PCB thickness measurement Device  - Page 1
Forum section: electronics / projects

author: thx130
author_rank: Newbie
timestamp: October 02, 2015, 02:38:38 pm
content: Hi all, long term follower, first time poster here.As part of a collage course, I am trying to make a device that will measure the thickness of a PCB within 0.01mm accuracy.I've been looking at ways of measuring thickness and I thought laser displacement sensors would be a good option.Like this:http://www.lap-laser.com/fileadmin/_migrated/pics/messprinzipien-05_dicke.jpgI looked at capacitance and ultrasonic sensors but decided they wouldn't be suitable because PCBs have a varying dialectic constant depending on material and I thought that variations in boards would also throw of ultrasonic sensors accuracy.Does anyone who has experience of these sensors know if that is true?If I go with the laser sensor option, does anyone know how they output data, and could this easily be read by an arduino, picaxe or similar?Thanks in advance,Ewan

author: lukier
author_rank: Unknown
timestamp: October 02, 2015, 02:44:23 pm
content: Is contact OK?Then this is probably the simplest way:https://en.wikipedia.org/wiki/MicrometerIf you want nm accuracy then maybe:https://en.wikipedia.org/wiki/Michelson_interferometer

author: amyk
author_rank: Super Contributor
timestamp: October 02, 2015, 02:46:32 pm
content: A micrometer...?Edit: beaten to saying "micrometer", I suppose everyone reading this thread is thinking of that.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 02, 2015, 02:47:40 pm
content: First thought that sprang to my mind was also 'micrometer'Presumably this is for measuring as it passes by on a production line or something though? What about two rollers, one of which is free to move, then measure the displacement of the roller with an encoder as the board passes through them and pushes them apart.

author: GNU_Ninja
author_rank: Regular Contributor
timestamp: October 02, 2015, 02:52:29 pm
content: Quote from: amyk on October 02, 2015, 02:46:32 pmNah. I first thought Vernier Calliper

author: 128er
author_rank: Regular Contributor
timestamp: October 02, 2015, 03:01:14 pm
content: Keyence has great devices for that task. I have a optical micrometer from keyence. It even captures a human hair.http://www.keyence.com/products/measure/laser-1d/index.jsphttp://www.keyence.com/products/measure/micrometer/index.jsp

author: thx130
author_rank: Newbie
timestamp: October 02, 2015, 03:12:02 pm
content: A Micrometer is what we use at the moment! The issue with it is that you can skew the results depending on how hard you apply it to the board.Plus I have to come up with something that is reasonably impressive for college.Maybe I could make a micrometer that interfaces with a PC and shows if it is within temperance etc?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2015, 03:42:25 pm
content: As the carrier material is often partially transparent, a direct optical method will not work. So you need some kind of mechical contact if you are after the mechanical thickness. Reading the small distance in the instrument itself can be done in various ways: e.g. capacitive, LVDT, magentiv, optical.0.01 mm seem like a high resolution, but typical copper clad is only 0.017 - 0.07 mm. So this is not that good.An alternative would be measuring the elektrical conductivity. If you want to include a lot of maths - the van der Paw method would be fun. Also resistance of a defined thin line might be an option.The more practical method might be magnetic through eddy currents. This could be as little as a special inductor with ferrite core you place somewhere in a large enough piece of copper area. Worst case one may need some kind of calibration samples to adjust before use. At least for raw material this is rather simple, but still quite effective. Something like 10% accuracy (e.g. 3 µm) schould be still ratehr easy. However one could not tell the difference between 35 µm pure copper and a thicker layer of the same conductivity.

author: MatthewEveritt
author_rank: Unknown
timestamp: October 02, 2015, 04:19:56 pm
content: Quote from: thx130 on October 02, 2015, 03:12:02 pmEvery micrometer I've ever seen has a small knob at the end - this is theonlybit you should turn. It has a sort of clutch like ratchet affair built in that only allows you to apply up to a certain pressure. Turning the main body will, as you say, give imprecise readings, and can potentially wear the mechanism introducing a systematic error.Now if I'm honest I don't think I've ever remembered that until after I've taken the measurement wrongly, but still, worth knowing...

author: daqq
author_rank: Super Contributor
timestamp: October 03, 2015, 06:43:29 am
content: For real bulk ammounts these guys might help:http://www.me-inspection.sk/?q=en/node/153or these for individual sensors:http://www.micro-epsilon.com/optical-micrometer/index.html(look over the site, there are more solutions there)

author: rx8pilot
author_rank: Super Contributor
timestamp: October 03, 2015, 07:07:20 am
content: . 01mm is hilariously over board. It is a challenge to hold that on machined metals.You can also get just about any guage with an SPC output to feed a pc or other device. A small surface plate, dial indicator and some guage blocks for reference should be enough to slide a pcb through.Sent from mobile device.... Keeping it short and mis-spelled

author: AndyC_772
author_rank: Super Contributor
timestamp: October 03, 2015, 07:13:12 am
content: It's not over the top for a measurement device, though, if we use the rule of thumb that the measurement accuracy should be 10x more precise than the tolerance of whatever we're trying to measure.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 03, 2015, 01:35:23 pm
content: Do you want .01mm resolution or accuracy?Because temperature will have effect on the calibration, of both the tool and the material.

author: thx130
author_rank: Newbie
timestamp: October 05, 2015, 12:30:30 pm
content: Wow, thank you all for the detailed responses, very helpful. Sorry I could respond earlier, I was offline over the weekend.Perhaps I should explain the situation a little more. I work for a PCB manufacturer and the thickness measurement will be taken after multilayer boards have been pressed, so they will be all over copper film on both sides.This is to ensure that the boards are within tolerance, and so that us CAM engineers can check that our stackup designs are coming out OK. It also flags up if the operator has used the wrong prepregs or cores.I would like 0.01mm accuracy as most boards have a tolerance of +/-10%, which on a 0.80mm board is 0.08mm so 0.01mm accuracy is reasonable.The problems I have identified with the existing method are:Because of the physical limitations of micrometers, only the edge of the board is measured, but this does not give a true representation of the finished board because there is always 100% copper on all layers where we measure.As previously mentioned, it is open to false readings due to operator error.evb149 raised some very valid points. There will always be some bow and twist on every panel measured.I would have thought my initial idea of a laser sensor would have been relatively simple? All I would need would be the distance from the sensor and a reference plane, like this:Anyway, thank you so much for the helpful replies. I will no doubt be back with further questions.If anyone has any more ideas, I would be happy to hear them.

author: Kalvin
author_rank: Super Contributor
timestamp: October 05, 2015, 12:38:54 pm
content: A cheap USB microscope or inexpensive FullHD-capable USB microscope, and OpenCV. With the FullHD resolution you should get to 1um resolution. With proper calibration the accuracy should be good. Contactless and fast, no moving parts.