The Basic Elements of a Flexible Printed Circuit

A flexible printed circuit (FPC connector), also known as flexible circuit, flexible PCB, flex circuit, and flexi-circuit, is a type of connector that is used for electronic equipment. Some applications include communication devices, personal computer components, home entertainment, and information devices, to name a few.

 

Paul Eisler, an engineer, inventor, and scientist born in Austria, is generally considered to be the inventor of the flexible printed circuit board. It was in 1936 when he created circuit patterns with the use of conductive epoxy.

 

In 1943, Isler made the first operational printed wiring board, which was used to replace bulky radio tube wiring. His patent application was eventually split into three separate patents, namely: Three-Dimensional Printed Circuits, Foil Technique of Printed Circuits, and Powder Printing. All of these were published in June of 1950.

 

Because of his invention, Isler received the Nuffield Silver Medal from the Institute of Electrical Engineers, and the Pur Le Merite distinction from the French Government. Up to this day, his invention is being used to connect billions of devices all over the world.

 

By structure, a FPC connector is comprised of a thin polymer film that contains circuit patterns with a thin polymer coating. This coating protects the circuits of the conductor.

 

There are three basic elements of an FPC. Each element must function properly in order to guarantee ease of manufacture and connection reliability. The three basic elements are the following:

 

Base material. From its name, the base material provides the base foundation for the laminate. It is a flexible polymer film, which provides primary physical and electrical properties of the flexible circuit.

 

While base material thickness generally ranges from ½ mil to 5 mils, using thicker or thinner materials is also possible.

 

As a general rule, thinner materials are more flexible, while thicker materials are stiffer. Some of the materials that are used as base films are polyester, polyimide, polyethylene naphthalate, polyetherimide, floropolymers, and copolymers.

 

Bonding adhesive. This element is the bonding medium. Thus, it is crucial in creating a laminate. As in the base material, bonding adhesives come in different thickness.

 

Electrically conductive adhesives come in two types: isotropic and anisotropic. An isotropic conductive adhesive (ICA) is electrically conductive in all directions and is used in chip contacting. An anisotropic conductive adhesive (ACA), on the other hand, contains special particles that conduct electricity only in one direction.

 

Metal foil. The conductive element of a flexible laminate, this is the material where the circuit paths are etched.

 

Because copper is cost-effective, it is more generally used than other metals. Copper comes in two different categories: electrodeposited and wrought, each on with four different sub-types. To improve its adhesive quality, a thin layer of treatment is applied on one side of the foil.

 

While copper is the most widely used metal in a FPC connector, other metals may also be used, such as copper alloy or aluminum.

 

Aluminum carries almost the same conductivity and carrying capability as copper. It is also highly pliable, which makes it an excellent choice for flexibility requirements. To add to these, aluminum is also less corrosive, making it the more durable, reliable, and cost-effective option in the long run. It is also available in a wide range of thickness.

 

While these properties make it a popular alternative to copper, aluminum also presents limitations. It is not ideal for use in applications that require component assembly, multilayer configuration, or high density.

 

In addition to this, long-term compatibility with the application environment should also be considered, as well as metal corrosion in the long run.