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Are there any low-cost and accurate ways to measure small dimensions on a physical PCB? Physical is critical here, since it's easy to measure dimensions in a CAD tool. However, actual dimensions will diverge somewhat from the design dimensions due to manufacturer tolerances. In particular, I'm interested to know how closely the width of my \$50\,\Omega\$ traces are to the design specs.

Here's what I've tried: I've taken a picture of a PCB trace adjacent to a set of calipers measuring 1.01mm (idea taken from here). I can then measure the pixels in GIMP (though I'm sure other image editors can do this too). The ratio of the trace to caliper pixel widths times the caliper width gives me the trace width. However, the image is a bit blurry. I think the reason for this is that my iPhone camera cannot support a very short focal length and does not have an optical zoom. I did rest the camera on a stable surface so it wouldn't move. I'm not a photographer, so if there's a way to get better results please let me know. Moreover, the result is a bit far off what I'd expect. I designed these trace widths to be 0.38mm, but am getting about 0.43mm (bear in mind the blurriness makes it difficult to be accurate here). Here's the image. The PCB is manufactured by oshpark 4-layer.

enter image description here

Any thoughts on this process, or better processes available? I know there are dedicated tools that will do this (e.g. some dinolight microscopes will measure distances), but from what I've seen, those are all expensive solutions.

MattHusz
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    There are loupes with measurement marks. Could be cheaper than your dinoscope due to simplicity, or more expensive due to superior quality to a dinoscope. Will need a unpopulated board though because it must be held flat. – DKNguyen Apr 09 '20 at 19:50

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I like your optical comparison method; what you need is a small microscope for your cameraphone to plug into, that would work.

Foldscopes come in at 5€ and do that (German shop, sure there's shops outside of Europe, too), since you don't need high magnification.

Also, replace your caliper by a nice thin sheet metal ruler; you can make one yourself or get one in the shape of square pins that come in pretty well-defined widths; I'd probably do something like getting a nice flat, thin piece of sheet metal, like a razor blade, cutting it down and then filing it until it's of some defined width, according to your caliper (0.3 mm sounds doable by hand, if you support it from below while filing), and then holding it flat onto my trace to be measured – that eradicates the need for the camera optics to have a large focus depth.

Marcus Müller
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  • Or just get a leaf gauge. – DKNguyen Apr 09 '20 at 20:01
  • ah, that's what these "Lehren" are called in English, thanks! Problem with them is you'd have to look very straight from above down to them to judge the width of a trace? – Marcus Müller Apr 09 '20 at 20:23
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    Yeah, unless there is a rod equivalent . But I feel cutting a leaf gauge to not be as wide would be easier than filing down a ruler to be a leaf gauge. – DKNguyen Apr 09 '20 at 20:24
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Microscope calibrators or rulers is what I have used in the past, they can come on a glass slide or something transparent, there are many different types. You can set them on the PCB and take a picture. This also allows you to measure distances to some degree away from the measuring tool if you can start counting pixels. Many microscope camera apps have the ability to calibrate to these and then automatically measure distance.

strong text Source: https://www.amazon.com/RIYIBH-Accessories-Calibration-Transparent-76x25-4x0-1mm/dp/B09STTV284

There are also small metal rulers that I have seen come with microscopes, but don't know where to purchase them

Voltage Spike
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