Semi Kelvin Versus True Kelvin
When using a fixture, the fastest method of connecting and disconnecting the component under test, while still maintaining a true, four-wire Kelvin connection to the component, is to use Kelvin blades. Kelvin blades consist of two spring blades held in an insulating body (Figure 3b).
As previously explained, true Kelvin provides the most ideal connection method when measuring resistances <1Ω. However, when designing a test fixture, the mechanical aspect of the connection method must be considered. In this case, spring probes may provide an alternative to Kelvin blades. However, the current through the component under test must then also pass through the spring probe itself, introducing an additional, undesirable voltage drop.
Fixtures made using spring probes have the advantage of being easier to construct, easier to maintain, and they have a longer life span than Kelvin blades, which are subject to wear from the action of inserting and removing the test component. However, because spring probes can only offer semi-Kelvin connection, they should not be used when measuring a resistance of less than 1Ω.