Currently, flexible circuit boards are mainly classified into four types: single-sided, double-sided, multilayer, and rigid-flex. ① Single-sided flexible boards are the lowest cost and are suitable for applications with low electrical performance requirements. They should be used for single-sided wiring. They feature a layer of chemically etched conductive patterns, with the conductive pattern layer on the flexible insulating substrate being rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fiber ester, or polyvinyl chloride.
② Double-sided flexible boards have etched conductive patterns on both sides of the insulating base film. Metallized vias connect the patterns on both sides of the insulating material to form conductive paths, satisfying the design and functional requirements of flexibility. A cover film protects the single- and double-sided conductors and indicates the placement of components.
③ Multilayer flexible boards are made by laminating three or more layers of single-sided or double-sided flexible circuitry together. Metallized vias are formed through drilling and electroplating, creating conductive paths between different layers. This eliminates the need for complex soldering processes. Multilayer circuits offer significant functional differences in terms of higher reliability, better thermal conductivity, and easier assembly. When designing the layout, the interplay between assembly dimensions, number of layers, and flexibility should be considered.
④ Traditional rigid-flex boards are composed of rigid and flexible substrates selectively laminated together. They have a compact structure with conductive connections formed by metallized holes. Rigid-flex boards are a good choice if components are present on both sides of a printed circuit board. However, if all components are on one side, using a double-sided flexible board with an FR4 reinforcement layer laminated on the back is more economical.
Hybrid flexible circuits are multilayer boards where the conductive layers are made of different metals. An 8-layer board uses FR-4 as the inner dielectric and polyimide as the outer dielectric, with leads extending from three different directions on the motherboard, each made of a different metal. Constantan, copper, and gold are used as independent leads. This hybrid structure is mostly used in situations where the relationship between electrical signal conversion and heat conversion is critical, and in low-temperature conditions where electrical performance is demanding; it is the only feasible solution.
The performance can be evaluated based on the ease of the internal design and the total cost to achieve the best price-performance ratio.
