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I have created Gerber files for one of my application circuit. While giving it for development, manufacturer is asking for thickness in microns.

My question is why thickness is given expressed in microns, not in mils/mm?

And what is start and finish copper thickness in PCB layout.

Thanks.

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    \$\begingroup\$ Are you sure he's asking for PCB thickness and not copper thickness? I my part of Europe, it's very common to express PCB thickness in milimeters and copper thickness in microns. \$\endgroup\$ – AndrejaKo Jun 13 '15 at 7:13
  • \$\begingroup\$ It's worth asking what his common practice is, and adopting that unless you have specific requirements (high current for example) \$\endgroup\$ – Brian Drummond Jun 13 '15 at 10:34
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He is probably referring to copper layer thickness. He will probably start with FR4 plated by 18um Cu at both sides (start thickness) and eventually amends it with electroplating to 35 um (finish thickness), as he creates vias etc.

You probably need to specify the finish thickness only (depends on current density in the traces, 35um or 70um are the most common) and the manufacturer will then select the material that best suits his technology.

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    \$\begingroup\$ ... and in case you're wondering about the "odd" numbers, copper thickness was traditionally specified in terms of "ounces of copper per square foot". 1 oz/sf corresponds to a thickness of 35 um. \$\endgroup\$ – Dave Tweed Jun 13 '15 at 13:04

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