Stiffener is used to rigidize the selective areas of FPC in order to enable electronic components insertion or mounting. Other usage including thickness adjustment to suit ZIF (Zero Insertion Force) connectors.

POLYESTER
Polyester is a relatively inexpensive material compared with Polyimide. With a upper operating temperature limit of 80¡ãC and a maximum short temperature duration of 150¡ãC, this material has good electrical and mechanical properties with low thermal expansion rate at low and normal temperatures.
Upper heat resistance limitation of 150¡ãC generally restricts its use to applications where mechanical connections are made for soldered joints. Soldering can cause delamination, and requires a high degree of skill with the soldering iron. Direct contact with the heated iron will shrink or melt the material. Apart from its low cost, the greatest advantage of polyester is in common with other thermo plastic materials.
Polyester is not a suitable material for multilayer flex and reflow process but they are fit for antenna coil of smart card application.

POLYIMIDE
(RAW MATERIAL FOR COVERLAY, BASED MATERIAL & POLYIMIDE STIFFENER)
The combination of polyimide film and a high temperature thermo-setting resin adhesive produces a flexible system having excellent mechanical and electrical properties and is substantially unaffected by temperature change. Heat applied during manufacture and subsequent reflow or soldering processes tends to increase the adherence rather than weakening the conductors.
    Although the adhesives used do not have the same temperature properties as the polyimide film itself, the construction offers a continuous operating temperature of 150C and a maximum short duration temperature of 300C Subject to material type). Polyimide has a low thermal expansion (0.02-0.04mmC-1m-1)and very high tensile strength (1.69MNm-2) and is compatible with conventional epoxy-glass laminates. Thus, polyimide is applicable for rigid/flexible combinations and to multilayer applications using both plated through hole and clearance layer techniques.
    Polyimide thermoset is readily hand, dip or wave soldered and it can be unsoldered and resoldered many times. Before wave or dip soldering it is desirable to oven bake for one hour at 80Celsius to 110Celsius. The most commonly used thicknesses are 12.5 microns(0.0005") and 25 microns (0.001") and 50 microns (0.002"); other thicknesses are available.
This material can be used in the majority of applications, is obtainable in a flame retardant grade and is highly recommended.