A flexible printed circuit actually is making use of flexible laminate. The raw materials and properties of the laminate are crucial not only to its manufacturing process but also to the performance of the finished circuit. The flexible laminate consists of a conducting foil and also the dielectric substrates.
The dielectric substances are of two types which are used for flexible printed circuits:
A) Thermoplastics: The materials in which, after curing, will soften by heat input, such as polymers, polyester, fluorinated hydrocarbon, etc.
B) Thermosetting Plastics: Materials such as polyimide, polyacrylate, etc.
Now, let us review the copper material, copper as the conductor is commonly used and come in foil form, while almost all flexible PCB is built on polyimide or polyester film. For some of the special purposes, aramid and fluorocarbon films are also used.
The choice of selection of a particular film depends upon a number of factors. These are listed below.
– Cost-down or cheaper flex circuits are built on polyester films that provide performance at a lower cost, but with reduction on thermal resistance.
– Super High performance flex PCB, especially those for military applications, are manufactured with polyimide films because they offer the best overall performance.
– Aramid non-woven fiber material is not costly and also has a good mechanical and electrical property, but the problem is this material will absorb moisture.
– Fluorocarbons, is an expensive material and also very difficult to handle, however it offers the best dielectric properties. This material mostly use for controlled impedance applications.
As far as application wise, the common material choice of flexible circuits is polyimide film. This is due to the average performance and cost factor of chemical characteristics, electrical and thermal performance. This material can even withstand the temperatures in manufacturing soldering operations. The material is also used in wire insulation and as insulation in motors and transformers.