VARIOUS STIFFENER MATERIALS FOR FLEXIBLE CIRCUIT BOARDS

Flexible circuit boards are necessary in numerous applications WHERE a design requires the circuit to be bent within the electrical equipment or electronic device. However, it is not desired to have the flexible circuit board bend adjacent to connectors, mounted components, solder joints, and hole patterns. In these instances, a stiffener needs to be designed in to add rigidity and stability so that the flex circuit performs reliably.

Stiffener materials may also be used for other purposes. There may be an area in the design that requires a specified thickness to meet a connector specification when joining with other components. An area on the flexible circuit board may also need to be stiffened to constrain a bend to a specific location based on the application.

Additionally, working components in the machinery or product may rub against the circuit board and cause excessive wear and tear. Stiffener material can be used to add addition durability and abrasion resistance. Another use for a stiffener is to facilitate handling when installing it into the final assembly, or to provide a heat sink to draw excessive temperatures away from other components.

Main Stiffener Types

When designing a flexible circuit, selecting the right stiffener material is essential as it can provide a range of desired benefits based on the application. Four main stiffener materials used are FR4, Polyimide, Aluminum and Stainless Steel.

FR4

FR4, also referred to as G10, is a woven glass-fiber laminate hardboard with epoxy impregnated into the material. When used for flex circuit boards, the stiffener material typically comes unclad with no copper added. For some applications, copper pads or plated holes may be placed onto the FR4 for component mounting purposes.

FR4 can be bonded using a flex adhesive, that is thermally set, or a with a pressure sensitive adhesive (PSA). Standard thicknesses for FR4 range from 0.008" up to 0.059". When utilizing an FR4 stiffener on a circuit board with a plated-through holes, the stiffener needs to be placed on the same side of the flex as the connector or component to allow for access to the solder pads on the opposing side. On surface-mount technology (SMT) flex circuit boards, the FR4 stiffener needs be placed on the opposite side of the components.

Polyimide

Polyimide is made from a high-temperature polymer crafted into a thin film layer.  The film is available in a variety of thickness or multiple layers can be stacked to achieve a greater thickness. Stiffeners made from polyimide can vary from 0.001" up to 0.009", although flexible circuit boards typically require a thickness ranging around 0.005".

Polyimide film stiffeners are attached using a thermal set adhesive. They can be utilized with many types of components and or connectors but are specifically required for zero insertion force (ZIF) connectors. The polyimide stiffener allows the design to meet the necessary thickness level specifications of ZIF connectors.

Stainless Steel

Stainless steel is a stiffener alternative when more rigidity is required than what FR4 can provide. There may be areas on the flex circuit that requires support, yet polyimide and FR4 simply can't provide the necessary strength due to a space limitation that limits the thickness of the stiffener.

Stainless steel can provide the thinness that is required in these areas while providing added stiffness necessary. However, one thing to keep in mind is that stainless steel much more costly than other types of materials.

Aluminum

Aluminum is another alternative to the polymer or glass/epoxy stiffeners commonly used for flexible circuit boards. The main reason that aluminum is used involves applications where a lot of heat may be generated.

To move and dissipate that heat from components, aluminum is added as a stiffener as the metal acts as a heat sink. So, components and connectors can remain cool as the aluminum will draw away the heat. Just like with stainless steel, aluminum will cost more than when using either polyimide or FR4 materials.