sales@ipcb.com
What is Rigid flex PCB?
The trend in PCB design is towards lightweight and compact design. In addition to high-density circuit board design, there is also the important and complex field of 3D connection assembly for Rigid flex PCBs. With the birth and development of FPC, Rigid flex PCB, a new product, has gradually been widely used in various occasions. Therefore, Rigid flex PCB is a circuit board that combines flexible circuit boards and traditional rigid circuit boards through various processes according to relevant process requirements, forming a circuit board with both FPC characteristics and PCB characteristics. It can be used in products with special requirements, with both flexible and rigid areas, which greatly helps to save internal space, reduce finished product volume, and improve product performance.
It is very important to be well prepared when considering the design and production process of a Rigid flex PCB. But this requires a certain level of professional knowledge and understanding of the required material characteristics. The materials selected for Rigid flex PCB directly affect the subsequent production process and its performance.
Everyone is familiar with the material of Rigid PCB and often uses the material of FR4 PCB. But the hard board materials used for Rigid flex also need to consider many requirements. Suitable adhesion and good heat resistance are required to ensure consistent expansion and contraction of the rigid flexible joint after heating without deformation. Generally, manufacturers use resin series rigid board materials.
For FPC PCB (Flex) materials, choose substrates and cover films with smaller size expansion and contraction. Generally, materials made of harder PI are used, but there are also those produced directly using non adhesive substrates.
Polymide (PI): Kapton (12.5um/20um/25um/50um/75um). Good flexibility, high temperature resistance (long-term use temperature of 260 ° C, short-term resistance to 400 ° C), high moisture absorption, good electrical and mechanical properties, and good tear resistance. Good weather resistance, chemical resistance, and flame retardancy. Polyesterimide (PI) is the most widely used. 80% of them are manufactured by DuPont in the United States.
Polyester (PET): (25um/50um/75um). Cheap, with good flexibility and tear resistance. Good mechanical and electrical properties such as tensile strength, good water resistance and moisture absorption. But the shrinkage rate is high after heating, and the high temperature resistance is poor. Not suitable for high temperature soldering, melting point 250 ° C, less commonly used.
Coverlay: The main function of the covering film is to protect the circuit, prevent moisture, pollution, and soldering. The thickness of the covering film ranges from 1/2 mil to 5 mil (12.7 to 127um).
Conductive Layer: divided into several methods: Rolled Annealed Copper, Electrodeposited Copper, and Silver Ink. The rough crystal structure of electrolytic copper is not conducive to the yield of fine circuits. The crystal structure of rolled copper is smooth, but its adhesion to the base film is poor. Point solution and rolled copper foil can be distinguished from each other in appearance. Electrolytic copper foil appears copper red, while rolled copper foil appears gray white.
Additional Material&Stiffeners: Hard materials that are separately pressed onto local areas of FPC for soldering components or adding reinforcement for installation. Reinforcing film can be made of FR4, resin board, pressure-sensitive adhesive, steel sheet, aluminum sheet, etc.
Low Flow PP is used for the lamination of Rigid flex PCBs (Rigid and Flex Connection), typically consisting of very thin PP. Generally, there are specifications such as 106 (2mil), 1080 (3.0mil/3.5mil), and 2116 (5.6mil).
A rigid flexible composite board is a board with one or more rigid layers attached to it. The circuits on the rigid layers are interconnected with those on the flexible layers through metallization. Each rigid flexible composite board has one or more rigid zones and one flexible zone. The following is a simple combination of rigid and flexible boards, with more than one layer.
Rigid flex PCB
In addition, the combination of a flexible board and several rigid boards, as well as the combination of several flexible boards and several rigid boards, is achieved through drilling, plating holes, and laminating processes to achieve electrical interconnection. According to design requirements, make the design concept more suitable for device installation, debugging, and welding operations. Ensure better utilization of the advantages and flexibility of the rigid flexible combination board. This situation is quite complex, with more than two layers of wires. As shown below:
Layering is the process of laminating copper foil, Preperg sheets, inner flexible circuits, and outer rigid circuits into a multi-layer board. The lamination of rigid flex boards is different from lamination of only FPC or rigid boards. The minutes consider both the deformation of flexible boards during the lamination process and the surface flatness of rigid boards. Therefore, in addition to material selection, it is necessary to consider the appropriate thickness of the rigid plate during the design process to ensure that the expansion and contraction rates of the rigid and flexible parts are consistent and will not warp. Experimental results have shown that a thickness of 0.8-1.0mm is more suitable. At the same time, attention should be paid to placing through holes at a certain distance from the joint between the rigid and flexible boards, so as not to affect the rigid flexible joint.
Now we know that Rigid flex PCB is a combination of FPC and PCB, and the production of Rigid flex PCB should have both FPC production equipment and PCB processing equipment. Firstly, the electronic engineer draws the circuit and shape of the flexible bonding board according to the requirements, and then distributes it to the factory that can produce Rigid flex PCBs. After the CAM engineer processes and plans the relevant documents, the FPC production line is arranged to produce the required FPC, and the PCB production line is arranged to produce the PCB. After these two FPC and hard board are produced, according to the electronic engineer's planning requirements, the FPC and PCB are seamlessly pressed together by a laminating machine, and then go through a series of detailed steps to finally process the Rigid flex PCB.
