Soldering basics tutorial

- a summary, tutorial or instructions about the basics of manual soldering techniques used in the build of electronic equipment, and the soldering equipment that may be used.

Soldering is a technique that is essential in the construction of electronic equipment. The concept of soldering ahs been used since the very earliest days of radio and electronics, and the basic concept has remained unchanged, although many advances have been made, and the reliability and safety of the soldering process have greatly improved over the years.

Soldering in the production of electronic equipment can take many forms. In the mass production of electronic equipment, automated techniques are widely used, and techniques such as wave soldering, or infra-red reflow soldering are likely to be used. However the concept of manual soldering is still widely used both in commercial and home environments. The use of a soldering iron and leaded electronic components is still used in a number of situations.

Soldering equipment

There are some obvious requirements for soldering equipment. Some of the soldering equipment is less obvious, but equally important. A list of some of the more obvious soldering equipment is given below:

  • Soldering iron
  • Soldering iron holder or stand
  • Solder
  • Wire cutters
  • Small pliers
  • Vice or holding frame

This is a short list of some of the more important items of soldering equipment. There are many more items that could be included, from solder suckers for de-soldering and anti-static mats for ensuring that components are not damaged by static.

The soldering iron     This is obviously the key piece of equipment and it is essential that it is right for the job. For commercial work, temperature controlled soldering irons are essential to ensure that that good solder joints can be made reliably.

The much cheaper non-temperature controlled soldering irons may be suitable for occasional home construction work. However they are not ideal because their temperature regulation results simply from the cooling of the bit, and when being used for soldering heat will be extracted and the temperature will fall. Additionally the temperature of the soldering iron will rise and fall dependent upon a variety of other factors ranging from the location, the temperature of the day, whether the iron is in a draught, etc. Thus their temperature will only be approximately in the right region for soldering.

Fully temperature controlled soldering irons have a thermostat to control the temperature. This is also adjustable and enables the soldering iron temperature to be set and maintained at the correct value.

Soldering iron holder or stand     In view of the fact that soldering irons reach temperatures in excess of 200 C, it is essential for health and safety reasons that they are held in a proper holder or stand when not in use. This will offer protection from being accidentally touched - an essential requirement in any laboratory situation. Soldering iron stands are available for professional soldering irons.

Solder     Suitable solder is also required. This is normally in the form of a reel of solder "wire". Today lead free solder is usually required for commercial manufacturing applications to comply with health and safety as well as environmental directives for many countries. The solder that is used is the solder manufactured for electronic applications. This contains cores of flux to enable the joints to be cleaned so that good reliable joints are produced.

Wire cutters     A good pair of wire cutters is needed when soldering. Often wires need to be trimmed after components have been soldered in place. For most applications today, a small pair of cutters is best as components are small and wires are not very thick. Additionally a special pair of cutters for trimming the ends of wires that have been passed through a printed circuit board and soldered.

Pliers     A small pair of pliers is also useful when soldering because they may be needed for pulling wires, forming them and holding them in place. As most components these days are small, a small pair of pliers is all that is needed.

Vice or holding frame     The first stage of any soldering job is to put the components into position. Often a vice or other instrument is needed to hold the work if the unit is not large enough to sit on a workbench. Often small assemblies need to be held in position and a suitable vice enables the work to be held at the correct angle to enable it to be assembled and soldered. Printed circuit boards that us conventional leaded components normally have the components on one side of the board and the soldered joints on the other. It is therefore necessary to load the board (commonly called stuffing the board) from one side and then turn it over to perform the soldering. To prevent the components falling out the leads can be bent outwards slightly. Alternatively a holding frame can be used. The frame consists of the basic frame with a sheet of metal of board covered normally in foam rubber. Once the components are loaded, the board can be placed over the components to hold them in place as the board is turned over. This makes the process of inserting the components and soldering them far easier.

Other soldering equipment     In addition to the very basic items mentioned above, a fully equipped soldering and re-work area will have a variety of other tools. One of the major elements that is always required in any electronics production area is the ability for re-work. There will always be board failures and as a result, components need to e removed and replaced. This needs to be done using skill and the right equipment so that no damage is caused. While de-soldering tools are helpful a full solder station is an essential item. The solder station comprises equipment for solder and de-soldering. The de-soldering elements of the solder station normally comprise a heated element with a central hole that is connected to a vacuum pump. In this way the solder station can be used to effectively apply heat and remove the solder from the joint. This means that a solder station will be the most effective way of ensuring that any rework is undertaken with the minimum of risk to the board.

