I only have experience with low-speed boards that are 4 layers at most. I want to get into full-speed USB 2.0 and 100Mb/s ethernet, so I need some impedance controlled traces. Instead of doing something professional first, I want to put together a hobby board that does these speeds.

Advanced circuits, the PCB house that I use, has this pdf about stackup thickness. I've spent about an hour googling, but I haven't come up with clear reputable results to the following questions related to stackup thickness:

  • What is pre-preg? What is its purpose in manufacturing? I always thought that we just glued copper to an FR-4 sheet and then photoexposed it.
  • Why does pre-preg thickness change depending on how much of the pcb is filled up?
  • If I order an "impedance controlled" board, will the pre-preg thickness be better controlled? How is this process different than "regular" boards?
  • Is there a textbook or nice, long app note that talks about all the nitty-gritty of pcb manufacture?
  • 1
    \$\begingroup\$ Core thickness is well controlled. Prepreg thickness is less well controlled. Prepreg is used to glue pairs of cored layers together, and to glue foil to other layers. At 100MHz, you shouldn't have too much of a problem. \$\endgroup\$
    – Neil_UK
    Jul 16, 2017 at 16:04
  • \$\begingroup\$ I figure I wouldn't have an issue at these speeds, but I'm still curious. If I go on to design a USB 3.0 board or something professionally, I want to make sure I understand what's going on. \$\endgroup\$
    – John M
    Jul 16, 2017 at 20:43
  • \$\begingroup\$ possibly related application note: Multilayer PCB Stackup Planning \$\endgroup\$ Jul 17, 2017 at 0:14

1 Answer 1


"Pre-preg" is a sheet of FR4 which is pre-impregnated with uncured epoxy resin. So it is flexible and somewhat plastic in nature.

Pre-preg is used in between pieces of "core". The core is, as you say, made with copper on both sides. The copper is then etched. Basically, core is like a two-sided PCB. Once all the layers are stacked together, the epoxy and glass fiber of the pre-preg fill the gaps between the copper areas in the core. Because of its plastic nature, the pre-preg oozes into the gaps. This is why the finished thickness of the pre-preg depends somewhat on the fill factor of the copper layers adjacent to it.

On a controlled impedance board, the manufacturer will manipulate several variables to achieve the desired impedance. This may even include slight changes to the trace width. I do not believe the control of pre-preg thickness is any different in a controlled impedance board. Controlled impedance is not magic. The layout still needs to be designed to achieve the target impedance. The manufacturer will just guarantee that it ends up within a certain range.

  • \$\begingroup\$ Thanks! Can you edit your answer to include a response to my bullet-point asking for a textbook ;) \$\endgroup\$
    – John M
    Jul 16, 2017 at 20:31
  • 1
    \$\begingroup\$ I don't know of a textbook. Whatever I know I picked up here and there. \$\endgroup\$
    – user57037
    Jul 16, 2017 at 20:56

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