"Tom Del Rosso" wrote in news:qvlccr$kcc$ snipped-for-privacy@dont-email.me:
Add a half mil for the adhesive used to bond to foil. Add 1 mil (IIRC) for the mask. The PCB house can tell you the exact bonded thickness of the mask.
Then measure the pcb at a location that is bare. Measure again over a trace. Subtract.
Measuring thickness is easy. Here's an example for a thin aluminum film:
The resistance of a pure aluminum film 26 nanometers thick is
1.038 ohms per square.
This means you can have a ribbon of material 1 cm wide and 10 cm long, and the resistance between the ends will be 10 * 1.038 = 10.38 ohms. The resistance is calculated as follows:
Ohms/square = Bulk Resistivity / Thickness
The bulk resistivity of pure aluminum is about 27e-9 Ohms, and the film is
26 namometers thick. So the resistance is
Ohms/square = 27e-9 / 26e-9 = 1.038 ohms per square
Similarly, you can measure the resistance of the ribbon, and determine the average thickness. The thickness is calculated as follows:
Thickness = Bulk Resistivity / Ohms per square
For example, if you measure 10.38 ohms between the ends of a ribbon 1 cm wide and 10 cm long, the resistance is 10.38 / 10 = 1.038 Ohms per Square. The thickness of the film is then
Thickness = 27e-9 / 1.038 = 26 nm
A Kelvin probe can be used to measure the resistance when it is very low.
The next problem is to extend this to copper, and figure out the weight from the thickness.
Indeed. I reckon peeling the stuff off and micing it will give an erroneous oversize result. The most accurate method is to etch it away. But if precision isn't an issue then fine I suppose.
Part of the foil on common FR4 boards includes a terrible "black oxide" adhesion treatment. It adds thickness but is a horrible conductor. Skin effect is especially awful.
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So measure resistance to get the effective low-frequency copper thickness.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
"Tom Del Rosso" wrote in news:qvm12t$umd$ snipped-for-privacy@dont-email.me:
a
No etch. Just take a reading and subtract the KNOWN mask thickness. One can assume the mask thickness better than the copper (obviously).
Another way is to get and keep handy some samples of 4oz 2oz 1oz, and maybe even a cutaway of each.
With experience, one could tell easily by how difficult the peeled strip is to bend or curl.
Or you could use a known cut strip size and use weight. Known values for each copper weight sould allow you to peel a specifically sized strip and simple weight it to tell what the copper oz weight is.
Peel up a bit and grab it with tweezers. Now pull and run a hot soldering iron on the top of the trace, just ahead of the pull point. The epoxy softens and the whole trace zips off easily.
--
John Larkin Highland Technology, Inc
The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Incidentally, John, that quote on your sig about the triumph of ignorance over reason or whatever it is, who is the author - PG Woodhouse perchance? I really should read more Woodhouse.
Wodehouse - of course! It's been far too long; over 40 years since I finished The Mating Season. His satirical observations on the English class system are matchless. The Grossmiths' Diary of a Nobody came close-ish, but.... not close enough.
Does this mean that the copper layer thickness in a via is just 0.3 oz ((1.2-1.06)*2)? I always thought it's strange that vias are so reliable, especially given that they are supposed to make contact with inner layers as well as top/bottom.
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