lead to flow better than tin on its own. Lead is toxic however and lead free solders are mostly tin with addition of other metals such as solver and copper. They do however take a bit of practice getting used to if you haven't soldered before.
Buying a Soldering Iron
If you want a soldering iron that'll last you can't go wrong with Weller. Like all tools, if you buy a professional model, you can get parts when they wear out. Spare parts such as various sizes and styles of tips, handles, elements, thermostats etc are all available for these irons. This Weller iron, is a 70 watt model, with a temperature range of 200°F to 850°F (100°C to 450°C), suitable for use with leaded and lead-free solder. It has temperature control and a 1.6mm (0.062") tip for general purpose soldering. If you need to do finer work, it's advisable to get a smaller diameter, more pointed tip like this 0.8 mm one.
Note that this a 120 volt soldering iron station and the iron itself is 23 volts. A 50 watt iron is also available for less demanding applications.
If you only want a soldering iron for occasional, hobbyist use, this cheaper Atten iron is another option. It's a 50W/120 volt model with a pointed tip for electronic/electrical work. The tip is iron plated to reduce heat corrosion. Tip temperature can be adjusted using a knob on the iron's handle (392° F - 932° F) to cope with lead or lead-free solder.
Mudder rosin cored lead-free solder wire, from Amazon is ideal for soldering electronic components and wires. The 0.6mm wire is a perfect size for electrical repairs, joining wires to connectors, and general electronic circuit board assembly. Mudder also sell thicker wire up to 1.5 mm and thinner 0.3mm wire for delicate soldering applications (e.g. on surface mount boards). Soldering temperature is lower for this product than some lead free solders.
Solder containing lead is easier to solder with and flows better and is more suitable for beginners, however lead is toxic, so it's use could be harmful over time.
- Weight 0.22 lbs (100g)
- Wire diameter of .0024" (0.6mm)
- Alloy composition Sn/ Ag/ Cu: 99%/ 0.3%/ 0.7%; Flux: 2%
- Melting point 422.6°F (217°C)
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2020 Eugene Brennan
KHK on April 27, 2020:
This article is mainly for electrical soldering.
However, soldering is also commonly used to join two metals together in one joint.
Commonly known is copper to copper soldering for HVACS etc.
There are different grades of solder and different alloys for different purposes. Common for all is: They need some form for flux material in order to flow into the joint, they need a heat source (a flame is common), and they are an alloy of mostly tin and other metals, like silver or copper.