PCB Stacking Structure & Four Key Parameters of Materials

April 20, 2025

PCB Stacking Structure:A PCB is typically made from a prepreg layer and a core substrate. Below is a typical cross-sectional view of a 6-layer PCB.

1.Prepreg (PP): The full name is "prepreg," which refers to a semi-cured material (usually FR4: fiberglass-epoxy resin). When heated or pressed, the prepreg dissolves into the epoxy resin adhesive. Material-wise, it consists of fiberglass cloth impregnated with resin, generated through a series of physical and chemical reactions. There are four important parameters to understand: Dk, Tg, Caf, and Df.
Dk (Dielectric Constant): This parameter is frequently used in impedance calculations. A higher dielectric constant results in a lower impedance, following an inverse relationship.
Tg (Glass Transition Temperature): This is the temperature at which the PCB material transitions from a glassy state to a more elastic state. A higher Tg indicates greater stability of the board. For instance, our company selects a higher Tg value for automotive electronics.
Caf (Resistance to Ion Migration): When PCB routing is dense and the voltage is high, we need to pay attention to insulation calculations.
Df (Dissipation Factor): This factor reflects the material's response to an electric field and the extent of energy loss, closely related to signal transmission quality. A lower Df value indicates better material response to electric fields, reducing signal attenuation, delay, and distortion, thereby improving transmission quality.

2.Core: A cured substrate (usually FR4: fiberglass and epoxy resin). Both surfaces of the core are laminated with copper foil, which can serve as signal layers, power layers, or ground layers. The solid material filling the space between the upper and lower surfaces can be understood as prepreg with copper attached on both sides.