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PCB Core Laminate Thickness

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mayd85

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Hi,

I am designing a PCB for which I have to mention the stack up and do some impedance calculations. I have a basic question. When I say that I have a core with a thickness of 200 um, does this 200 um include the thickness of copper as well? I mean if i am using 35 um copper, is the cross section 35-200-35 or 35-130-35 (making it 200 um)?
 

The thickness that is specified is typically the thickness of the dielectric substrate, and does not include the outer copper clad thickness. So a 200um controlled thickness board with 35um (0.5oz) copper will be 270um thick overall. (Cross section = 35-200-35)
 
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    mayd85

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mayd85 said:
Hi,

...does this 200 um include the thickness of copper as well? ...
Click to expand...

Within a multilayer, copper sinks into the adjacent Prepreg. The core is still 200µ after pressing, but a 200µ prepreg has 130µ provided you have almoust no copper, which is true for layers with just a few routes. If you have copper planes with near 100% copper area, the prepreg will have 200µ - the difference is proportional to the percentage of copper, and this must be calculated for the layer stack, else the resulting thickness will be wrong: for a ml with 4 inner signal layers by about 140µ.

sy Reinhard
 

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