Introduction to Soldering

Soldering is the process of a making a sound electrical and mechanical joint between certain metals by joining them with a soft solder. This is a low temperature melting point alloy of lead and tin. The joint is heated to the correct temperature by soldering iron. For most electronic work miniature mains powered soldering irons are used. These consist of a handle onto which is mounted the heating element. On the end of the heating element is what is known as the "bit", so called because it is the bit that heats the joint up. Solder melts at around 190 degrees Centigrade, and the bit reaches a temperature of over 250 degrees Centigrade. This temperature is plenty hot enough to inflict a nasty burn, consequently care should be taken.

It is also easy to burn through the PVC insulation on the soldering iron lead if you were to lay the hot bit on it. It is prudent, therefore, to use a specially designed soldering iron stand. These usually incorporate a sponge for keeping the bit clean.

Soldering irons come with various ratings from 15W to over 100W. The advantage of a high wattage iron is that heat can flow quickly into a joint, so it can be rapidly made. This is important when soldering connectors as often there is a quite a large volume of metal to be heated. A smaller iron would take a longer time to heat the joint up to the correct temperature, during which time there is a danger of the insulation becoming damaged. A small iron is used to make joints with small electronic components which are easily damaged by excess heat.

Always use a good quality multicore solder. A standard 60% tin, 40% lead alloy solder with cores of non-corrosive flux will be found easiest to use. The flux contained in the longitudinal cores of multicore solder is a chemical designed to clean the surfaces to be joined of deposited oxides, and to exclude air during the soldering process, which would otherwise prevent these metals coming together. Consequently, don't expect to be able to complete a joint by using the application of the tip of the iron loaded with molten solder alone, as this usually will not work. Having said that, there is a process called tinning where conductors are first coated in fresh, new solder prior to joining by a hot iron. Solder comes in gauges like wire. The two commonest are 18 swg, used for general work, and the thinner 22 swg, used for fine work on printed circuit boards.

Soldering Skills

Soldering skills are essential in the construction of electronic circuit boards.  It often requires a certain amount of skills, and often students will acquire them after a bit of practice in making a few connections.  Remember soldering fulfils two fundamental requirements : one, to connect two conductors electrically and two to connect such conductors in a secure mechanical way.  Note: a bad soldering join may fulfil one of the two requirements, hence it is a “bad” joint.  Making good solder connections requires a few basic rules:

·        The soldering iron should be a 25 to 40 watt pencil type with a chisel or cone-shaped tip. When first using a new tip, coat the tip with a good coat of solder and wipe off the excess with a damp sponge. Always the leave the tip in good clean condition when you are finished with soldering by wiping it on a damp sponge each time before making a connection.

·        Always wait at least five minutes for the soldering iron to come up to operating temperature {CAUTION: do NOT touch the iron to find out whether it is hot enough, you will get “burnt”} Also, remember to unplug the soldering iron when you are finished using it.

·        The solder should be rosin core 60/40 Tin-Lead content. Do not use acid core or the type used for plumbing {CAUTION: Lead is a poison and has been proven to cause health problems. Never leave solder around where young children may handle or eat it.}

The first step in soldering a component to the stripboard or pc board is to make absolutely sure that you have the component leads in the right holes and that the part is properly oriented. (usually flush with the surface with minimum conducting leads exposing, often done with bending the wire with a  pair of long nose pliers to ensure a proper right-angled bend.  A range of good and bad examples are shown below:

If you built the circuit on the breadboard first, it is a simple matter of pulling parts off the breadboard and placing them one by one onto the pc board. Push leads of a component through the holes and spread the leads apart to keep the component from falling out when you turn the pc board over to solder. You can work one part at a time, but the job will go faster if you place several parts close together at the same time and then solder.

To facilitate a good and easy solder, position yourself as if you are writing using a pen. [law of gravity : solder flows down when melted]

To make a solder joint, touch the soldering iron tip to both the pc board copper pad/strip and the component wire at the same time and allow the connection to heat for a second :

Now, bring the solder to the hot connection, not the iron tip directly, and allow enough solder to flow around the connection.

Remove the solder and iron together and allow the connection to cool for a few seconds before you move anything.  The solder should be shiny and look almost wet when it first cools

Poor solder joints, such as shown below, are usually the result of insufficiently heating the joint or excessive solder.  The joints may have voids, or have a massive blob-like appearance and then to look dull.  Just reheat the joint and add/remove solder as necessary. You may have to cut off excess component lead close to the connection with diagonal cutters.