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78 The PCB Magazine • October 2014 by Todd Kolmodin gardien serViCes usa What is 4-Wire Kelvin? C o l u m n testiNg todd I've been asked many times, "What is 4-Wire Kelvin?" So, this month I will explain the 4-Wire Kelvin Test and how it can help uncover defects that normally would go undetected in standard electrical test methodology. Most of us have used an ohmmeter to mea- sure voltage, resistance and current. The typical meter has two wires; you probe the two termi- nals, trace ends or put the leads in line with the circuit for measuring current. A standard ohm- meter is shown in Figure 1 below. However, with this type of measurement, the resistance of the leads and contact are add- ed to the measurement. In Figure 2 you can see that the two leads with 0.5 ohms of resis- tance are added to resistance of the resistor be- ing measure and providing a final result of 2.0 ohms, where the expected reading would be 1.0 ohms. 4-Wire Kelvin 4-Wire Kelvin testing is a methodology where high resolution measurements are taken to determine finite changes in resistance. These finite changes in resistance can then be used to locate plating defects or variations in plat- ing thickness. The Kelvin test is highly accurate because of a four terminal system that negates all current sources, lead and contact resistances. This allows for the finite measurements to only be measured on the PCB circuitry. Typically these measurements are in the milliohm range. Figure 3 shows the typical Kelvin circuit. Now the question, "What can Kelvin de- tect?" In the PCB industry, the main focus of the Kelvin test is to identify plating defects in the drilled holes. The higher the aspect ratio of the drilled hole the higher risk of defect. The figure 1: a standard ohmmeter. figure 3: a typical Kelvin circuit. figure 2: two leads with 0.5 ohms resistance.