FPC (Flexible Printed Circuit)

Flexible circuit materials
Because there are only a few elements involved construction of a substrate for flexible circuits, none of them is lacking in importance. Each element of the flex circuit construction must be able to consistently meet the demands placed upon it for the life of the product. In addition, the material must play reliably in concert with the other material elements of the flexible circuit construction to assure ease of manufacture and reliability. Following are brief descriptions of the basic elements of flex circuit construction and their functions.


Base material
The base material is the flexible polymer film which provides the foundation for the laminate. Under normal circumstances, the flex circuit base material provides most primary physical and electrical properties of the flexible circuit. In the case of adhesiveless circuit constructions, the base material provides all of the characteristic properties. While a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimension from 12 µm to 125 µm (1/2 mil to 5 mils) but thinner and thicker material are possible. Thinner materials are of course more flexible and for most material, stiffness increase is proportional to the cube of thickness. Thus for example, means that if the thickness is doubled, the material becomes eight times stiffer and will only deflect 1/8 as much under the same load. There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene napthalate (PEN), Polyetherimide (PEI), along with various fluropolymers (FEP) and copolymers Polyimide films are most prevalent owing to their blend of advantageous properties electrical, mechanical, chemical and thermal.


Bonding adhesive
Adhesives are used as the bonding medium for creating a laminate. When it comes to temperature resistance, the adhesive is also typically is the performance limiting element of a laminate especially when polyimide is the base material. Because of the earlier difficulties associated with polyimide adhesives, many polyimide flex circuits presently employ adhesive systems of different polymer families. However some newer thermoplastic polyimide adhesives are making important in-roads. As with the base films, adhesives come in different thickness. Thickness selection is typically a function of the application. For example, different adhesive thickness is commonly used in the creation of cover layers in order to meet the fill demands of different copper foil thickness which may be encountered.

Metal foil
A metal foil is most commonly used as the conductive element of a flexible laminate. The metal foil is the material from which the circuit paths are normally etched. A wide variety of metal foils of varying thickness are available from which to choose and create a flex circuit, however copper foils, serve the vast majority of all flexible circuit applications. Copper’s excellent balance of cost and physical and electrical performance attributes make it an excellent choice. There are actually many different types of copper foil. The identifies eight different types of copper foil for printed circuits divided into two much broader categories, electrodeposited and wrought, each having four sub-types.) As a result, there are a number of different types of copper foil available for flex circuit applications to serve the varied purposes of different end products. With most copper foil, a thin surface treatment is commonly applied to one side of the foil to improve its adhesion to the base film. Copper foils are of two basic types: wrought (rolled) and electrodeposited and their properties are quite different. Rolled and annealed foils are the most common choice, however thinner films which are electroplated are becoming increasingly popular.
In certain non standard cases, the circuit manufacturer may be called upon to create a specialty laminate by using a specified alternative metal foil, such as a special copper alloy or other metal foil in the construction. This is accomplished by laminating the foil to a base film with or without an adhesive depending on the nature and properties of the base film.


Manufacturing processes
The processing of flexible circuits varies with the type of structure desired. The following series of flow charts provide a general idea of the processes required for common structures.

Flexible circuit industry standards and specifications
Specifications are developed to provide a common ground of understanding of what a product should look like and how it should perform. Most standards are either developed directly by the industry which produces the product specified (as is the case with IPC specifications) or they are developed by an end user with significant industry input (such as with military specifications).Following is a listing of key standards and specifications relevant to the manufacture and use of flexible printed wiring and the raw materials used in their construction. The documents are briefly summarized here for quick referral purposes and reference to content only. These documents should be kept at hand for reference when designing, inspecting or testing flexible PCBs. There are many other documents other than those listed here which are commonly referred to in the manufacture of flex circuits but which have general interest level and much broader in scope. These documents are normally referenced in the documents about to be referenced here and for the sake of brevity, they will not all be listed here.

Primary industry specifications
The IPC has been the primary progenitor of standards and specifications for the electronic interconnection industry. Recently the IPC has been restructuring of its specifications and documents in an effort to make them more compatible with other industry standards. As a result new standards are being generated that will be replacing some of the older standards. However, since the older standards are likely to remain on blueprints and in other relevant documentation for some time to come it is worth knowing both the new and the old document numbers. As a result, the various flex circuit related documents are described here.