Dielectric constant for trace parasitic calculations?

Discussion in 'General Electronics Chat' started by Shocker33, Sep 4, 2015.

  1. Shocker33

    Thread Starter New Member

    Oct 28, 2013

    I'm designing a high-frequency circuit - 0.5-1GHz and i want to calculate the parasitics that the PCB traces will have.

    In calculations, the dielectric constant, Er, is included. Eg. This online calculator

    This is the stackup of the PCB that i'm using from the manufacturer i'm using.

    The Er for FR4 is 4.2-4.8 and the Er for 2116 HR Prepreg is 3.6-3.8.

    Signal traces are on layer 1 and and ground layer is on layer 2.

    If i'm calculating trace parasitics for the outer layer, layer 1, should i use 3.6-3.8 for the Er or 4.2-4.8?

    It seems intuitive to use 3.6-3.8 since it's for the medium between the signal trace and ground layer. If it's not 3.6-3.8, why is the core FR4 of the PCB used?

  2. WBahn


    Mar 31, 2012
    Yes, use the prepreg Er. The ground place shields the signal from the lower layers.