Where I mostly seen this approach is with 3D printers where we just want to know if the nozzle is touching the print base.

But if we can quantify the general worse case variation between most PCBs then maybe we can create a recommended strain sensor element with a semi-quantified level of accuracy so it's not just an on/off sense.

The temperature coefficient of resistance of the strain element seems like a concern, though, and so do thermal EMFs. My kitchen scale zeroes when I turn it on, a procedure that should be able to cancel one of these two but not both. Maybe you could have a diode thermal sensor, as an MCXO does, to measure the temperature so you can cancel both?

This project seems to do the first-order temperature correction thing:

> The included sample firmware will wait until a serial console is opened, perform a 5 second offset calibration, then sample continuously at the lowest gain setting. The graph.py script can be used to display the output.

> For high sensitivity measurements, it's important to let the board reach a stable operating temperature for at least 5 minutes before calibrating

The thing I intuitively worry about here is creep. Does FR4 creep enough to worry about? Normally you make strain gauge bodies out of steel because it doesn't.

You measure the change in resistance with a wheatstone bridge tuned correctly.

You basically just need a strain gague (a few dollars), 4 resistors, an op-amp, and a microcontroller with an ADC.

Calibration is important and you'll run into things like the metal bar creeping, permanently bending as a result of weight being put on and off.

But also, milligram accurate scales are $20 on Amazon https://www.amazon.com/GRAM-PRES-Precision-Milligram-Reloadi...

The device effectively measures mechanical strain at the surface of the PCB. The surfaces of a thicker board will experience more strain from bending because the radius of the curve (at the surface) is greater.