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I'm trying to get a rough idea of the quality of PCB traces for a little project where my students will work on developing their own single sided PCB traces, and also play around with different substrates, if they want.

I can only think of using a LCR meter and measuring the characteristics of a long snaking PCBtrace. Can anyone think of an affordable and simple way of getting a rough estimate of PCB trace quality that goes beyond simply measuring continuity with a multimeter?

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    \$\begingroup\$ What do you mean by 'quality?' Adherence to the substrate? Conductivity? Characteristic impedance at a given frequency? \$\endgroup\$
    – JYelton
    Jun 28, 2022 at 22:18
  • \$\begingroup\$ @JYelton Nothing mechanical like adherence to the substrate, more like conductivity and impedance. I'm not trying to get an the "performance" for an exact application, just a couple measurements to get a rough idea of how it would perform in general. \$\endgroup\$ Jun 28, 2022 at 22:20
  • \$\begingroup\$ To measure the characteristic impedance, the easiest measurement method is to use a TDR. Just measuring with an LCR won't tell you if there is any irregularity in the impedance along the length. Also the L and C values are pretty small and might be hard to measure accurately with an LCR. \$\endgroup\$
    – The Photon
    Jun 28, 2022 at 23:12

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PCB fabricators will run electrical tests on boards by subjecting them to a "bed of nails" test (actually a much more complicated robotic approach these days).

The idea is that every net is tested electrically for continuity between different end-points. (Note that "continuity" in such cases is defined as "resistance below some threshold.") A comprehensive test might also inject test signals and measure impedance, capacitance, etc. to validate board performance.

A simplified version of this might be to use a digital multimeter with some probes attached to some sort of holder so that you can ensure consistent results between boards. (Meaning, you are removing variables like pressure and probe position.) If your budget allows for it, you might even consider something like the PCBite probe kit which is essentially a probing kit for small pins or just holding something like an oscilloscope probe to a given pad hands-free. (I won't link it because I am not affiliated and don't want this to come across as an endorsement.) You could probably make something to hold the board and probes in a repeatable way.

You would place a student board into a testing fixture, then connect the DMM probes to the relevant pads, and make a resistance measurement. You could also measure impedance if you have a function generator and oscilloscope, allowing you to send an AC signal along the track and measure it at another point. In all cases, you will need reasonable quality test equipment — an inexpensive DMM may give you acceptable results, but an extremely cheap one will be inaccurate and just give you headaches.

If your students are making variable-width tracks or having differing results etching copper, this may be a usable objective way to quantify track performance. I'm not sure what processes you are wanting to check; if your students are trying different substrates, then you might need more advanced tests to measure how that affects performance. (A copper track on fiberglass versus, say, plastic, will perform the same at DC, all else being equal.)

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  • \$\begingroup\$ Thank you for your response. Given that I'll be supplying multiple substrate materials, I suppose I should measure AC characteristics. My biggest issue is price right now. Besides a handful of cheap multimeters and arduinos, I don't really have anything else available right now. Would buying I2C DACs to produce sine waves work, then using the ADCs on the arduino as an oscilloscope produce any sort of meaningful result for measuring output impedance? \$\endgroup\$ Jun 29, 2022 at 0:15
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    \$\begingroup\$ @itisyeetimetoday You could endeavor to use Arduino and the various AD-DA conversions but you'd still need some equipment to verify and measure that what you're generating is correct. I'm aware some schools/teachers have no budget whatsoever, but I'd strongly recommend an entry-level scope and function generator. It would save a lot of time getting to a working solution. \$\endgroup\$
    – JYelton
    Jun 29, 2022 at 0:24
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There's no single measure of quality. To answer your question, we need to know what aspects of the production process you're judging, and why.

Imaginary part of the complex impedance is mostly a factor of geometry and substrate/coating choice, and has little to do with how the track conducts as long as it maintains the geometry and the conductance isn't outrageously out of spec. So, using a network analyzer to get complex impedance would mostly matter if controlled impedances were a specified aspect of the process.

Real part of the complex impedance, and its reciprocal - conductance - will depend on the material choice and processing during manufacturing. For example, sintered powder traces - like thick film - can be quite sensitive to process parameters, while electrolytically deposited metals are usually more forgiving.

Personally, I'd judge not quality in absolute terms, but the team's ability to define viable product specifications, and then manufacture the product that is within those specs. The specs would usually all be ranges, and they need to be "reasonable" - the students should be able to derive those in an engineering process, taking into account what is possible for them to do. Sure, one could specify properties only achievable with say monocrystals, but it's clearly not feasible (unless you're not telling us something).

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If you want to measure the resistance of a PCB trace, you need a ohm meter with four wire Kelvin measurement to measure some milliohm.

A 35 µm copper plating may be only about 25 µm and up to 45 µm. So you should be able to measure a difference. I would use a fairly wide, about 1 mm and long, about 400 mm trace for that purpose. The long trace may be a rectangular spiral or meander. I would calculate the resistance before and compare it with the specification of the milliohm meter. The result should be much bigger than the resolution and the error of the meter. There are some meters capable of measuring microohm.

Place two vias at each end of the trace to connect the probes of the four wire kelvin method. The current probes at the outer vias and the voltage probes at the inner vias. Two students may be needed to make a good contact.

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