At present, flexible circuits mainly include four types of single-sided, double-sided, multi-layer and rigid flexible circuit boards.
1. Single-sided flexible board is the lowest cost, when the board is not required for electrical performance. For single-sided wiring, a single-sided flexible board should be used. It has a chemically etched conductive pattern, and the conductive pattern layer on the surface of the flexible insulating substrate is a rolled copper foil. The insulating substrate may be polyimide, polyethylene terephthalate, aramid fiber ester and polyvinyl chloride.
2. Double-sided flexible board is a conductive pattern made by etching each layer on both sides of the insulating base film. The metallized holes connect the patterns on both sides of the insulating material to form a conductive path to meet the design and use functions of the flexibility. The cover film protects the single and double-sided wires and indicates where the components are placed.
3. The multi-layer flexible board is formed by laminating three or more layers of single-sided or double-sided flexible circuits, and forming metallized holes by drilling and electroplating to form conductive paths between different layers. This eliminates the need for complex welding processes. Multilayer circuits have tremendous functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance. When designing the layout, the interaction between assembly size, number of layers, and flexibility should be considered
4. Conventional rigid flexible sheets are composed of rigid and flexible substrates that are selectively laminated together. The structure is tight, and the metallized holes form an electrically conductive connection. If a printed board has components on both the front and back sides, a 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 board and laminate a layer of FR4 reinforcement on the back.
The flexible circuit of the hybrid structure is a multilayer board, and the conductive layer is composed of different metals. An 8-layer board uses FR-4 as the inner layer medium, using polyimide as the outer layer medium, and the leads are extended from three different directions of the main board, each of which is made of a different metal. Constantan, copper and gold are used as separate leads. This kind of hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion and the low temperature condition with relatively strict electrical performance, which is the only feasible solution.
It can be evaluated by the convenience and total cost of the interconnect design to achieve the best performance-price ratio.