Measurement Mechanism
A highly sensitive electronic instrument designed to measure electrical resistance in the milliohm and microohm range utilizes a controlled current source and a sensitive voltmeter. This micro-ohmmeter is essential for verifying the integrity of printed circuit board traces and copper-plated vias. By driving a known current through the test point and measuring the voltage drop, the instrument calculates the resistance according to Ohm’s law.
Kelvin Sensing
The elimination of test lead resistance is achieved by using a four-wire measurement configuration instead of a standard two-wire probe setup. When a micro-ohmmeter is connected to a circuit board trace, one pair of leads delivers the precise test current while a separate pair of high-impedance leads measures the voltage directly across the target area. Since almost no current flows through the voltage sensing leads, there is no voltage drop across them to distort the measurement.
This design allows the technician to obtain stable readings even when measuring resistances below one milliohm, where lead resistance would otherwise dominate the measurement. Handheld or benchtop instruments utilizing this technique can detect hairline fractures in vias that standard multimeters miss.
Production Testing
In high-volume printed circuit board fabrication, automated flying probe testers integrate these specialized circuits to verify plating consistency. The automated micro-ohmmeter detects micro-cracks and variations in plating thickness by taking fast, localized measurements. This screening provides a quantitative pass or fail decision before boards proceed to assembly.