Rigid flex PCB design specifications
The design of rigid flexible PCB is much more complex than traditional PCB design, and there are also many areas that need attention. Especially in the rigid transition area, as well as related wiring, via holes, and other design aspects, corresponding design rules and requirements need to be followed.
1. Hole position
In dynamic use, especially when frequently bending FPC, it is advisable to avoid through holes on FPC as they are easily damaged and cracked. However, it is still possible to drill holes in the reinforcement area on the FPC, but it is also necessary to avoid the edges of the reinforcement area. Therefore, when drilling holes in Rigid flex PCB design, a certain distance should be avoided from the bonding area.
The design rules to be followed for the distance requirements between the via and the rigid flex area are:
A distance of at least 50mil should be maintained, and high reliability applications require at least 70mil.
The vast majority of Rigid flex PCB manufacturers will not accept a maximum distance below 30mil.
Follow the same rules for vias on FPC PCB.
This is the most important design rule that must be followed in Rigid flex PCB.
2. Design of solder pads and vias
Solder pads and vias achieve maximum value while meeting electrical requirements. Smooth transition lines are used at the connection between solder pads and conductors to avoid right angles. Independent solder pads should be equipped with toe pads to enhance support.
In Rigid flex PCB design, vias or pads are easily damaged. The rules to be followed to reduce this risk are:
The larger the copper ring exposed on the solder pad or via, the better.
Add tears as much as possible to the through-hole routing to increase mechanical support.
Add toe plates for reinforcement.
3. Wiring design
If there are wiring on different layers in the Flex zone, try to avoid having one wire on the top layer and another wire on the bottom layer following the same path. When bending the FPC in this way, the uneven force on the copper foil of the upper and lower layers can easily cause mechanical damage to the circuit. Instead, they should be arranged in a staggered manner, crossing and arranging the paths.
The routing design in the Flex zone requires a circular arc rather than an angular line. Contrary to the suggestion for the rigid area. This can protect the flexible board from damage during bending. The line should also avoid sudden expansion or contraction, and tear drop shaped arcs should be used to connect the thick and thin lines.
4. Copper laying design
For enhancing the flexible bending of flexible PCB board, it is best to use a mesh structure for copper or flat layers. However, for impedance control or other applications, the mesh structure is not satisfactory in terms of electrical quality. Therefore, designers need to make a reasonable judgment on whether to use mesh copper or solid copper based on the design requirements. However, for the waste area, it is still advisable to design as many solid copper layers as possible.
5. Distance between drilling and copper skin
This distance refers to the distance between a hole and the copper sheet, which we call the "hole copper distance". The FPC material is different from the material used for the hard board, making it difficult to handle the copper hole distance that is too tight. Generally speaking, the standard distance between copper holes should be 10 mil.
For the combination of rigidity and flexibility, the two most important distances must not be ignored. One is the Drill to Copper distance mentioned here, which follows the minimum standard of 10 mils. The other is the distance from the hole to the edge of the FPC (Hole to Flex), which is generally recommended to be 50mil.
6. Design of the rigid flexible combination zone
In the rigid flexible junction area, it is best to design the FPC to be connected to the hard board in the middle of the layer stack. And the via of FPC is considered as a buried hole in the rigid flexible bonding area. The areas that need attention in the combination of rigidity and flexibility are as follows:
The line should have a smooth transition, and the direction of the line should be perpendicular to the direction of the bend.
The wires should be evenly distributed throughout the entire bending area.
The width of the wire should be maximized throughout the entire bending zone.
Try not to use PTH design in the rigid transition zone.
7. Bending radius of the bending zone of the rigid flexible composite board
The flexible bending zone of the rigid flexible composite board should be able to withstand 100000 bending cycles without any open circuits, short circuits, performance degradation, or unacceptable delamination. The flexural resistance can be measured using specialized equipment or equivalent instruments, and the tested sample should comply with relevant technical specifications. The design of bending radius should be related to the thickness and number of layers of FPC in the flexible bending area. The simple reference standard is R=WxT. T is the total thickness of FPC. Single panel W is 6, double-sided board is 12, and multi-layer board is 24. So the minimum bending radius for single panel is 6 times the thickness of the board, for double-sided panel it is 12 times the thickness, and for multi-layer panel it is 24 times the thickness. All should not be less than 1.6mm.
Rigid flex PCB
For the design of Rigid flex PCB, it is particularly important to focus on the design of flexible circuit boards. When designing flexible boards, it is required to consider the different materials, thicknesses, and combinations of the substrate, bonding layer, copper foil, cover layer, reinforcement board, and surface treatment of the flexible board, as well as its properties such as peel strength, flexural resistance, chemical properties, working temperature, etc. Special consideration should be given to the assembly and specific application of the designed flexible board. The specific design rules in this regard can refer to IPC standards: IPC-D-249 and IPC-2233.
In addition, for FPC processing accuracy, PCB sample processing accuracy: line width: 50 μ m, aperture: 0.1mm, 20 or more layers of PCB. PCB mass production: Line width: 75 μ m, aperture: 0.2mm, 18 layer PCB or less. These all need to be understood and referenced during the specific rigid flex PCB design.