How to select the test point for PCBA Kelvin Sensing in an efficient way


Kelvin Four-terminal sensing, also called four-wire sensing or four-point probe method, is an electrical impedance measuring technique, which uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurement than two-terminal sensing.

The high-precision measurement is achieved by a special bridge in the sensing instrument. This method is commonly used for measurement of very small or high-precision resistor, for example, if you’re going to measure a resistor of 2.5Ohm by traditional measuring method, the accumulated loss of resistor contacting and wire measuring can be several Ohm, unable to measure precisely, but Kelvin sensing method can make it. The advantage of four-terminal sensing is that the separate current and voltage electrode will eliminate the impedance of wiring and contact resistance. Due to these features, Kelvin four-terminal sensing is extensively used in PCBA manufacturing industry for measurement of high-precision resistance, such as automotive electronics, instruments, communication products and so on.

To adopt Kelvin sensing method, the PCB circuit has to meet the following requirement: the network connected with the pin of to-be-measured resistor should be set up two test points, therefore there should be 4 test points for one resistor, while by traditional sensing method, there are only two test points for one resistor. The figure below shows the test point of a resistor for four-terminal sensing.

In order to meet the number of test point, it is required to select the points in the circuit network connected with the pin of target testing element. For engineers, it would be a hard work to manually select these points as it’s time-consuming and the selected point may not be the optimal, virtually increasing the cost of ICT fixture.

High efficiency and low cost are what enterprises pursue and will make a great difference to enterprise’s competitive advantage. Keeping the customers’ demand in mind, Vayo technology has been dedicated to the development of NPI software and helping enterprises improve their competitive edge over the past 16 years. VayoPro-Test Expert is such a tool able to reduce time cost and improve reliability. Here you will see how Vayo’s Test Expert completes the nail selection of Kelvin test for high-precision resistance in an efficient way.

First, to define the to-be-measured part in the software, for example, if the object for four-terminal sensing is the resistor of 0Ω<R≤10Ω, then click “Add” button in the Search Parts dialog box and choose RES* for TYPE.

Second, to set parameters for searching;

With above steps, the part number for Kelvin sensing will be filtered out and the searching result.

Up to now, the to-be-measured part has been defined, then the software programme will execute nail selection process to choose two test points in the network connected with the pin of those selected resistor. The details can be found in Nail report.

Note: The default nail selecting rules has been configured with Kelvin sensing parameters.

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