Rigid-Flex Circuit Boards: A Primer

Rigid-flex circuits have been used for the last 30 years primarily in the military and aerospace markets. Rigid-flex circuits have a distinctive advantage over conventional rigid circuits. They provide a flexible medium that delivers signals and power to areas of a system that may be around a corner or located in a tight space. Engineers and designers have the freedom to position components and other devices so that the system builder does not have to change the package configuration (a costly task).

Due to the size and packaging constraints of today's products, engineers are being forced to seek alternatives in trying to cram high tech function into less space (remember ENIAC)? Eniac was replaced by a handheld calculator. In most rigid-flex boards the circuit consists of multiple flex layers bonded together. The flexible substrate layers are usually a polyimide film with a cast adhesive, using either a modified acrylic or epoxy adhesive. The rigid portion of the board is typically bonded using an epoxy no-flow prepreg. The flexible portions consist of a base material in varying dielectric thickness .001 through .005 thousand's of an inch. Adhesive and rolled annealed copper foils are laminated onto the polyimide base on one or both sides to form the first surface that can be processed. The copper side of the base material is then coated with a photoimagible material.

The coating allows for an image to be developed and then exposed to etching process. This process is the same for rigid boards except that flex requires a certain amount of special handling. Etched layers are then laminated with a protective cover layer of adhesive backed Kapton base material that offers insulation, dielectric and oxidation protection.

Rigid core standard material is then bonded to the outer surfaces of the flex to form a sandwich that adds stability and strength to the final assembly of the complex board. The package is now ready for further processing and is treated like many standard boards. Drill, copper plating, etching. Etc... Increased real-estate on PC boards demands total utilization of the packaging surface area.

Rigid-flex gives you the technology to capture and maximize cross-coupling technologies of packaging and processing of special materials. The advantage to this is a solution of flexibility, reduction of package size, cost-effectiveness, and reliability for technology customers.