SOLDERING PRACTICES

Among the foremost of reasons an electronic project frequently fails to work properly is due to "poor" soldering practices. This is usually caused by "dry joints" when soldering. Here I discuss the correct procedures for soldering electronic projects.

Dry joints when soldering

At first glance many solder joints appear to be quite "O.K." but on closer examination many are in fact defective. The insidious problem with dry joints in soldering is that the circuit frequently performs alright for a period of time, even years before failure.

This problem even occurs with manufactured equipment. Ask any TV / Video repair technician who has torn a lot of hair out over an elusive fault ultimately traced back to a dry joint.

Good soldering practices for your electronic project

The cause of dry joints in soldering is mostly the improper application of heat. Both the component leg and the PCB need to be both heated simultaneously to the correct temperature to allow the solder to flow freely between BOTH surfaces. Obviously this requires practice and most newcomers inevitably get it wrong.

Improper heating while soldering and its consequences can be seen below.

Figure 1 - correct soldering procedures to avoid dry joints

Here in figure 1 entitled "correct soldering procedures to avoid dry joints" we have three examples of soldering depicted. The first example indicates the component lead was heated while the PCB wasn't heated. As a consequence the solder only flowed onto the component lead.

In the second example of soldering in figure 1 we find the PCB was correctly heated while little or inadequate heat was applied to the component lead. This is the most treachorous example because although I have made it very obvious in the diagram, in practice it is not always particularly obvious. Often this type of dry joint "just" allows the solder to "touch" the component lead while not actually being "soldered" to the lead. Of course it might work for a period of time depending upon environmental conditions of heat and cold.

In the final example of "correct soldering procedures to avoid dry joints" I have depicted the solder bridging both the PCB and the component lead. In this case the PCB and the component lead were both heated "simultaneously" AND the solder was applied to either the component lead or the PCB to "flow" freely from one to the other to provide a good "electrical" joint. Such a joint is always "bright and shiny", dull looking joints are often suspect.

You never apply the solder to the soldering iron "tip". Solder is always applied to the "job", never the soldering iron. Allow the solder to "set" and cool before proceeding to the next joint.

Other cases of soldering

We have discussed soldering components to a PCB yet this is not the only case of soldering. Often we need to connect wires to switches and other components. A common misconception is that soldering is designed to provide a good mechanical joint. - It isn't!

Any connection should have it's own mechanical strength perhaps by twisting wires together or twisting the wire around a binding post or through a hole provided for the purpose. The solder is only intended for a good "electrical" connection. Never provide a connection which can't stand mechanically on it's own merits.

What's soldering flux?

Modern quality electronics solders contain a "flux" resin within the solder. This flux is designed to flow over the job and prevent contact with the atmosphere. Metals, particularly copper when heated tend to "oxidise" and prevent the alloying or good electrical bond between the copper and the solder.

Good solder containing the resin will have resin flowing over the leads and prevent this oxidisation process and as the solder flows the resin is displaced allowing the solder to form an "atomic" bonding with the items being soldered together. A good resin helps to keep the surfaces clean.

Rules for good soldering

Of course some of these rules might seem very obvious but are worth repeating.

 Use a reasonable quality iron of the correct wattage for the job.
 Only use "electronic" resin cored solder of fine gauge.
 Make sure all surfaces to be soldered are "bright, shiny" and thoroughly clean.
 If a mechanical joint, make sure it can "stand alone" before soldering.
 Make sure the solder tip is clean, shiny and properly "wetted".
 Remember the soldering iron tip is only to heat up the surfaces to be soldered.
 Apply the resin cored solder to the heated "job", not to the soldering iron tip.
 Remember to visually inspect ALL of your soldered joints, preferably with magnifying glasses.
 Consider using your multimeter to provide an "electrical continuity" check between various parts of the circuit.