What is Prepreg in a PCB?

What is Prepreg? What is its purpose?

PCB Fabrication 
1 Answer
Can you answer this question?

Editorial Team - PCB Directory

Jul 20, 2022

Prepreg is an insulating layer within the PCB that helps bind multiple layers of a PCB. It can bind two cores or a core and a copper layer in a mult-layer printed circuit board. The Prepreg is an insulation layer that is made by combining glass fiber and a resin bonding agent. The name 'prepreg' comes from pre-impregnated. In this case, Glass fiber is impregnated with a resin bonding agent to create prepreg materials used in PCBs.

The thickness of a Prepreg material depends on the amount of resin content bound with the glass fiber. Based on the amount of resin content, prepregs can be divided in to three types - Standard Resin (SR), Medium Resin (MR) and High Resin (HR). High resin results in thicker prepreg materials that are more expensive.

The thickness and resin content in a prepreg regulates the impedance, dielectric constant, Coefficient of Thermal Expansion (CTE), and drilling / etching quality.

In a PCB, the Prepreg is sandwiched between two copper layers (top & bottom) and is then stacked, heated, and pressed together to form a laminate. Laminates are known as the core of a PCB which is used to bind, strengthen and protect the layers of a printed circuit board.

Prepregs are made of  Resin (epoxy or polyimide), and glass Fabric. The glass fibers are subjected to a weaving mechanism where they are interwoven to form a glass fabric. This glass fabric is then impregnated with resin in a special machine known as the prepreg treaters under high temperature and pressure to create prepreg sheets.  

Prepregs provide excellent control over laminate thickness, and good environmental and corrosion resistance as well. The resin content in a prepreg sheet is kept uniform throughout.

However, preparing prepregs are expensive. Prepregs in their raw form have a shelf life of around six months at room temperature (75°F), however, the shelf life can be extended by a month or two by freezing the materials or keeping them at low temperatures.


Web Analytics