Soldering basics

There are a number of requirements for a soldered joint. The main two are:

  • It should provide a reliable electrical connection
  • It should provide sufficient mechanical strength to hold the component(s) in place

To achieve these aims it is necessary that the joint is made properly. There are four main points to note when making a joint. If these are correct when the solder joint is made, it should remain reliable as long as the product is in use. If the soldering is not carried out correctly, then what is termed a dry joint may result, and these give poor electrical conductivity, sometimes giving intermittent performance, and also the mechanical strength will be much less.

The four keys to producing a good joint are:

  • Ensure the joint surfaces are clean and free from grease.
  • Ensure the temperature of the solder when making the joint is correct.
  • Ensure the heat is applied to the joint for the correct amount of time
  • Ensure the correct amount of solder is used.

Each of these keys is important in producing good quality joints. In fact it is particularly important to ensure that all joints are made to the highest standard because it only takes one joint to cause problems in a circuit.

In view of the fact that dry joints can cause these problems it is necessary to look for them as the individual joints are made, examining each one as the solder cools to ensure it is satisfactory. A good solder joint appears to have a shiny surface to the solder. On the other hand a dry joint can be identified from the fact that the surface looks dull, having a "matt" finish. It also does not have the same mechanical strength as a properly made joint, and provided the wires to the joint are not mechanically would round each other to provide strength, a dry joint will pull apart more easily than a properly made solder joint. When any dry solder joint are found the old solder should be removed and the joint remade.

Making a solder joint

When soldering, it is necessary to ensure that the right steps are taken to ensure that the joints will be of a sufficiently high quality. The iron should be set to the correct temperature. The melting point of most traditional solder containing lead is in the region of 180C and in order that sufficient heat can be transferred to the joint the iron tip temperature should typically be set to the range 330C - 350C. However the new lead-free solders typically require a slightly higher temperature.

One of the major requirements to ensure that a joint is made well is to ensure that everything is properly clean. The soldering iron bit should be cleaned. Many soldering iron stands have a holder for a damp sponge. This should be moistened and the bit for the soldering iron wiped over it, and any residue removed. A small amount of solder can then be melted onto the iron bit so that it is suitably "tinned".

The surfaces of the components to be soldered should be clean and free from grease. To ensure that the surfaces will take the solder, it is often wise to tin the surfaces. Often printed circuit boards come ready tinned and the solder will flow onto them easily. In addition to this it is sometimes necessary to tin the other areas that will take the solder. Typically this may include the leads of wired components and any other surfaces such as tag strips, or areas on a connector to be soldered. Tinning is accomplished simply by applying the soldering iron to the item to be tinned and melting some solder onto the iron bit in such a way that it flows onto the work piece. This should leave only a thin film of solder over the area to be tinned. Once this has occurred the soldering iron should be removed, and not held on the area any longer than is needed. It is then wise to check that the solder has adhered to the item properly.

With the items tinned as required, the soldered joints can be made. The components should be secured in position. When leads are being soldered to tags, some like to wind the leads around the tag to give increased mechanical strength. If this is done then it is considerably more difficult to unsolder if there is a problem later.

With the surfaces to be soldered brought together the soldering iron should be applied. The solder should then be brought to the joint and solder melted over the area to be soldered. One of the keys to making a good joint is to use the correct amount of solder. Experience tells how much should be used in a given situation, but it should be sufficient to cover the area to be soldered. The contour of the solder should be slightly concave.

It is important not to hold the iron on the joint for too long. It should be retained there long enough for the solder to flow freely onto the joint and then removed. If it is held on the joint for too long then the flux will burn off and the solder will oxidise and a dry joint will be formed.

Although certain guidance can be set down to summarise the best methods for soldering, it is a skill that is learned by actually dong it. Fortunately soldering is not difficult to master and with the right equipment and tools it is quickly possible to make reliable joints with little chance of error.

By Ian Poole

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