Flexible metal-clad dielectrics are typically used as wiring for various electronic devices including movable parts, such as hard disc drive, floppy disc drive, printer head and the like. It is necessary for the flexible metal-clad dielectrics to have a satisfactory bending characteristic, because a poor bending characteristic tends to cause failure of wiring of the movable parts, thereby giving rise to fatal damage to the main body of the device. Moreover, in view of future devices having a miniaturized dimension and a higher operational speed, there is a demand for achieving an improved reliability in terms of the bending characteristic.
As a base member of such flexible printed circuit board, there are known two-layer type flexible metal-clad dielectric and three-layer type flexible metal-clad dielectric. The two-layer type is comprised of a resin film and a copper foil as a circuit conductor, which are directly laminated one above the other, and is relatively costly due to complicated manufacturing steps, though it is advantageous in terms of enriched bending characteristic. In contrast, the three-layer type is comprised of a copper foil which is bonded onto a resin film with an adhesive material, and has somewhat poor bending characteristic in comparison to the two-layer type, though it is advantageous in terms of low cost.
There have been various proposals for satisfying the demand for an improved bending characteristic, such as use of a conductive material for the circuit board in the form of a rolled copper foil having a satisfactory fatigue-resistance characteristic, instead of conventional electrolytic copper foil, reduction in thickness of copper foil, base film layer and adhesive material, and refinement in the adhesive material.
However, even with a flexible substrate having the best bending characteristic, the durable bending cycle is only about 1.times.10.sup.7 times for the two-layer type and about 2-7.times.10.sup.6 times for the three-layer type, both at a bending radius of 2 mm. Such a bending characteristic is still insufficient for the desired miniaturization and/or increased operation speed of future devices.