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How much variability is there in the thickness of a standard PCB?

Let's say I order a 1.6mm FR-4 board. Will the actual thickness range 1.5mm-1.7mm? 1.4mm-1.8mm? 1.59mm-1.61mm? What is the (approximate) value of x in "1.6mm ± x%"?

What if the board is ordered thinner? (say 0.8mm)

How does controlled impedance affect this range? It's supposed to specify that the dielectric thickness must be controlled, but how much does it actually reduce the variance?

I'm assuming this all varies somewhat by manufacturer, I'm just looking for some reasonable guidelines for what to expect.

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The board thickness tolerance depends on how good your fabrication house is. 10% is a typical value.

As far as controlled impedances, for professional work the actual impedance is specified in order to give flexibility to the fab house to tweak the trace width to achieve the desired impedance. Good fab houses will do a good job on that. Depending on the geometry involved impedance tolerance of +/-10% is typical. Typically the fab house will actually measure the impedance on a special coupon built on the same panel as the PCB or on the actual trace of interest using TDR in order to make sure they met the specification.

For cheap hobbyist PCBs you're left to you own luck. In practice, the impedance may not be that all off, as long as the trace is not super narrow (the narrower the trace, the higher the tolerance).

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  • \$\begingroup\$ Interesting. And they would be able to deal with modifying trace widths even when that would affect other components, e.g. a -3db microstrip coupler to the transmission line? Would I have to specify the target performance of these kinds of stripline devices in order that they would be handled correctly? Thanks for the quick answer by the way! \$\endgroup\$ – T. K. Jun 22 at 17:03
  • \$\begingroup\$ Typically you provide them a table specifying which traces widths on which layer are controlled impedance so that they would only be allowed to tweak those. \$\endgroup\$ – joribama Jun 22 at 17:31
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    \$\begingroup\$ For a coupler, you'd want to do some tolerance analysis as part of your design and choose materials with variability in mind. The fab house isn't going to deal with the fact that adjusting the trace width also changes the trace separation in the coupled section. They're going to target characteristic impedance and nothing else. \$\endgroup\$ – The Photon Jun 22 at 19:26

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