Flexural and reliability of flexible pcbs

At present, there are four kinds of flexible circuits(FPC): single-sided, double-sided, multi-layer flexible pcb and rigid flexible pcb.

(1)Single-side flexible board is the lowest cost, when the electrical performance requirements are not high printed board. When wiring on one side, one side flexible pcb should be used. It has a chemically etched conductive pattern layer, and the conductive pattern layer on the surface of the flexible insulating substrate is calendered copper foil. The insulating substrate may be polyimide, polyethylene terephthalate, arylamide fiber ester and polyvinyl chloride.

(2)Double-sided flexible pcb is a conductive pattern etched on both sides of the insulating base film. Metallized holes connect the two sides of the insulating material to form a conductive path to meet the flexural design and use function. The covering film can protect the single and double-sided wire and indicate the position of the element.

(3)Multi-layer flexible pcb is the single or double sided flexible circuit of 3 layers or more layers laminated together, through drilling, electroplating to form a metallized hole, forming a conductive path between different layers. In this way, there is no need to use complex welding process. Multi-layer circuits have great functional differences in terms of higher reliability, better thermal conductivity and more convenient assembly performance. The interaction of assembly size, number of layers and flexibility should be taken into account when designing layout.

(4)The traditional rigid flexible pcb is composed of rigid and flexible substrate pressed together in selective strata. The structure is tight and metallized to form conductive connection. If a printed board has components on both sides, rigid flexible board is a good choice. But if all the components are on one side, it is more economical to use a double-sided flexible pcb and layer a FR4 reinforced material on the back

(5)The flexible circuit of hybrid structure is a kind of multi-layer pcb, the conductive layer is composed of different metals. An 8-ply board uses FR-4 as the medium for the inner layer and polyimide as the medium for the outer layer, with leads extending from three different directions of the motherboard, each made of a different metal. Copper alloy, copper and gold are used as independent leads. This kind of hybrid structure is the only feasible solution when the relationship between electric signal conversion and heat conversion and electrical properties are harsh.