PCB Technical - What is FPC

FPC is the abbreviation of Flexible Printed Circuit, also known as a flexible circuit board, flexible printed circuit board, or flexible circuit board, abbreviated as soft board or FPC, which has the characteristics of high wiring density, lightweight and thin thickness. Mainly used in mobile phones, notebook computers, PDAs, digital cameras, LCMs and many other products.

FPC main raw materials

Its main raw materials right: substrate,cover film,reinforcement,other auxiliary materials.

Substrate

Adhesive substrate

Adhesive substrate mainly consists of three parts: copper foil, glue, and PI, there are two categories of single-sided substrate and double-sided substrate, only one side of the copper foil material for the single-sided substrate, there are two sides of the copper foil material for the double-sided substrate.

Non-adhesive substrates

Non-adhesive substrate is the substrate without adhesive layer, it is relative to the ordinary adhesive substrate, less adhesive layer in the middle, only copper foil and PI two parts of the composition, than the adhesive substrate has a thinner, better dimensional stability, higher heat resistance, higher resistance to bending, better chemical resistance, etc., and now has been widely used. 

Copper foil: The thickness of commonly used copper foil has the following specifications, 1OZ, 1/2OZ, 1/3OZ, now introduced 1/4OZ thickness of the thinner copper foil, but currently the domestic use of such materials, in the making of ultra-fine road (line width and pitch of 0.05MM and below) products. As the customer's requirements become higher and higher, this kind of material will be widely used in the future.

Covering film

There are three main components: release paper, glue, and PI, ultimately retained in the product only glue, and PI two parts, the release paper will be torn off in the production process is no longer used (its role in the protection of the glue on the foreign body).

Reinforcement

Specific use of materials for FPC, a specific part of the product used to increase the support strength, to make up for the pcbs softer characteristics.

Currently used reinforcement materials are the following:

(1) FR4 reinforcement: the main components of glass fiber cloth and epoxy resin glue composition, the same as the FR4 material used in PCB;

(2) steel reinforcement: composed of steel, with strong hardness and support strength;

(3) PI reinforcement: the same as the cover film, PI and adhesive release paper composed of three parts, only its PI layer is thicker, from 2MIL to 9MIL can be proportionate production.

4.Other auxiliary materials

(1) Pure adhesive: This adhesive film is a heat-curing acrylate adhesive film with protective paper/release film and a layer of adhesive composition, mainly used for layered boards, hard and soft combination of boards, and FR-4/steel sheet reinforcing boards, to play a role in bonding.

(2) electromagnetic protection film: pasted on the board surface to play a shielding role.

(3) Pure copper foil: only composed of copper foil, mainly used for hollowing board production.

Basic structure

Copper Film

Copper foil is basically divided into two types: electrolytic copper and calendered copper. The common thicknesses are 1oz 1/2oz and 1/3 oz.

Substrate Film: Common thicknesses are 1 mil and 1/2 mil.

Adhesive: Thicknesses are determined by customer requirements.

Cover Film

Cover Film: For surface insulation. Common thicknesses are 1 mil and 1/2 mil.

Adhesive: Thickness depends on customer's requirement.

Release paper: To avoid foreign substance from adhesive before pressing; to facilitate the work.

PI Stiffener Film

PI Stiffener Film: reinforce the mechanical strength of the FPC to facilitate the surface mounting operation. Common thicknesses range from 3mil to 9mil.

Adhesive: Thickness depends on customer's requirement.

Release paper: to avoid adhesive foreign matter before pressing.

EMI: Electromagnetic shielding film to protect the circuit board from outside interference (strong electromagnetic area or interference-prone area).

Currently there are four types of flexible circuit boards: single-sided, double-sided, multilayer flexible circuit boards and rigid-flexible combination of circuit boards.

1.Single-sided flexible circuit board is the lowest cost, when the electrical performance requirements are not high printed circuit board. In the single-sided wiring, should choose a single-sided flexible circuit board. It has a layer of chemical etching out of the conductive pattern, in the flexible insulating substrate surface of the conductive pattern layer for the calendered copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fiber ester and polyvinyl chloride. 

2.Double-sided flexible circuit boards have a conductive graphic layer etched on each side of the insulating substrate. Metallized holes connect the graphics on both sides of the insulating material to form conductive pathways to meet the design and function of the flex. The cover film protects single and double-sided wires and indicates the location of component placement.

3.Multilayer flexible circuit boards are 3 or more layers of single-sided or double-sided flexible circuit boards laminated together, through the drilling of solenoids, plating the formation of metallization holes in the different layers to form a conductive path. This eliminates the need for complex soldering processes. Multi-layer circuits make a huge functional difference in terms of higher reliability, better thermal conductivity and easier assembly performance. The interaction of assembly size, number of layers and flex should be considered when designing the layout.

4.Conventional rigid-flex circuit boards consist of rigid and flexible substrates selectively laminated together. The structure is tightly packed with metallized solitons to form conductive connections. If a printed board has components on both the front and back sides, rigid-flex circuit boards are a good choice. However, if all the components are on one side, it is more economical to choose a double-sided flexible circuit board with a layer of FR4 reinforcement laminated on its back side.

5.A hybrid structure flex board is a multilayer board with conductive layers made of different metals. An 8-layer board uses FR-4 as the dielectric for the inner layer and polyimide as the dielectric for the outer layer, with leads protruding from the main board in three different directions, each made of a different metal. Conoco alloy, copper and gold are used as separate leads. This hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion and the electrical properties of the more demanding low-temperature situation, is the only feasible solution. It can be evaluated by the convenience and total cost of the inline design to achieve the best performance-price ratio.

FPC product features

1.It can be freely bent, folded, and wound, and can be moved and stretched freely in three-dimensional space.

2.The heat dissipation performance is good, and the FPC can reduce the volume.

3.Realize lightweight, miniaturization, and thinning, so as to achieve the integration of component devices and wire connections.

FPC is a highly reliable and excellent flexible printed circuit made of polyimide or polyester film as the base material.