copper thickness test

Measuring thickness is easy. Here's an example for a thin aluminum film:

The resistance of a pure aluminum film 26 nanometers thick is

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

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.

Resistivity

Aluminum : 2.65e-8 Copper : 1.68e-8

The following is the formula for cpw (in oz) to thickness (in mils) conversion:

Thickness(in oz) = thickness (in mils) /1.37

The following is the formula for thickness (in mils) to cpw (in oz) conversion:

t (in mils) = t (in oz) * 1.37

Weight Thickness 1/2 Oz. .7 mils 1 Oz. 1.4 mils 2 Oz. 2.8 mils

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.

So measure resistance to get the effective low-frequency copper thickness.

What if you peel some off and wash it in solvent to remove the mask from the top and the glue from the bottom?

But since the real goal is to get the resistance you want, measuring that is more direct than direct measurement of the thickness.

If you etch it away, the glue could partly dissolve away too. And a micrometer might compress the glue as well.

"Tom Del Rosso" wrote in news:qvm12t$umd$ snipped-for-privacy@dont-email.me:

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.

Heat it with a reducing flame (ie one with excess fuel) the resin will let go long before the copper melts

The redusicng flame will prevent the copper from oxidising.

From JLPCB, you may have just inherited the weight of other jobs, processed at the same time.

If you'd opted for ROHS, they'd have back-ordered you to the

RL

I'm oblig'd.

Why do the Chinese proto board houses have such insane default stackups?

I doubt the Chinese proto houses are doing something that isn't common practice

and there are numerous app notes, saying that keeping the signal layer close

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.

I'm oblig'd. :)

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.

Instinct over Reason!

That was Dorothy Sayers, in one of the Lord Peter mysteries. She was really smart. She invented the Guinness toucan.

Wodehouse was an amazing writer. A Damsel In Distress is astonishing, especially the last three chapters.

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.

It really is remarkable that within 120s of reading that, it was downloaded onto my machine ready to be copied to my kindle.

Back in the good old days it would have taken a week or so to get it from the public library.

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.