How to perfect your solder joint formation

Firstly, for a seamless soldering process, it is imperative to have clean and un-oxidised parts prior to soldering. Providing all the pre-requisites are in place it should then be a  simple matter of  positioning the soldering iron at the appropriate spot, applying heat for the required time and feeding the correct gauge solder wire in a sufficient quantity. Consideration should always be given to the size of pads to be soldered.

Naturally the smaller areas will heat up faster. Furthermore, molten solder will flow to an area with a higher temperature because of its fluid characteristics so to aid thermal conductivity it is always advisable to apply a small amount of solder to the soldering tip prior to soldering. So, without further ado, let’s examine effective solder joints in a five question format:

How do you know if you have completed a good solder joint?

Good solder joint formation is a pre-requisite in electronics manufacturing. Poorly executed solder joints can significantly reduce the lifetime of any device and cause intermittent faults both of which can lead to embarrassing and expensive field failures.

Whilst an immediate failure of a joint is hard enough to contend with, it can at least be quickly addressed and remedied. The biggest issue is a more dormant, underlying fault or intermittent breakdown which is difficult to diagnose so can be even worse as it will cause the equipment to fail once it’s in service.

Fortunately, the vast majority of solder joints are well produced and do not pose any problems. There are obviously many aspects to the production of a high quality soldered joint. Cross-sectional or X-ray analysis will offer a high level of confidence when trying to ascertain if soldered joints are acceptable.

However, in reality, most applications will rely on visual inspection by a trained operative. Inspection criteria will include the degree of wetting, amount of solder, shape of the soldered joint and physical appearance. For example, if there is too much solder on a joint, this could be a poor joint, i.e. It is difficult to inspect, and also increases the risk of solder bridges, which will result in a short circuit.

Can you use too much flux when soldering?

In reality, the simple answer is no, however, most solder wires are flux cored and contain a small percentage of flux (typically two to three percent). In difficult applications where heavy oxidation is present, the integral flux may be insufficient to produce a good quality joint. In this case, additional flux may be required.

However, care should be taken to use a flux chemistry that is compatible with the solder wire and the amount applied should be kept to a minimum, especially if the assembly is to be cleaned at a later stage.

Is it possible/a good idea to solder new on top of old solder?

As long as the solder alloy is the same, there is no problem mixing old solder with material that has been freshly applied. Make sure that your soldering iron is set to the correct temperature, clean the soldering tip using a damp sponge or Tip Cleaning Wire and then apply a small amount of solder to the tip to aid thermal conductivity.

Once ready, touch the solder tip to the component pin/copper on one side and touch the new solder wire to the other. Once you are happy with the quantity of solder wire applied, remove the solder wire shortly followed by the solder tip. Special care should be taken to ensure that Tin/Lead and Lead Free Solders are not mixed as this would create a new alloy whose electrical and mechanical properties could be compromised.

What are the key steps required to test a solder joint?

The ultimate test of a soldered joint will always be the functionality of the unit produced. However, this is purely a snapshot of the joint integrity at that moment in time and does not give any indication of long term reliability. If there are any concerns regarding the quality of a soldered joint, the favoured tests would incorporate cross-sectional and/or x-ray analysis.

These can be offered by either your materials manufacturer or specialist establishments, e.g. International Tin Research Institute. Most manufacturers would usually rely on a visual inspection by skilled/trained operatives and a functional test during the manufacturing process. In latter years, the inspection process has been automated by the use of Automatic Optical Inspection (AOI) equipment but there would still be a requirement, in most cases, for an additional functional test.

Some solder joints can appear dull in colour – why is this?

Traditionally, most people in the electrical/electronics industry have been trained with Tin/Lead Solders and to look for a ‘nice, bright, shiny joint’. However, if there was movement of the joint during the cooling process this could produce a dull finish, which would need reworking. Lead-Free solders tend to have a coarser grain structure in the first place.

As a result, light is refracted in numerous different directions giving the impression of a duller finish. It is not a defect in this case, just an accepted difference between the different alloys.

Practice makes perfect when it comes to making good solder joints, as well as thorough inspection afterwards. As a rule of thumb, ensure that all surfaces to be soldered are clean, free from grease and as secure as possible as any movement during the soldering process can result in a dry joint.

If the iron is left on the joint for too long, as well as risking damage to the component and PCB, this may also exhaust the flux content of the solder wire. This will cause the solder to oxidise, which in turn could result in a dry solder joint forming. Allow the solder on the joint to cool and solidify before allowing any movement.

Hakko hopes the advice on perfecting your soldering joints has been useful. Look out for Hakko Tech Tips next month and in the meantime feel free to contact us for any advice with your soldering or extraction equipment questions.