Materials used in Flexible PCBs

Flexible Printed Circuit Boards (PCBs) are printed circuit boards that can be bent or twisted without damaging the circuit. Like regular Rigid PCBs, flexible PCBs also consist of a conduction layer, substrate and laminate. The flexible property of these PCBs is due to the various materials used for the construction of their components. Their adaptability and efficient usage of space make these PCBs very significant in modern electronic devices.

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

Material used for Various Components

Substrate

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 a tensile strength (can not tear easily) and can withstand temperatures from -200 °C to 300 °C. It is even tolerant enough to handle multiple solder reflow cycles and temperature fluctuations. 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 and 3.48.

Polyimide film

Polyester (PET) is another common material for flexible PCBs, but it can not withstand high temperatures and therefore is an issue when soldering. It finds use mostly in low-cost applications where flexibility and heat resistance are less critical. Polyester has a dielectric constant of 2.4.

Polyamide: This material is flexible even at very low temperatures and is therefore recommended for devices used in use cases involving freezing temperatures.

Fluoropolymer (PTFE) is highly resistant to chemicals and suitable for high-frequency applications. Although it is relatively expensive, in some cases, it provides excellent performance.

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 1 GHz. 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 the most popularly used three substrates are summarized below:

Conductors

Like in other Printed Circuit Boards, the conducting layer in Flexible boards is also Copper. Ultrathin Copper foils are laminated on the substrate using various techniques in the shape and dimensions of the required electrical circuit. Typical foil thickness in flexible PCBs ranges from 12 μm to 35 μm (0.5 oz to 1 oz) and is chosen based on current carrying needs to maximize flexibility.

For flexible PCBs, Copper can be implemented depending on the usage of the flexible circuits. If the application of flexible PCBs is to reduce manufacturing time and costs by removing the cables and connectors, Copper foils are laminated onto the substrate through an electro-deposition process where a comparatively heavier layer of copper is needed to manage high-current carrying conductors. But for flexible PCBs that undergo repeated creasing or movement of the flex circuit, a higher-grade Rolled Annealed (RA) copper foil is preferred. The rolling annealing process elongates the grain structure of the copper in the planar direction. The added step of annealing the foil adds to the cost considerably, but the annealed copper can stretch more before fatigue cracking occurs and is springier.

Rolled Annealed (RA) Copper Foil

Cheaper and smaller electronics may also use flexible PCBs that have printed conductors made of carbon film or silver-based ink.

Adhesives

Adhesive materials that consist of resin are used to bind the Copper layer with the polyimide core. This is shown in the figure below depicting the structure of a flexible PCB. PI Resin, PET resin, modified epoxy resin, and acrylic resin are used to form the adhesive material. The resin, copper foils (Top and Bottom), and substrate materials i.e. polyimide or Liquid Crystal Polymer (LCP) together form the “Laminate” of the flexible PCB and is also called Flexible Copper Clad Laminate (FCCL). These layers are bonded together by applying high temperature and pressure using an automatic press under a controlled environment.

Flexible PCB Structure

Coverlay Films

Coverlay films are thin insulation layers applied to protect the outer conductors from corrosion and mechanical damage. They ensure the integrity of the circuit and a longer lifetime. Coverlay thickness is usually 25 to 50 μm and is made of a layer of polyimide or polyester film with a layer of either epoxy or acrylic-based flexible adhesive.

Coverlay in a Flexible PCB.

Finish and Coatings

An epoxy liquid called solder mask is used to coat the conductor pattern for insulation and anti-oxidation. Hot air solder leveling (HASL) or other surface finishes are also applied to exposed pads.

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