Contents
What is Prepreg Material?
Figure 1: Toroidal Inductors on a PCB
A prepreg is a conducting material that separates the PCB’s copper layer from its core layers or, in some cases, the dual core. But, the PCB core can be bonded to the copper sheet in most cases.
Thus, the prepreg will act as the binding material in multilayer boards that also feature special insulation properties.
It’s also possible to convert the FR4 Prepregs from a non-conducting dielectric material to a conductor via chemical additives.
What is PCB Core Material?
A PCB with no components mounted.
As the name suggests, it is the basic PCB manufacturing material with a copper foil coating. Furthermore, the circuit board cores are essential in single-sided and multi-structure PCB fabrication.
Primarily, the core constituents include FR4 substances such as glass fiber cloth with titanium epoxy laminates & copper traces. The core and prepreg are the key structural components of circuit boards.
The Difference Between Prepreg and PCB Core
Figure 3: A PCB without components
The table below lists the main differences between prepreg and PCB cores and the respective material properties of each.
The Effective Dielectric Constant of Core and Prepreg
Figure 4: Several Electronic components on a PCB
Most of the datasheet dialectic constants of the prepreg and its actual physical properties rely on the following:
- Experimental conditions/ Experimental design
- Structural conditions
- Structural adhesives
- Routing geometry
- Type of adhesive used
- Test method
- Salt spray conditions
- Material thickness
- Symmetry conditions
Thus, the datasheet value may not match the actual value as it is primarily a factor of the experimental design.
Also, the mechanical properties of the Honeycomb core or prepreg depend on the copper roughness and weave pattern, among other manufacturing parameters. The equation below gives the expression of finding the copper roughness of a prepreg:
The differences in the effective dielectric constant and the response in terms of the PCB load behavior are accurately accounted for by the equation. Thus, in a nutshell, the key determinant of the effective dielectric constant is your experimental model.
Applications
The two are primarily useful in the following two applications:
- Complex PCBs
- Multilayered circuit boards
Conclusion
In a nutshell, the prepreg and the core are complimentary composite structures in the PCB manufacturing process. That’s all for today but reach out to us if you have queries on the two components and the material properties.
