# Is there an easy way to measure the thickness of the foil on FR-4

Sometimes I find myself trying to guess or recollect what foil thickness was specified for the given piece of FR-4. Shops do not always readily give it - and it appeared that what they write is not always reliable. And sometimes I, regretfully, simply could not remember what was specified.

I do not care about much precision, but it would be good to be able to roughly distinguish between 70, 35 and 18 um foils (to select more suitable for intended traces width). I have few ideas in mind but none deserves to be called "simple"...

• Measure the resistance, do calculations Dec 7 '15 at 7:40
• This idea have two complicated steps. Measuring small resistance and performing calculations given that we have measured resistance of the square foil, not long and thin wire... Dec 7 '15 at 7:48
• shouldnt be that hard to run a current through it and measure the voltage drop to a few percent. Also the resistance per square for various copper clad thicknesses is given everywhere, so its only a bit of division to do Dec 7 '15 at 10:01
• "resistance per square for various copper clad thickness" - thank you for the guidance. I read a few interesting things googling for this. Now let's see: typical resistance is 5e-4 Ohm for square, which for current of 1A will give a drop of 5e-4 Volts right? well, it do not seem easy to measure with usual voltmeter. Though perhaps the method may work with long and narrow strips... Dec 7 '15 at 10:48
• It of course all depends on what you have available and what you call "usual voltmeter". Having a resolution of 100µV is not that uncommon and you can always up that by increasing the current and amount of squares you measure. Dec 7 '15 at 11:06

I'm not sure how interested you are/what your budget is, but what you're looking for is a simple profilometer. I have done lots of work in cleanrooms and these are one of the basic tools of metrology. If you have a budget, something like this is really what you should look at getting.

However, a much more inexpensive way I think would likely work would be to purchase a basic microscope ($200 - the ones you likely had in high school biology class) and get a few foil or plastic pieces as "standards". Then you could place them on the board next to the foils and focus on the standards. If you choose a microscope with a narrow depth of field lens (see link), you can find the height by seeing if the foils are in focus next to the standard. If the standard and the foil is in focus then the heights are within the depth of field of the lens. This the closest simple equivalent to doing 3D microscopy I can think of. Of course you'd have to calibrate this method quite a bit, but with practice and the right foil "standards" I think you could get within a few microns and make very quick measurements as well. • Huh, that looks like the direct measurement method, but I'm sorry to say this also do not sound easy / cheap. Though I think its advantage is that the measurement could be very precise and could be conducted even with small parts of material. Should be valuable for reverse-engineering also :) Dec 7 '15 at 8:19 • If you're worried about obtaining a old microscope, you can probably pick up one from a local high school for next to nothing. I just did a quick search of ebay and saw some microscopes that would suffice for$30. Now you probably wouldn't get the same accuracy as a new scope but if you're trying to distinguish 3 levels I bet a used one from a school or online would do the trick. The standards could be scraped off old boards or just measured once by someone with the right equipment. Dec 7 '15 at 8:24
• @RodionGorkovenko as far as ease, I think once you had the standards it would take about 15 minutes of experimenting with a few good boards to get the method figured out. It's as simple as sweeping the focus through around the standard and seeing where the focus becomes crisp. Dec 7 '15 at 8:27
• No-no, I do not worry - I even have one... And I understand that after some exercise I probably can get along with this method. Though handling opaque pieces with it always was bit painful to me - though it should be the matter of practice. :) Dec 7 '15 at 8:28
• @RodionGorkovenko Opaque pieces are a pain if the illumination source is from below, but just throw a nice bright 2W LED on a clamp and you're set! Dec 7 '15 at 8:32

Suddenly (reading loosely related "related questions" on the right of the page) I came to the idea which after testing seems better than others.

I've used kitchen scales to measure the weight of the sheet. Then I used the ruler to calculate the area and vernier caliper (if it is proper translation for what we locally call "shtangentzirkule") to check the overall sheet thinkness roughly.

Then I calculate the expected weight of laminate and subtract it from the total weight. Now I have the weight of two copper sheets (in my case it was about 10g for area of about 160 cm2) so I get the answer (0.0069 cm for two sheets total) looking very close to 0.035 mm per sheet.

e.g.

S - area of the sheet (163 cm3 in my case)
W - total weight of the sheet (55 g)
d - thickness of the sheet (0.14 cm)

p0 - laminate density (1.85 g / cm3) - googled, of course
p1 - copper density (9 g / cm3)

w0 = S * d * p0 (I get about 45.6 g)
w1 = W - w0 (i.e. 9g)
d0= w1 / (S * p1) / 2 layers


This should be bit less effective for single-side sheets, but I hope it will still work well, since the copper is almost 5 times more dense compared to the laminate...

Other ideas I have were either try to measure resistance or the time of solving in the FeCl or something like this. But I have no idea how to make them easy or reliable...

• "Stangenzirkel" == "callipers" Dec 7 '15 at 9:24
• Roger, thank you, that seems to be a good direction. "Vernier caliper" is precisely the instrument I meant. "Stangenzirkel" was wrong anyway as in German it appears to mean something different compared to Russian. Sorry for this mess :( Dec 7 '15 at 10:57
• "Stangenzirkel" (de.wikipedia.org/wiki/Stangenzirkel) = Compass (en.wikipedia.org/wiki/Compass_%28drawing_tool%29) Vernier Caliper (en.wikipedia.org/wiki/Calipers)= Messschieber (de.wikipedia.org/wiki/Messschieber)
– JRE
Dec 7 '15 at 11:32
• Not simply "Compass" but "Beam compass"... Oh my... We call simple "compass" (the drawing tool) the "zirkel" to distinguish from magnetic pointing device. And yes, I like this triple "s" in the German word :) Dec 7 '15 at 12:17

If you have access to both sides of the foil, then the easiest way might be to directly measure the thickness using a micrometer. It is possible to measure 0.1um accurately with a good one, and even about 3um accuracy is not too difficult to get.

• Yes, I thought of this, but it requires to detach the foil mechanically which probably is not the easiest thing to do... Dec 7 '15 at 8:21
• If you don't have access to both sides of the foil, I don't see much better methods than electrical testing. PCB's appear to have a flatness of about 0.75%, which over a $1cm^2$ square piece is about 100um variation in height. Dec 7 '15 at 8:36