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Rigid-Flex Board Application for PCB Design

Posted by TekRevue Contributor on March 7, 2019

The trend of PCB design is to develop in a light and small direction. In addition to the high-density board design, there are also important and complex areas of three-dimensional connection assembly of flex-hard boards. The rigid-flex circuit board, with the birth and development of FPC, is gradually being widely used in various occasions.

The rigid-flex board is a flexible circuit board and a conventional rigid circuit board, which are combined in various processes and according to relevant process requirements to form a circuit board having both FPC characteristics and PCB characteristics. It can be used in some products with special requirements, both a certain flexible area and a certain rigid area, which helps save internal space, reduces finished product volume, and improves product performance.

Flexible Board Material

As the saying goes: “When a worker wants to do something good, he must first sharpen his tools.” Therefore, it is very important to fully prepare for the design and production process of a rigid-flex board. However, this requires a certain amount of expertise and an understanding of the characteristics of the materials required. The materials selected for the rigid-flex plates directly affect the subsequent production process and its performance.

Rigid materials are familiar to everyone, and FR4 type materials are often used. However, rigid-flex material also needs to take into account many requirements. It is suitable for sticking and offers good heat resistance to ensure that the degree of expansion of the rigid-flexed joint portion after heating is uniform without deformation. The general manufacturer uses a rigid material of the resin series.

For flexible (flex) materials, select a substrate with a smaller size and a cover film. Generally, materials made of harder PI are used, and those produced by using a non-adhesive substrate are also directly used. The flex material is as follows:

Base Material:FCCL(Flexible Copper Clad Laminate)

Polyimide PI. Polymide: Kapton (12.5 um / 20 um / 25 um / 50 um / 75 um). 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, good tear resistance. Good weather resistance and chemical properties, good flame retardancy. Polyimide (PI) is the most widely used. 80% of them are manufactured by DuPont, USA.

Polyester PET

Polyester (25um/50um/75um). Cheap, flexible and tear resistant. Good mechanical and electrical properties such as tensile strength, good water resistance and hygroscopicity. However, after heat, the shrinkage rate is large and the high temperature resistance is not good. Not suitable for high temperature soldering, melting point 250 ° C, less used.

Coverlay

The main function of the cover film is to protect the circuit from moisture, contamination and soldering. Cover film thickness from 1/2 mil to 5 mils (12.7 to 127 um).

The Conductive Layer is rolled annealed copper, electrodeposited copper, and Silver Ink. Among them, the electrolytic copper crystal structure is rough, which is not conducive to fine line yield. The copper crystal structure is smooth, but the adhesion to the base film is poor. The point solution and the copper foil can be distinguished from the appearance. The electrolytic copper foil is copper red, and the rolled copper foil is grayish white.

Additional Material & Stiffeners

Auxiliary materials and stiffeners are hard materials partially pressed together in order to weld components or add reinforcement for mounting. Reinforced film can be reinforced with FR4, resin plate, pressure-sensitive adhesive, steel sheet and aluminum sheet.

Non-flowing/low flow adhesive prepreg (Low Flow PP). Rigid and Flex Connection for rigid-flex boards, usually very thin PP. There are generally 106 (2 mil), 1080 (3.0 mil / 3.5 mil), 2116 (5.6 mil) specifications.

Rigid-flexible plate structure

The rigid-flex board is one or more rigid layers adhered to the flexible board, and the circuit on the rigid layer and the circuit on the flexible layer are connected to each other by metallization. Each rigid-flex panel has one or more rigid zones and a flexible zone. The combination of simple rigid and flexible plates is shown below, with more than one layer.

In addition, a combination of a flexible board and a few rigid boards, a combination of several flexible boards and several rigid boards, using holes, plating holes, lamination process to achieve electrical interconnection. According to the design requirements, the design concept is more suitable for device installation and debugging as well as welding operations. Ensure that the advantages and flexibility of the rigid-flex board are better utilized. This situation is more complicated, and the wire layer is more than two layers. As follows:

Lamination is to laminate copper foil, P-piece, memory flexible circuit, and outer rigid circuit into a multi-layer board. The lamination of the rigid-flex board is different from the lamination of only the flex board or the lamination of the rigid board. It is necessary to 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, it is also necessary to consider the thickness of the rigid plate in the design process, and to ensure that the shrinkage rate of the rigid-flex portion is consistent without warping. The experiment proves that the thickness of 0.8~1.0mm is more suitable. At the same time, it should be noted that the rigid plate and the flexible plate are placed at a certain distance from the joint portion so as not to affect the rigid joint portion.

Rigid-Flexible Combination Board Production Process

The production of rigid-flex should have both FPC production equipment and PCB processing equipment. First, the electronics engineer draws the line and shape of the flexible board according to the requirements, and then delivers it to the factory that can produce the rigid-flex board. After the CAM engineers process and plan the relevant documents, the FPC production line is arranged. The FPC and PCB production lines are required to produce PCBs. After the flex board and rigid board comes out, according to the planning requirements of the electronic engineers, the FPC and the PCB are seamlessly pressed through the press machine, and then through a series of detailed steps, the final process is rigid-flex board.

For an example, take the Motorola 1+2F+1 Mobile Display and Side Keys 4-layer board (two-layer rigid board and two-layer flex board). The plate making requirements are an HDI design with a BGA pitch of 0.5 mm. The thickness of the flex board is 25um and there is an IVH (Interstitial Via Hole) hole design. The thickness of the whole plate: 0.295 +/- 0.052 mm. The inner layer LW/SP is 3/3 mil.

