Electronic Design - How the rigid-flex circuit board is designed

The trend of PCB design is to develop towards lightness, thinness and smallness. In addition to high-density circuit board design, there is also an important and complex field such as the three-dimensional connection and assembly of rigid and flexible boards. Rigid-flex board is also called rigid-flex board. With the birth and development of FPC, the new product of rigid-flex circuit board (flex-hard combined board) has gradually been widely used in various occasions. Therefore, the flexible and rigid combination board is the flexible circuit board and the traditional rigid circuit board. After many processes, they are combined according to the relevant process requirements to form a circuit board with both FPC characteristics and PCB characteristics. It can be used in some products with special requirements. It has both a certain flexible area and a certain rigid area. It is of great help to save the internal space of the product, reduce the volume of the finished product, and improve the performance of the product.

Flexible board material

As the saying goes: "Workers must first sharpen their tools if they want to do their jobs well", so when considering the design and production process of a rigid-flex board, it is very important to make full preparations. But this requires a certain degree of professional knowledge and an understanding of the characteristics of the required materials. The materials selected for the rigid-flex board directly affect the subsequent production process and its performance.

Everyone is familiar with Rigid materials, and FR4 type materials are often used. However, the rigid board materials used for the combination of rigid and flexible also need to consider many requirements. It needs to be suitable for adhesion and good heat resistance to ensure that the rigid-flex joint part has the same expansion and contraction without deformation after being heated. General manufacturers use rigid board materials of the resin series.

For the flexible board (Flex) material, choose a substrate and cover film with a smaller size expansion and contraction. Generally, materials made of harder PI are used, and some are directly produced using non-adhesive substrates. The soft board materials are as follows:

Base Material: FCCL (Flexible Copper Clad Laminate)

Polyimide PI. Polymide: Kapton (12.5um/20um/25um/50um/75um). Good flexibility, high temperature resistance (long-term use temperature is 260°C, short-term resistance to 400°C), high moisture absorption, good electrical and mechanical properties, and good tear resistance. Good weather resistance and chemical resistance, and good flame retardancy. Polyesterimine (PI) is most widely used. 80% of them are manufactured by the American DuPont company.

Polyester PET. Polyester(25um/50um/75um). Cheap, good flexibility, tear resistance. Good mechanical and electrical properties such as tensile strength, good water resistance and moisture absorption. However, the shrinkage rate after heating is large, and the high temperature resistance is not good. Not suitable for high temperature soldering, melting point is 250°C, so it is rarely used.

Coverlay

The main function of the cover film is to protect the circuit, prevent moisture, pollution and solder resistance. Cover film thickness From1/2mil to 5 mils (12.7 to 127um).

Conductive layer (ConducTIve Layer) is divided into rolling copper (Rolled Annealed Copper), electrodeposited copper and silver sputtering/spraying (Silver Ink) these methods. Among them, the electrolytic copper crystal structure is rough, which is not conducive to the yield of fine lines. The rolled copper crystal structure is smooth, but the adhesion to the base film is poor. It can be distinguished from the appearance of point solution and rolled copper foil. The electrolytic copper foil is copper-red, and the rolled copper foil is gray-white.

Auxiliary materials and reinforcement boards (AddiTIonal Material & STIffeners). The hard material is additionally pressed on the local area of the soft board for welding components or adding reinforcement for installation. Reinforcing film can be reinforced with FR4, resin board, pressure sensitive adhesive, steel sheet and aluminum sheet.

Non-flowing/low-flow adhesive prepreg (Low Flow PP). Rigid and Flex ConnecTIon is used for Rigid and Flex ConnecTIon, usually very thin PP. Generally there are 106 (2mil), 1080 (3.0mil/3.5mil), 2116 (5.6mil) specifications.

The structure of the rigid-flex board

The rigid-flex board is to glue one or more rigid layers on the flexible board, and the circuit on the rigid layer and the circuit on the flexible layer are connected to each other through metallization. Each rigid-flex board has one or more rigid areas and a flexible area.

In addition, the combination of one flexible board and several rigid boards, the combination of several flexible boards and several rigid boards, uses drilling, plating holes, and lamination processes to achieve electrical interconnection. According to the design needs, the design concept is more suitable for the installation and debugging of the device and the welding operation. To ensure that the advantages and flexibility of the rigid-flexible combined board can be better utilized. This situation is more complicated, with more than two wire layers.

Laminating is to press copper foil, P sheet, memory flexible circuit, and outer rigid circuit into a multilayer board. The lamination of the rigid-flex board is different from the lamination of only the soft board or the lamination of the rigid board. Minutes consider the deformation of the flexible board during the lamination process and the surface flatness of the rigid board. Therefore, in addition to material selection, the thickness of the rigid board needs to be considered in the design process to ensure that the expansion and contraction rate of the rigid-flex part is consistent without warping. The experiment proves that the thickness of 0.8~1.0mm is more suitable. At the same time, pay attention to the placement of via holes at a certain distance between the rigid board and the flexible board from the joint, so as not to affect the rigid-flex joint.

Rigid-flex board production process

Everyone knows that the rigid-flex board is a combination of FPC and PCB, and the production of rigid-flex board should have both FPC production equipment and PCB processing equipment. First, the electronic engineer draws the circuit and shape of the flexible board according to the requirements, and then sends it to the factory that can produce the flexible and hard board. The CAM engineer processes and plans the relevant documents, and then arranges the FPC production line production site FPC and PCB production lines are required to produce PCB. After these two soft and hard boards come out, according to the planning requirements of the electronic engineer, the FPC and PCB are seamlessly pressed by a pressing machine, and then a series of details are passed to the final process. Soft and hard board.

Take Motorola 1+2F+1 Mobile Display&Side Keys this 4-layer board (two-layer hard board and two-layer soft board) as an example. The plate-making requirement of this board is HDI design, and the BGA pitch is 0.5mm. The thickness of the flexible board is 25um with IVH (Interstitial Via Hole) hole design. The thickness of the whole board: 0.295+/-0.052mm. The inner LW/SP is 3/3mil.