Defining the PCB Layers

[garypang]

Hi,

Here is a question regarding the PCB Layers. If I have a 4 layers PCB, top layer with high speed (10GHz) signal trace, middle layer 1 is ground plane, middle layer 2 is power plane, and the bottom layer is other normal trace. We can define the thickness of the first layer of substrate (FR4 or Roger) by calculating the impedance of the high speed signal trace (50ohm for single ended and 100ohm for differential pairs).
How about the second and third layers of the substrate? What kind of consideration we need to make to define the thickness of the second and third substrate layers? Are there any guideline in defining the PCB stack?

Thank you and hopefully someone can help in this problem.

 

[tony_lth]

The 3rd layer is also be calculated according to the bottom trace impedance, because the power plane is the ground plane for the bottom trace.
The 2nd layer can be defined according to the requirements of the whole PCB robust, rigidness, etc.

 

[RCinFLA]

It sounds like you are using a conventional two 2 layers PCB substrates with prepreg between. Basically two double sided PCB's sandwiched together.

Connections between these two double sided boards requires a via that goes through the whole stack from bottom to top surfaces. Prepreg layer has poorer tolerance then the two double sided substrates and you should avoid using it for a transmission line unless you can accept the poorer tolerance.

Other method for 4 layer is two layer base center substrate with an HDI layer on top and bottom. HDI layer is a thin dielectric layer applied followed by a top metal plated on and fine resolution etched on surface. This allow blind via on center core with independent vias to top HDI layer. It is called a 1-2-1 HDI board.

You should contact your PCB vendor to get their specs for the board layers. Different vendor can have difference thickness and tolerances for the layers. You can request just about any standard thickness for the two base layers.