What are Solder flags or Spikes?

Solder flags or spikes are due either to inconsistent flux application or poor control of solder drainage from the wave.

If poor flux application is the cause there will be other evidence on the surface of the board like thin whisker of solder similar to snail trails on a garden path.

Poor control of separation from the solder wave would tend to be a random fault not on the same contacts every time. This is due to the back flow of the wave not being set correctly on a lambda style wave. The solder should flow at the same speed and direction as the board during separation from the wave.

Running slightly faster will not normally cause spiking but running slow or not flowing at all will increase spikes.

If the spikes are always on the tip of the leads it may be a solderability issue due to bare cut ends. If leads are cut by the supplier or cut and stored for a long period of time the bare end of the lead will oxidise and be difficult to wet with solder.

If the lead is slow to wet it will also be slow to drain hence a spike can form.

On some occasions the spiking can be a thermal problem simply eliminated by a longer immersion time in the wave or increase pre heat.

A component that has a high mass may have a lead of the same size as other parts. During pre heat of the board there may not be an opportunity for heat to be absorbed by the lead to overcome this thermal load.
As the leads of the component separate from the wave they will cool much more quickly leaving either spikes or shorts.

Incorrect hand soldering may also cause spike to occur.
It may be due to limited flux application or too long soldering time with a low residue cored wire.

The use of low residue wire often requires some re-training of prodution staff.