Flexible PCB Materials and Construction

What materials are used to fabricate a flexible printed circuit board?

PCB Basics PCB Fabrication 
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Editorial Team - PCB Directory

Jul 9, 2022

Flexible PCBs are boards that can be bent or twisted without damaging the circuit. Like regular Rigid PCBs, Flexible printed circuit boards consist of a conduction layer, substrate and laminate. They most often use copper for the conductive layer and Polymide for the substrate. There are other substrate options that include clear polyester and liquid crystal polymers. The substrate is responsible for the flexibility and tensile strength of the board.

Note: Click here to see a list of companies who fabricate flexible PCBs.

Types of Substrate Materials for Flexible PCBs

Polyimide is most often used as the substrate of a flexible PCB. It is easily available and cost-effective compared to other substrates that can be used in flexible boards. It has an excellent tensile strength (can not tear easily) and can withstand temperatures from -200C to 300C. However, it has a high rate of moisture absorption which can lead to irreversible degradation of polymer structure in high moisture environments. The dielectric constant of polyimide ranges between 2.78 – 3.48.

Polyester (PET) is another common material for flexible PCBs, but it can not withstand high temperatures and therefore is an issue when soldering. Due to this, its application as a substrate in Flexible PCBs is limited. Polyester has a dielectric constant of 2.4.

Liquid Crystal Polymer (LCP) is another suitable alternative to polyimide as a substrate for flexible PCBs. This material has all the properties of polyimides with greater resistance to moisture and humidity. Its resistance toward humidity and moisture is 0.04% and has a dielectric constant of 2.85 at 1GHz. This makes it highly functional in High-speed digital circuits and has a very high frequency. However, it is more expensive and difficult to procure compared to polyimide.

The key differences between these three substrates are summarized below:

Properties

Polyimide (PI)

Polyester (PET)

Liquid Crystal Polymer (LCP).

Temperature Resistance

High (-200C to 300C)

Low

High

Cost

Inexpensive

Inexpensive

Expensive

Dielectric Constants

2.4

2.4

2.85

Melting Point

250C to 300C

180C to 250C

320C

Moisture Resistance

Low

Low

High

Tensile Strength

High

High

High

Conducting Materials in Flexible PCBs

Like in other Printed Circuit Boards, the conducting layer in Flexible boards is also Copper. The PCB tracks, pads, holes, and other PCBs are made using Copper. For flexible PCBs, Copper can be implemented depending on the usage of the flexible circuits. If the application of the flexible PCBs is to reduce manufacturing time and costs by removing the cables and connectors, then a laminated copper foil can be used to optimize conductivity. 

There might also be a case where the flexible circuits are subjected to repeated creasing or movement. To sustain the frequent wear & tear caused by these activities, a high-grade rolled annealed (RA) copper foil is used. This process increases the overall cost considerably but makes the copper layer more flexible and springier in the Z deflection plane. This is used when a flex circuit is often bent and rolled,

Rolled Annealed (RA) Copper Foil

 How are these materials Bonded Together?

The structure of a flexible PCB can be seen in the figure below. The adhesive materials consist of Resin which is used to bind the Copper layer with the polyimide core. PI Resin, PET resin, modified epoxy resin, and acrylic resin can be used to form the adhesive material. The resin, copper foils (Top and Bottom), and substrate materials i.e polyimide or Liquid Crystal Polymer (LCP)  form the “Laminate” of the flexible PCB.

These layers are bonded together by applying high temperature and pressure using an automatic press under a controlled environment. The laminate is called Flexible Copper Clad laminate (FCCL). This can be used to develop different types of flexible PCBs depending on the required application.

The flex PCB circuits are coated with a Coverlay film that encapsulates the external circuitry of the flex PCBs. Coverlay is a sheet made from a material comprised of a layer of polyimide and a layer of either epoxy or acrylic-based flexible adhesive.

Diagram

Description automatically generated

Coverlay in a Flexible PCB. Source: Altium

Click here to see a list of companies that fabricate flexible PCBs.

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