Fillet Lifting

What is it?

Fillet lifting refers to the lifting of the solder fillet around a component lead from the solder pad on the board surface to which it is supposed to be attached. The result is a visible gap between the solder fillet and the PCB pad after the soldering process.

Why does it happen?

Fillet lifting occurs during the solder solidification stage as a result of differential solder solidification across the solder fillet due to a varying alloy composition through the joint. It occurs mainly on larger joints such as through hole joints created mainly by wave soldering, but can also occur during reflow soldering.

It is particularly associated with the presence of certain elements that can form low melting point eutectic alloys with the solder components. These elements include indium and bismuth. The effect is more likely to occur when lead has been used as a component of the circuit board finish.

Although fillet lifting is a particular problem for Lead-Free assembly, it can also occur with tin-lead based assembly processes and in particular with complex circuit boards having high layer counts and a high thermal mass.

What problems can it cause?

In many cases fillet lifting need only be regarded as a visible, i.e. cosmetic, defect and it is unlikely to contribute to reliability issues because there is still a good electrical and mechanical connection on the barrel of the joint. However, where possible, steps should be taken to avoid its occurrence.

How can it be prevented?

There are several factors that are known to contribute to the occurrence of fillet lifting and if these are understood it is often possible to avoid the phenomenon. Steps that can be taken to avoid fillet lifting include;

• Avoiding the use of bismuth containing alloys. These alloys are known to contract substantially on cooling and this can enhance the occurrence of fillet lifting.
  
• Avoid the use of lead containing circuit board and component finishes. Lead contamination in most Lead-Free solders will tend to change the cooling characteristics, increase the degree of contraction on solidification and thus increase the likelihood of fillet lifting.
  
• Increase the cooling rate immediately after soldering. More rapid and forced cooling minimises the temperature difference between the circuit board and the solder joint and this is known to reduce the likelihood of fillet lifting.