How to Solder

How to Solder

First a few safety precautions:

• Never touch the element or tip of the soldering iron.
  They are very hot (about 400°C) and will give you a nasty burn.
• Take great care to avoid touching the mains flex with the tip of the iron.
  The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock.
• Always return the soldering iron to its stand when not in use.
  Never put it down on your workbench, even for a moment!
• Work in a well-ventilated area.
  The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work.
• Wash your hands after using solder.
  Solder contains lead which is a poisonous metal.

Preparing the soldering iron:

• Place the soldering iron in its stand and plug in.
  The iron will take a few minutes to reach its operating temperature of about 400°C.
• Dampen the sponge in the stand.
  The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet.
• Wait a few minutes for the soldering iron to warm up.
  You can check if it is ready by trying to melt a little solder on the tip.
• Wipe the tip of the iron on the damp sponge.
  This will clean the tip.
• Melt a little solder on the tip of the iron.
  This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge.

You are now ready to start soldering:

• Hold the soldering iron like a pen, near the base of the handle.
  Imagine you are going to write your name! Remember to never touch the hot element or tip.
• Touch the soldering iron onto the joint to be made.
  Make sure it touches both the component lead and the track. Hold the tip there for a few seconds and...
• Feed a little solder onto the joint.
  It should flow smoothly onto the lead and track to form a volcano shape as shown in the diagram. Apply the solder to the joint, not the iron.
• Remove the solder, then the iron, while keeping the joint still.
  Allow the joint a few seconds to cool before you move the circuit board.
• Inspect the joint closely.
  It should look shiny and have a 'volcano' shape. If not, you will need to reheat it and feed in a little more solder. This time ensure that both the lead and track are heated fully before applying solder.

Using a heat sink

Some components, such as transistors, can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the component body. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper.

The circuit diagram (schematic)

The picture that you can see is known as a circuit diagram or a schematic diagram. This is the standard way of showing an electronic circuit so that you can see how the circuit should work. Each component has its own symbol which indicates what it's function is. There are a number of different electronic component circuit symbols in the electronics reference section. The symbol on the left is for a battery, at the top there is the symbol for a switch and at the right the circle with a cross in it is the symbol for an lamp (or indicator lamp).

Note that when a circuit is created there is often a component layout diagram which shows how the components are installed onto the circuit board. This is useful if you are creating a replica of a circuit that has already been designed, but it is the schematic (circuit diagram) that is most useful for understanding how and why a circuit works as it does.

Unfortunately circuit diagrams / schematics do not always look exactly the same as there are differences in the circuit symbols used depending upon region and preference. For example the resistor in the IEC circuit symbols is shown as a rectangle, but in the US a resistor is normally shown as a zig-zag line. In most cases the differences are only small and it's still possible to recognise the symbol even if it is not the one you are familiar with, but in the worst case it usually means there are a couple of extra symbols to remember / look-up.

The picture representation of the switch is not part of the circuit diagram but is provided as a means to interact with the circuit. Also circuit diagrams are normally static and the switch symbol would not normally change to the closed position, or the lamp change colour.

Requirements

This interactive circuit diagram is written in the Processing programming language. You need a recent working Java plug-in to be able to view this demonstration. If this does not load correctly look for "Active Content" blocked and enable if necessary.

Complete circuit

This is an example of a real circuit used in battery operated torches.

For any electronic circuit to work there must be a complete circuit. This means that there must be a connection made out of conducting material that goes in a circle from one terminal of the battery through the equipment and then back to the other terminal of the battery. If there is a gap at any point then we have air which is a bad conductor and as a result nothing will happen.

This is how a switch works.

When the switch is in the open position then it creates a break in the circuit and the light is off. When the switch is closed the metal contacts inside the switch join and complete the circuit.

Press the switch button using your mouse to see the lamp light up.

Conductors vs. insulators

Some common conductors and insulators are listed in the following table:

Conductor	Insulator
Copper wire	Most plastics
Other metals	Dry wood
Tap / rain water*	Glass and ceramics
People!	Air

Note that I specifically mention tap and rain water rather than just water. Pure distilled water is actually an insulator, but the impurities in most water turns it into a conductor. Never operate live mains equipment near water or outside in the rain, unless the equipment is specifically designed for that purpose

There is another type of material called a semi-conductor whose properties can change between an insulator and a conductor under certain conditions, but we'll look more at semiconductors later when we get on to active components.

