Quality soldering can make the difference between a successful project, and hours of frustrating fault-finding. This instalment of FUNdamentals should help you ensure smooth results for your next project.
As someone who has been involved in electronics for many years, I have had a great many kits and scratch projects presented to me with a plea for help in finding out why it won’t work. Without exaggeration, around 80 per cent of faulty circuits are due to poor soldering. Most often, it’s a result of dry joints, but sometimes there’s debris inclusion or even attempts to solder incompatible metals. While sometimes hard to spot, these issues are easily avoided with a little extra knowledge. So, if you are just starting out on your electronics journey, or if you’ve ever had a frustration with a circuit, this information may be exactly what you need.
WHAT IS SOLDER?
To understand soldering you first need to understand solder, itself. Solder is an alloy of certain metals – selected and blended – to give certain properties. Traditional solder is an alloy (mix) of two metals, Tin and Lead originally, mixed because together the melting temperature is reduced lower than the melting temperature of either metal. The 60/40 mix used in electronics has the least plastic stage. That is called the eutectic point, the point when liquid alloy converts most quickly into a solid metal.
While lead solder has been popular for many years, lead free solder uses other metals that are less toxic than lead. It is slowly becoming more popular for hobbyists.
Elements in their natural, pure state, which are already balanced, are said to be inert. Atoms of materials that have too many electrons are called negative ions, and too few electrons leading to a positive charge from the excess of unhappy protons, results in a positive ion. It is the opposite ions that seek each other out, so that they can hold hands and be happy! However, this isn’t always easy. Many pure metals don’t have anything around them to bond with, so they snatch ions from the air. Oxygen is great at taking up this offer, hence oxidisation, which is the term for corrosion from free oxygen atoms. For this reason, metals are blended to become as neutral as possible, and the result is an alloy. However, it is sometimes not possible to produce an inert alloy, when other factors in the end use of the alloy are considered. Other materials may be used too – such as carbon – which when alloyed to iron, produces steel.
What this means is that not all solders are equal. Plumbers’ silver solder, for example, is meant for high-strength, high-pressure situations, and so contains silver. This makes it very hard and makes the alloy’s melting point very high; high enough to need a hot gas flame to melt it. Of course, both of these factors make it unsuitable for use in electronics. What is needed instead, is a solder with good resistance to oxidisation that stays somewhat flexible, and has a low melting point. Traditionally, this has been an alloy of 60% lead and 40% tin, referred to as 60/40 solder.
HOW IS SOLDER USED?
Before you even turn on your soldering iron, there are several factors to think about. Solder in electronics is not intended to be an adhesive. Instead, it is there to reliably create a current path for electricity to flow along.
The purpose of solder in electrical/electronic circuits is fourfold:
- To provide an electrical connection.
- To provide a chemical (metallic) connection.
- To provide a mechanical connection to keep the component/wire in place.
- To avoid corrosion effects on the connection.
Occasionally there is a fifth reason. To soak away heat from heat sensitive components.
With this in mind, the first step is to make sure all surfaces are clean and brightly finished, which means free of corrosion. Most electronic components have legs coated with a thin layer of tin. While solder bonds well to copper, copper rapidly oxidises on its surface when in contact with air. The tin applied by manufacturers, bonds with the copper, and this protects it, while also providing a nice bond to solder. However, tin does eventually corrode, going dull. If your components are old, they will need to be cleaned first.
You may have heard of flux when talking about soldering. Solder meant for electronics is also rosin-cored.
Flux does two jobs:
- It cleans the surfaces, which, by reacting with light oxidisation and cleaning away contaminants, results in a bright surface; and
- It excludes oxygen from the joint.
However, just as all solders are not equal, neither are all fluxes. Many people have seen plumbers’ or metal workers’ flux, which is a thin liquid that is applied separately before soldering copper pipes or galvanised metal. This should never be used near electronics.
It is a corrosive product, unsuitable for use near components, and needs to be cleaned after soldering to prevent corrosion, which is often impossible to do on a circuit board.
Electronics solder contains cores of something called rosin. Rosin is a blend of chemicals specifically formulated for use as a flux around electronics. It is important to always check that the solder you are using is rosin-cored. 
The next point to consider again relates to the fact that solder is not glue. Always try to make a mechanical connection before soldering. With wires, this may mean twisting them together. 
With components on a circuit board, this means bending the legs at a 45-degree angle once they’re inserted through the circuit board, so they cannot move easily. 
In the case of integrated circuits, it is often sufficient to bend one leg at each end, diagonally opposite, to hold the chip in place.
Once a mechanically sound, clean connection is ready, you can heat up your soldering iron. The choice of equipment here is critical. A 120W sheet metal worker’s soldering iron will dump so much heat that the circuit board will be destroyed. A 25W circuit board iron will never heat a 4-gauge car audio power cable. For reasons such as these, one of the best pieces of equipment you can own for electronics is a soldering station, which allows a variable temperature setting.
The aim is to introduce enough heat to heat the joint quickly and melt the solder, while not putting in so much that things get cooked.
Here is another critical point: insufficient heat means that the heat gets drawn away from the joint faster than it is put in. In this scenario, the joint never reaches temperature or takes a long time to do so; however, in the meantime the component is being heat-damaged. So ideally, you need to strike a balance between a temperature and power that will heat the joint quickly and then be removed, yet without introducing too much heat. Unfortunately, this is often a matter of experimentation. Remember, metals are good at conducting heat as well as electricity.
Adding the actual solder to the joint is the real art, and often another reason that people come unstuck. The correct procedure is to add a little solder to your iron’s tip – just enough to allow a better surface area to conduct heat. Wipe the soldering iron on the wet sponge, then hold the solder against the two surfaces to be soldered (not against the iron tip), and allow the joint to heat up. When it reaches temperature, the joint will melt the solder, allowing it to flow into the relevant places. 
Then, withdraw the iron and solder at the same time, and wipe the iron clean. What you should see now is a smooth, shiny surface that appears to be seamlessly joined to the circuit board surface and component leg, or to both wires. If there are cracks, a dull surface, or a volcano-like crater at the top or a line around the bottom, the joint is likely not making a good connection . In which case, reheat, remove the solder, and try again.
SOME HELPFUL TOOLS
There are some things that make soldering much easier. The first is a solder sucker. These are available in both spring plunger and squeeze bulb form. Simply reheat the joint, position the desolder tool near the joint and trigger. The solder should largely be removed.
The second helpful tool is a set of helping hands. These are a metal frame with clips to hold the circuit board or pair of wires, allowing both your hands to be used for holding and carefully applying the iron and solder.
The final piece of equipment is a fan or fume extractor. This can be as simple as a fan directed so as to suck air away from your face, to help ensure you avoid breathing in the toxic lead soldering fumes, or it can be as complex as a carbon-filtered desktop fume extractor. Whatever you choose, remember that most solder has lead in it (note: the wisps of vapour you often see from a soldering operation is actually the rosin).
All that remains now, is to trim the component legs off your board. This is best done with a pair of small side cutters that have a flush cutting surface (not a bevelled one). You should trim closely to the solder but not cut into the solder.
This information will hopefully help you succeed in your soldering. I could wax lyrical about it, but for now I’ll await your feedback, which may well result in more information being provided down the track. I certainly intend to visit the topic of lead-free solder soon, as it is vastly different to work with. In the meantime however, the points here should get you well on your way. With a bit of practise, you’ll be creating sound, solid, shiny solder joints in no time!