Introduction To Flexible Printed Circuit Boards

Introduction to flexible circuit boards

FPC: Flexible Printed Circuit. It is made of conductive circuit patterns by using light imaging pattern transfer and etching processes on the surface of a flexible substrate. The surface and inner layers of double-sided and multi-layer circuit boards are electrically connected to the inner and outer layers through metallized holes.The surface of the circuit pattern is protected and insulated by PI and glue layer.
It is mainly divided into single-sided board, hollow board, double-sided board, multi-layer board, and rigid-flex board.

• Features of flexible circuit boards
  ⒈Short: short assembly hours, all lines are configured, eliminating the need for redundant cable connection work;
  ⒉Small: The volume is smaller than PCB (hard board), which can effectively reduce the product volume and increase the convenience of carrying;
  ⒊Light: lighter than PCB (hard board) can reduce the weight of the final product;
• 4.Thin: The thickness is thinner than PCB (hard board), which can improve the flexibility and strengthen the assembly of three-dimensional space in a limited space.

Advantages of flexible circuit boards

Flexible printed circuit boards are printed circuits made of flexible insulating substrates, and have many advantages that rigid printed circuit boards do not have:

1. It can be bent, wound, and folded freely, can be arranged according to the space layout requirements, and can be moved and expanded in three-dimensional space, so as to achieve the integration of component assembly and wire connection;
2. The use of FPC can greatly reduce the volume and weight of electronic products, and is suitable for the development of electronic products in the direction of high density, miniaturization and high reliability. Therefore, FPC has been widely used in aerospace, military, mobile communications, laptop computers, computer peripherals, PDAs, digital cameras and other fields or products;
3. FPC also has the advantages of good heat dissipation and solderability, easy assembly and low overall cost, etc. The design of soft and hard combination also makes up for the slight deficiency of the flexible substrate in the component carrying capacity to a certain extent.

FPC main raw materials

The main raw materials are right: 1. Substrate, 2. Cover film, 3. Reinforcement, 4. Other auxiliary materials.

1. Substrate
   1.1 Adhesive substrate
   Adhesive substrates are mainly composed of three parts: copper foil, adhesive and PI. There are two types of single-sided substrates and double-sided substrates. The material with only one side of copper foil is single-sided substrate, and the material with double-sided copper foil is double Surface substrate.

1.2 No glue substrate
The non-adhesive substrate is the substrate without the adhesive layer. Compared with the ordinary adhesive substrate, the middle adhesive layer is missing, and only composed of two parts: copper foil and PI, which is thinner than the adhesive substrate , Better dimensional stability, higher heat resistance, higher bending resistance, better chemical resistance and other advantages, it has now been widely used.

Copper foil: At present, the commonly used copper foil thickness has the following specifications, 1OZ, 1/2OZ, 1/3OZ, and now a thinner copper foil with a thickness of 1/4OZ is introduced, but this material is already being used in China and is making ultra-fine roads. (Line width and line spacing are 0.05MM and below) products. With the increasing requirements of customers, materials of this specification will be widely used in the future.

2. Cover film
   It is mainly composed of three parts: release paper, glue, and PI. Only the glue and PI remain on the product. The release paper will be torn off during the production process and will no longer be used (its role is to protect the glue with foreign matter).
3. Reinforce
   It is a specific material used for FPC and is used in a specific part of the product to increase the support strength and make up for the “soft” characteristic of FPC.

At present, the commonly used reinforcement materials are as follows:

▶FR4 reinforcement: The main component is composed of glass fiber cloth and epoxy resin glue, which is the same as the FR4 material used in PCB;
▶Steel sheet reinforcement: The composition is steel, which has strong hardness and support strength;
▶ PI reinforcement: the same as the cover film, it consists of three parts: PI and release paper, except that the PI layer is thicker, and it can be produced from 2MIL to 9MIL.

4. Other auxiliary materials

▶Pure glue: This adhesive film is a heat-curing acrylic adhesive film composed of protective paper/release film and a layer of glue. It is mainly used for layered boards, flexible and rigid boards, and FR- 4/Steel sheet reinforcement board plays a role of bonding.
▶Electromagnetic protective film: Pasted on the board surface for shielding.
▶Pure copper foil: only composed of copper foil, mainly used for hollow board production.

Types of FPC
FPC types have the following 6 distinctions:

A. Single panel: Only one side has wiring.
B. Double panel: There are lines on both sides.
C. Hollow board: also known as window board (opening the window on the finger surface).
D. Layered board: two-sided circuit (separate).
E. Multilayer board: more than two layers
F. Rigid-flex board: a product that combines soft board and hard board.

FPC process flow
FPC will continue to innovate from three aspects in the future, mainly in:

1. Thickness: The thickness of FPC must be more flexible and thinner;
2. Folding resistance: the ability to bend is an inherent feature of FPC, and the FPC in the future must be more resistant to folding;
3. Process level: In order to meet various requirements, the FPC process must be upgraded, and the minimum aperture and minimum line width/line distance must meet higher requirements.