SECURING SUCCESS WITH SOLID
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. - by Daniel Koch
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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:
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:
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.