Flexible Printed Circuit

Flexible circuits are used in a multitude of applications, ranging from the lowest end consumer products to the highest end military and commercial systems. It is no coincidence that the ranges of materials used to fabricate these circuits are as diverse in performance as the range of products in which they are used.
Materials such as polyester (Mylar), and polyimide (Kapton) have been industry standards for many years. Epoxy based laminates, both glass reinforced and non-reinforced, are also being used where cost advantages can be realized. Other materials are used for specific applications where certain performance characteristics are desired.
And other materials we can support are Adhesiveless Materials, Flame Retardant, Laminates and Coverlay, Other Materials Upon Request. formulations.

Single Side Flexible Circuits

Single-sided flexible circuits have a single conductor layer made of either a metal or conductive polymer on a flexible dielectric layer. Component termination features are accessible only from one side. Holes may be formed in the base film to allow component leads to pass through for interconnection, normally by soldering.

Double Side Flexible Circuits

Double-sided flex circuits are flex circuits having two conductor layers. Theses flex circuits can be fabricated with or without plated through holes, though the plated through hole variation is much more common. When constructed without plated through holes and connection features are accessed from one side only. It is not a common practice but it is an option. Because of the plated through hole, terminations for electronic components are provided for on both sides of the circuit, thus allowing components to be placed on either side.
Depending on design requirements, double-sided flex circuits can be fabricated with protective coverlayers on one, both or neither side of the completed circuit but are most commonly produced with the protective layer on both sides. One major advantage of this type of substrate is that it allows crossover connections to be made very easy. Many single sided circuits are built on a double sided substrate just because they have one of two crossover connections.
An example of this use is the circuit connecting a mousepad to the motherboard of a laptop. All connections on that circuit are located on only one side of the substrate, except a very small crossover connection which uses the second side of the substrate.

Mutilayer Flexible Circuits

Flex circuits having three or more layers of conductors are known as multilayer flex circuits. Commonly the layers are interconnected by means of plated through holes, though this is not a requirement of the definition for it is possible to provide openings to access lower circuit level features.
The layers of the multilayer flex circuit may or may not be continuously laminated together throughout the construction with the obvious exception of the areas occupied by plated through-holes. The practice of discontinuous lamination is common in cases where maximum flexibility is required. This is accomplished by leaving unbonded the areas where flexing or bending is to occur.(http://www.jesen.com.cn)