Conductors vs. insulators

Electrons (ie. electricity) can move through some materials much more easily than others. The wires connecting the mains electricity to a mains appliance is normally made of copper as this allows the electrons to pass very easily, but to save you getting electrocuted every time you touch the power lead the copper wire is covered in a plastic coating which does not allow the electrons to pass through.

Materials that allow the electrons to move easily are called conductors, whereas those that prevent electrons from flowing are called insulators. It is these properties that allow us to control where electricity is allowed to pass and to be able to turn devices on and off. The insulating properties of a material will differ depending upon the material and the thickness, so an appropriate insulator should always be used when dealing with electricity especially with mains electricity

Electricity, electric current and electrons

We all know of electricity as the energy that makes our lights shine, powers the TV and for which the energy companies like to charge us lots of money for using, but to understand electronics we need to look at what electricity is. Essentially electricity is caused by subatomic particles called electrons which move around the electronic circuit interacting with the various components. The electrons always flow in a full circuit needing to get back to where they started (ie the battery), although as we will see to different terminals of that battery. This also sounds kind of complicated, but from the point of view of designing electronic circuits (rather than designing the components themselves) you don't need to go any deeper, but I will be referring to electrons again in future. With that out of the way we can now look at how we control these electrons.

Simple electronic switch circuit

To get started with the basics of electronics we are going to look at a very basic circuit with a switch that can turn a light on and off. Okay this may not be exactly what you are hoping to achieve in terms of creating the latest computer controlled electronic gadget, but that will come a bit later. For the moment we need to look at the basics and learn to walk before we can run.

Lets first look at some basics about electricity and how it works. I've kept this to the very minimum so that we can get on to creating our first circuit.

Desoldering

Desoldering

At some stage you will probably need to desolder a joint to remove or re-position a wire or component. There are two ways to remove the solder:

Using a desoldering pump (solder sucker)

1.  With a desoldering pump (solder sucker)

• Set the pump by pushing the spring-loaded plunger down until it locks.
• Apply both the pump nozzle and the tip of your soldering iron to the joint.
• Wait a second or two for the solder to melt.
• Then press the button on the pump to release the plunger and suck the molten solder into the tool.
• Repeat if necessary to remove as much solder as possible.
• The pump will need emptying occasionally by unscrewing the nozzle.

Solder remover wick
Photograph © Rapid Electronics

2.  With solder remover wick (copper braid)

• Apply both the end of the wick and the tip of your soldering iron to the joint.
• As the solder melts most of it will flow onto the wick, away from the joint.
• Remove the wick first, then the soldering iron.
• Cut off and discard the end of the wick coated with solder.

After removing most of the solder from the joint(s) you may be able to remove the wire or component lead straight away (allow a few seconds for it to cool). If the joint will not come apart easily apply your soldering iron to melt the remaining traces of solder at the same time as pulling the joint apart, taking care to avoid burning yourself.

Top of page | How to Solder | Advice for Components | What is Solder? | Desoldering | First Aid

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First Aid for Burns

Most burns from soldering are likely to be minor and treatment is simple:

• Immediately cool the affected area under gently running cold water.
  Keep the burn in the cold water for at least 5 minutes (15 minutes is recommended). If ice is readily available this can be helpful too, but do not delay the initial cooling with cold water.
• Do not apply any creams or ointments.
  The burn will heal better without them. A dry dressing, such as a clean handkerchief, may be applied if you wish to protect the area from dirt.
• Seek medical attention if the burn covers an area bigger than your hand.

To reduce the risk of burns:

• Always return your soldering iron to its stand immediately after use.
• Allow joints and components a minute or so to cool down before you touch them.
• Never touch the element or tip of a soldering iron unless you are certain it is cold.

Soldering Advice for Components

Soldering Advice for Components

It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this!

1. Stick all the components onto a sheet of paper using sticky tape.
2. Identify each component and write its name or value beside it.
3. Add the code (R1, R2, C1 etc.) if necessary.
   Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given.
4. Resistor values can be found using the resistor colour code which is explained on our Resistors page. You can print out and make your own Resistor Colour Code Calculator to help you.
5. Capacitor values can be difficult to find because there are many types with different labelling systems! The various systems are explained on our Capacitors page