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In Eurocircuits I can select between 12um (end + -30um) to 105um (end + -130um), so in the worst case if I choose 12um can I have a thickness of 12-30 = -18um?

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I believe they're using "+/-" to mean "approximately" here. I've seen some people use it in that manner before, though it's always struck me as a bit odd since "~" is less ambiguous.

The way a PCB is manufactured starts with a copper foil of some thickness on an FR-4 substrate, and then the thickness is increased by plating additional copper onto it.

It seems to me that what they're saying by "12 μm (end +/-30 μm)" is that the copper foil initially on the FR-4 is 12 μm thick, and by the end of the plating process, it will be approximately 30 μm thick.

You'd have to ask them to be sure, of course. I expect they'd be happy to tell you; they do want your business after all.

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