I know that lead-free press fit pins come in two flavors – solid and compliant pin. What are the differences between the two types?

The original attempts at a press-fit process used solid pins. The advantage was low cost and simple production. However, the application required very precise production of the holes and plating in the PCB. The application of solid pins was very simple, but had one major application drawback. The application of the pin would deform the PCB as the solid pin was inserted. This connection was not very robust as there was very little spring effect to help deal with conditions like vibration and temperature fluctuation.

The compliant pin was invented to directly deal with the drawbacks of the solid pin. With the compliant pin, insertion of the pin forces the material of the pin to be compressed and act like a spring. There are various configurations of compliant section – compliant “C”, eye-of-the-needle, Action Pin, etc – each with the concept of allowing the pin to deform to produce a spring force that produces the mechanical connection. This allows the tolerances on the PCB holes\plating to be much more forgiving. This also has the advantage of the absolute minimum of PCB deformation with a connection that can expand\contract as the application is subject to conditions such as vibration and temperature fluctuation.