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  • Etch strain gauge in copper layer to measure deformations during press process [How to?]

Etch strain gauge in copper layer to measure deformations during press process [How to?]

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#72115

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 14 May, 2014

Hello all!

For my mechanical engineering study I want to etch a strain gauge in the Wheatstone bridge configuration in a copper layer of an PCB. The goal is to measure the strain occurring in an PCB even during the press process! We can measure with X-ray before and after, but we want to measure DURING the press process.

I am wondering if anyone has any experience or information about how to do this. I’ve looked around on the internet and asked some teachers but I did not find anything useful except electrical schematics of the Wheatstone bridge configuration (which is wildly available).

I’d love to etch these gauges in both the copper layers of FR4 32 mil 18”x12” material with 17,5 or 35 micrometer copper layer thickness on both sides. Two of such layers are subsequently glued together using 2-3 thin layers of 3mil 1080 prepreg in a press with a temperature time cycle predetermined by the manufacturer which is something roughly like this:

Temperature will go to from 100 to about 200 Celsius in 30 minutes. Hold the temperature at 200 Celsius there for 1 hour. Cool down in a cool press with press plate temperature of 60 Celsius.

How to measure the output of the strain gauges is also not clear yet since there are no inputs for such measurement equipment inside the press. I think it’s not impossible to get measurement equipment that can work at ~200 Celsius (which is vacuum anyways so probably less) inside the press chamber. There are also 6 thermocouple inputs in the press to the outside control station. Maybe I can use those and do some fancy tricks with the output to transform them to the required strains.

I want to do this experiment so I can measure strains in the PCB’s during process in the press. Any help is appreciated.

Greetings,

Julian

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tmo

#72116

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 14 May, 2014

You should look in to this document:

IPC/JEDEC-9704A Printed Circuit Assembly Strain Gage Test Guideline

Regards

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#72119

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 15 May, 2014

Tmo,

Thanks for your reply. It gives good advise about how to perform tests.

The type of strain gauges that I want to design are inside the PCB at various layers and I want to read the data during the press step. Do you know if these prefabricated strain gauges would be able to do that?

Regards

reply »


tmo

#72120

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 15 May, 2014

I always made tests with gauges on top or bottom surface of the PCB, before, during and after the process that we wanted to check. But I never tried to do it on internal layers of PCB. Don't know if that is possible.

Regards

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#72121

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 16 May, 2014

Interesting. How did you connect the gauges in the press? Our press only has connections for thermocouples. But in essence, it's only a wire and an electical output. If you placed these prefabricated gauges on top and on the bottom surface, weren't they affected by the high pressure of the press?

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#72125

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 17 May, 2014

I hate to say this but in all probability this wont work. I have built real strain gages and they depend on the change(Co-effiecent) of resistence with strain(or Stretch). since you are connecting you elements up with copper wire it will frm part of your circuit. The changes are infinitely small, which is why the Wheatstone bridge configuration is used You then need the other elements to have no change to get the difference. Look up an older analog circuit reference to find the configuration. We then used a very low noise Op-Amp in a high gain configuration to measure the difference. Using the technology that was available the system was barely manufacture-able. Doing it with a circuit board, I think would take you into the land of rubbery figures.

sarason

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#72151

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 26 May, 2014

Thanks for your reply Sarason. I'm aware now that my goals seem very ambitious and infeasible.

Let me formulate my problem in an different way that might be more realistic.

Is it possible to measure a DIFFERENCE in strain between top and middle layers in a thick lay-up?

For this to be determined I don't need an absolute correct value of the strain but only a significant difference in output to be able to conclude that there is indeed a difference in strain. This difference in output should be more than the accuracy of the strain gauge. The difference with the old goals I had is that I don't require absolute perfect data but merely an indication that the effects occur.

A lay-up that I want to test is 4 thick FR-4 layers with 3x2 1080 prepreg inbetween the FR-4 layers. I want to measure the strains of each FR-4 layer with the strain gauges etched on each FR-4 layer to see the behavior of the layers before, during and after curing. Ultimately to see the effects of the press plates on the outer layers (do they stick, stick and slip, or are they free to move) and the inner layers (are internal stresses released when the 1080 prepreg become viscous?) and are they reintroduced when the prepreg is curing.

From your experience and expertise; do you think that is achievable?

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#72152

Etch strain gauge in copper layer to measure deformations during press process [How to?] | 26 May, 2014

Damn near impossible. You need to see a real change in the difference between the 2 elements you are reading. So to give yourself any hope run 2 long traces one top one bottom up the length of the board. Then compare to 2 wriggly traces at one end (of the same length as the long traces) hook these 4 trace resistors up in the wheatstone bridge arrangement. The logic which is explained in an old analog circuit reference. Now hook it up to the lowest noise Op-amp you can afford and mount it all on the one board, set it up with a gain from 1000 towards 100K and measure the difference with a precision low noise Multimeter like a HP 3458A. This will give you the greatest chance of success. After you have your results then you can plot your next step.

regards sarason

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