Electronic Design - Precautions for the design rules of the rigid-flexible version

Rigid-flex board is much more complicated in design than traditional PCB design, and there are many places to pay attention to. Especially the rigid-to-flex transition area, as well as the design aspects of related wiring, vias, etc., all need to follow the requirements of the corresponding design rules.

1. Via position

In the case of dynamic use, especially when the flexible board is often bent, the vias on the flexible board should be avoided as much as possible, and these vias are easily damaged and cracked. However, the reinforced area on the soft board can still be punched, but it must be avoided near the edge of the reinforced area. Therefore, a certain distance should be avoided when drilling holes in the design of the rigid-flex board.

2. Design of pads and vias

The pads and vias will win the maximum value if they meet the electrical requirements. The connection between the pad and the conductor adopts a smooth transition line to avoid right angles. The independent pad should be added with a toe to strengthen the supporting effect.

In the design of rigid-flex board, vias or pads are easily damaged. Rules to be followed to reduce this risk:

The soldering flux layer of the pad or the via hole exposes the copper ring, the larger the better.

Try to add teardrops to the via traces to increase mechanical support.

Add disk toe for reinforcement.

3. Layout design

If there are wires on different layers in the flexible area (Flex), try to avoid one wire on the top layer and the other wire on the same path on the bottom layer. In this way, when the flexible board is bent, the stresses of the copper skins of the upper and lower layers are inconsistent, which is likely to cause mechanical damage to the circuit. Instead, they should be staggered, and the paths should be arranged crosswise.

The routing design in the flexible area (Flex) requires the best to follow the arc line, not the angle line. Contrary to the suggestion in the Rigid area. In this way, part of the circuit of the flexible board can be protected from being 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 paving design

For the flexible bending of the reinforced flexible board, it is best to use a mesh structure for the copper or plane layer. But for impedance control or other applications, the electrical quality of the mesh structure is unsatisfactory. Therefore, in the specific design, the designer needs to make a reasonable judgment based on the design requirements, whichever is less, whether to use a mesh copper sheet or a solid core. copper. However, for the scrap area, as much solid copper as possible is designed.

5. The distance between the drill hole and the copper sheet

This distance refers to the distance between a hole and the copper skin, we call it "hole copper distance". The material of the soft board is different from that of the hard board, so that the too tight hole copper distance is difficult to deal with. Generally speaking, the standard hole copper pitch should be 10mil.

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

6. The design of the rigid-flex zone

In the rigid-flex area, the soft board is best designed to connect with the hard board in the middle of the stack. The vias of the soft board are considered as buried vias in the rigid-flexible junction area. The areas that need attention in the rigid-flex zone are as follows:

The line should transition smoothly, and the direction of the line should be perpendicular to the direction of bending.

The wires should be evenly distributed throughout the bending area.

In the entire bending zone, the width of the wire should be maximized.

Try not to use PTH design in the rigid-flex transition zone.

7. The bending radius of the bending area of the rigid-flex board

The flexible bending area of the rigid-flex board should be able to withstand 100,000 times of deflection without breaking, short circuit, performance degradation or unacceptable delamination. The flexural resistance is determined by special equipment or equivalent instruments. The tested sample should meet the requirements of relevant technical specifications. The design of the bending radius should be related to the thickness of the soft plate in the flexible bending zone and the number of layers of the soft plate. The simple reference standard is R=WxT. T is the total thickness of the soft board. Single-sided board W is 6, double-sided board 12, and multi-layer board 24. Therefore, the minimum bending radius of a single-sided board is 6 times the board thickness, a double-sided board is 12 times the board thickness, and a multilayer board is 24 times the board thickness. All should not be less than 1.6mm.

In short, for the design of the rigid-flex board, the design of the flexible circuit board is particularly important. Flexible board design requires consideration of different materials, thicknesses and different combinations of flexible board substrate, bonding layer, copper foil, covering layer and reinforcement board and surface treatment, as well as its properties, such as peel strength and flexural resistance. Curve performance, chemical performance, working temperature, etc. Special consideration should be given to the assembly and specific application of the designed flexible board. Specific design rules in this regard can refer to IPC standards: IPC-D-249 and IPC-2233.

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