To understand the difference between PCB core & prepreg material, it is important to understand what each of these are:
What is prepreg material?
Simply put, prepreg is a conductive substance that is used to offer the appropriate insulation between the copper and the core of the PCB. It is a dielectric material that is sandwiched between two cores of between a core and a copper foil. It is often referred to as the binding material as it binds two cores or a core and a copper foil.
As its name indicates, prepreg is a glass fiber impregnated with a resin bonding agent. All prepegs are B-stage materials. There are different types of prepregs depending on their thickness and the resin content that they hold. Prepregs are broadly available in 3 formats depending on their resin content:
- Standard Resin
- Medium Resin
- High Resin
A typical prepreg preparation process involves:
- Reinforcing with resins
- Removal of excess resins
- Preparation of curing with high and low-temperature settings
Why is it important?
Prepreg is an essential component of the PCB manufacturing process on account of the following:
- It is easy to apply and work with. It is easy to getting accustomed to and does not require too much technical expertise.
- It is not messy as opposed to wet style layups. The waste management during the manufacturing process is therefore, efficient.
- There is a wide variety of prepreg available to suit different requirements.
- It helps in reducing the weight of the device and therefore is particularly helpful in a milieu where miniaturization of devices has become a norm.
What is PCB core material?
The core of the PCB is a rigid base material, which is laminated with copper. It is used in manufacturing single or double-sided boards as also multilayer PCBs. A multilayer PCB has the following components:
- Copper foil
- Prepreg materials
- Copper foil
- Core materials
- Copper foil
- Prepreg materials
- Copper foil
The difference between PCB core vs prepreg:
Oftentimes on account of the similarity between prepreg and core, there is a confusion between the two. Simply put, the core is a product of the prepreg and lamination. It is therefore far more rigid compared to the prepreg. The core is made up of a fiberglass-epoxy laminate which is Flame Retardant and has copper on either side.
The prepreg, on the other hand is partially dried without lamination and isn’t as rigid as the core.
Another important difference between the PCB prepreg and the core is its dielectric constant. While the dielectric constant of the core does not vary, that of the prepeg varies before & after lamination. The different dielectric constants are also a function of the following:
- Resin content
- Type of resin
- Glass weave
This can become an issue where you need precise impedance matching. Also with not all prepreg and core materials being compatible with each other, and core/prepreg stacks with different doelectric constants, it is hard to predict the exact dielectric constants as well as losses in an interconnect.
The thickness of the PCB is a function of the prepregs used. In fact the required thickness can be achieved by a combination of prepreg plies i.e thin sheets of prepregs placed one over the other.
At high voltage, irrespective of the PCB core or prepreg material used, creepage as well as leakage currents are an issue. Creepage specifications for FR4 materials are required for electromigration of copper and growth of conductive filaments. On account of this issue and also with a view to increase glass transition as well as decomposition temperatures, there has been a move towards non-dicyandiamide resins.
To Sum Up
Both the core and the prepreg are essential components of the PCB. While the core is a base PCB material, a multilayer PCB is held together by the prepreg. The difference between the two lies in the fact that the core is cured, while prepreg materials are malleable which means it can be easily formed into sheets. When subject to high temperatures the material melts, helping to fuse the layers.
With over 40 years of combined experience in rendering high quality and standard PCB assembly and manufacturing services, we are not just an assembler, we can completely test at board level to box build level to provide the benefit of unparalleled technical expertise and the most progressive solutions at competitive prices. At Technotronix, we have adopted advanced PCB production techniques & manufacturing practices with specialization in a varied range of PCB fabrication processes, producing high-quality multi-layered PCBs from tailor-made layouts
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