Design Rules for Rigid-Flex Boards

The rigid-flex board is much more complicated in design than the traditional PCB design, and there are many places to pay attention to. In particular, rigid-transition transition areas, as well as related routing, vias, and so on are subject to the requirements of the corresponding design rules.

1. Via Location

In the case of dynamic use, especially when the flexible board is often bent, the through holes on the flexible board are avoided as much as possible, and the through holes are easily broken. However, the reinforced area on the flex board can still be perforated, but also avoid the vicinity of the edge of the reinforced area. Therefore, it is necessary to avoid a certain distance of the bonding area when punching holes in the design of the flex and hard board. As shown below.

For the distance requirements of the via and the rigid-flex, the rules to be followed in the design are:

  • A distance of at least 50 mils should be maintained, and a high reliability application requires at least 70 mils.
  • Most processors will not accept extreme distances below 30 mil.
  • Follow the same rules for vias on a flexible board.
  • This is the most important design rule in the rigid-flex board.

2. Pad and Via Design

Pads and vias win the maximum value when electrical requirements are met, and a smooth transition line is used at the junction between the pad and the conductor to avoid a right angle. Separate pads should be added to the toe to enhance support.

In the rigid-flex board design, the vias or pads are easily damaged. The rules to follow to reduce this risk:

  • The solder pad of the pad or via is exposed to a copper ring, the larger the better.
  • Through-hole traces add teardrops as much as possible to increase mechanical support.
  • Add a toe to strengthen.

3. Trace Design

If there are traces on different layers in the flex zone (Flex), try to avoid one wire at the top and the other at the same path at the bottom. In this way, when the flexible board is bent, the force of the upper and lower layers of the copper wire is inconsistent, which is likely to cause mechanical damage to the line. Instead, you should stagger the paths and cross the paths. As shown below.

The routing design in the flex zone (Flex) requires that the arc line be the best, not the angle line. Contrary to the recommendations in the Rigid area. This can protect the flexible board part section from being easily broken when bent. The line should also avoid sudden expansion or contraction, and the thick and thin lines should be connected by a teardrop-shaped arc.

4. Copper Plating Design

For the flexible bending of the reinforced flexible board, the copper or flat layer is preferably a mesh structure. However, for impedance control or other applications, the mesh structure is not satisfactory in terms of electrical quality. Therefore, in the specific design, the designer must make a judgement call that fits the design requirements. Is it using mesh copper or solid? However, for the waste area, it is still possible to design as many solid copper as possible. As shown below.

5. Distance Between Borehole and Copper

This distance refers to the distance between a hole and the copper skin. This is referred to as the “hole copper distance.” The material of the flex board is different from that of the rigid board, so that the distance between the holes and the copper is too difficult to handle. In general, the standard hole copper distance should be 10 mils.

For the rigid-flexible zone, the two most important distances must not be ignored. One is the Drill to Copper mentioned here, which follows the minimum standard of 10 mil. The other is the hole to the edge of the flex board (Hole to Flex), which is generally recommended to be 50mil.

6. Design of Rigid-Flexible Zone

In the rigid-flexible zone, the flexible board is preferably designed to be connected to the hardboard in the middle of the stack. The vias of the flex board are considered to be buried holes in the rigid-flexible bond area. The areas that need to be noticed in the rigid-flexible zone are as follows:

  • The line should be smoothly transitioned, and the direction of the line should be perpendicular to the direction of the bend.
  • The layout should be evenly distributed throughout the bending zone.
  • The width of the wire should be maximized throughout the bend zone.
  • The rigid-transition transition zone should try not to adopt the PTH design.

7. The Bending Radius of the Bending Zone of the Rigid-Flex Board

The flexible bending zone of the rigid-flex panel shall be capable of withstanding 100,000 deflections without breaks, short circuits, reduced performance, or unacceptable delamination. The flexural resistance is measured by special equipment, and it can also be measured by equivalent instruments. The tested samples should meet the requirements of relevant technical specifications.

In design, the bending radius should be referenced as shown in the figure below. The design of the bending radius should be related to the thickness of the flex board in the flexible bending zone and the number of layers of the flex board. A simple reference standard is R=WxT. T is the total thickness of the flex board. The single panel W is 6, the double panel 12, and the multilayer board 24. Therefore, the minimum bending radius of a single panel is 6 times, the double panel is 12 times thick, and the multilayer board is 24 times thick. All should not be less than 1.6mm.

In summary, it is particularly important for the design of the flexible and hard board to be related to the flexible circuit board design. Flexible board design requires consideration of the different materials, thicknesses and different combinations of the substrate, bonding layer, copper foil, cover layer and reinforcing plate and surface treatment of the flexible board, as well as its properties, such as peel strength and flex resistance. Flex properties, chemical properties, operating temperatures, etc. Particular consideration should be given to the assembly and specific application of the designed flex plate. Specific design rules in this regard can refer to the IPC standards: IPC-D-249 and IPC-2233.

In addition, for the processing accuracy of flex board, the processing precision abroad is: circuit width: 50μm, aperture: 0.1mm, and the number of layers is more than 10 layers. Domestic: circuit width: 75μm, aperture: 0.2mm, 4 layers. These need to be understood and referenced in the specific design.

One normal application of a rigid-flex board is the iPhone PCB design. Apple uses a rigid flex board to connect the device’s mobile display with the main board. If you want to know more about rigid-flex board applications for industries such as medical devices, military, or optoelectronics, visit RayMing.

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