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What is Four-Terminal Sensing: Micro Resistance Test for Advanced PCB

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Tues, Jun 15, 2021

Before distribution, PCBONLINE applies four-terminal sensing to PCBs to test via copper's and line copper's thickness and flatness, filling quality of plated filled-holes, resin plug holes, and silver paste plug holes, and resistance of the circuit. You may ask, what is four-terminal sensing? In this article, we will use the plainest language to explain four-terminal sensing.

What is Four Terminal Sensing?

Also called the Kelvin test or four-wire sensing, the four-terminal sensing is a necessary test to check the copper thickness, resistance, and hole filling quality of printed circuit boards that require high precision and high quality. Unlike the AOI or in-circuit test which uses two points to test the circuit, the four-terminal sensing uses four points.

PCBs used for aerospace/defense/automotive have low resistance components, and for these advanced PCBs, the two-point circuit tests can't check issues like thin-copper, small-scale open-circuit, contamination on PTH hole wall, and so on, which can be serious problems to the products. In this case, the four-terminal sensing is used.

Note
The four-terminal sensing is widely used for PCBs used for automotive, aerospace, and defense. It was proposed to put in the IPC 9252 standards.

The below image shows the circuit diagrams of the two-point test and four-terminal sensing. As you can see, in the two-point test diagram, the power supply and the voltmeter are connected in parallel and then connected in series with the PCB through two contact points. In four-terminal sensing, the power supply is connected in series with the PCB, and then the circuit is connected in parallel with the voltmeter.

two-point test and four-terminal sensing

If you ignore the resistances of the contact points and lines, the two-point test is okay to test the PCB's resistance. But when the PCB's resistance is micro, the resistance of the power supply line and contact points can't be ignored as they can impact the testing result, and the four-terminal sensing perfectly solves this problem.

Why Four Terminal Sensing Can Test Micro Resistance?

The lines, contact points, and the PCB all have their resistance. We use RL to stand for the line resistance, RC for resistance of the contact point, R for the PCB's resistance under test, RT for the total resistance of the circuit that is tested out, and RO for the voltmeter's resistance. And the equivalent circuit diagrams of the two-point test and four-terminal sensing are as below.

four-terminal sensing vs two-terminal sensing

In the the two-point test's circuit diagram, RT = 2RL + 2RC + R.

In the four terminal sensing's circuit diagram, RT = R (2RL + 2RC + RO) / R + 2RL + 2RC + RO.

When the PCB's resistance R is very small and the voltmeter's resistance RO is very large, the effect of 2RL, 2RC, and RO on the measuring result is beyond the effective number's range. In this case, the RT is nearly the R, and the tested result is accurate.

From the above, we have explained why the four-terminal sensing is accurate to test the micro resistance of the PCB. If the copper line on the PCB is unevenly wired or the copper is unevenly plated, the four-terminal sensing can check out different resistances of the test points and locate where the problem is.

Requirements of Four Terminal Sensing

For the high accuracy of the testing result, the voltmeter for the four-terminal sensing circuit must have extremely high resistance so that the current flows is nearly zero.

PCBONLINE Applies Four Terminal Sensing to PCBs and PCB Assemblies

PCBONLINE is a one-stop advanced PCB manufacturer. We apply the four-terminal sensing to PCBs and PCBAs for automotive, medical, communications, aerospace, and defense. Below are the technical specifications of the four-terminal sensing at PCBONLINE.

Four Terminal Sensing
Resistance Range
1mΩ~20KΩ
Testing Accuracy
±0.1mΩ
Testing Speed
within 3.5 seconds
Available Testing Points
2,000 - 32,000

Through the four-terminal sensing, your PCB and PCBA are free from these problems:

  • Poor plated holes and silver through holes
  • All kinds of loop circuit defects
  • Not qualified circuits that have specific resistance requirements

Through the four-terminal sensing, your PCB and PCBA are free from these problems:

  • Poor plated holes and silver through holes
  • All kinds of loop circuit defects
  • Not qualified circuits that have specific resistance requirements

PCBONLINE uses dedicated jigs, switches, and instruments for the four-terminal sensing, and for maintaining accuracy, they are never used for the two-point tests.

four-terminal sensing machine

PCBONLINE focuses on the high quality of products. Besides the four-terminal sensing, we are also featured with:

    • All PCBs are manufactured to the IPC-A-600 standards;
    • All PCBAs are assembled to the IPC-A-610 standards;
    • One-to-one engineering support and DFX are offered;
    • Free complete samples and functional testing to PCBA are offered to bulk orders with an amount at or above $5,000.

We provide 1 to 24 layers of complex PCB manufacturing and assembly. You can order online and get an online quote now! Sign up and get $100 coupons for online purchases.

Conclusion

In this post, we introduce what four-terminal sensing is and why it can test micro resistance. We also recommend PCBONLINE, a leading advanced PCB manufacturer for automotive, aerospace, and defense. If you are working on an advanced PCB project that requires precision, choose PCBONLINE as your PCB manufacturer from prototyping to batch production.