Design Rules for Rigid-Flex Circuits

- Feb 13, 2020-

 

Rigid and flexible panels are much more complex in design than traditional PCB design, and there are a lot of things to pay attention to. In particular, the transition area, as well as related lines, holes and other design aspects, need to follow the requirements of the corresponding design rules.


Through Holes position 01

In the case of dynamic use, especially when bending the soft plate frequently, the holes on the soft plate need to be avoided as much as possible, and these holes are easily damaged and cracked. However, the strengthening area on the soft plate can still be perforated, but also to avoid the edge of the strengthening area near. Therefore, in the design of hard and soft composite plate to avoid the combination of a certain distance. as shown below.


For the distance between the hole and the soft and hard area, the design rules are as follows:

 

shall keep at least 50 mil of distance, high reliability application occasions required at least 70 mil. the vast majority of processing manufacturers will not accept a limit distance below 30 mil.Follow the same rule for the overholes on the soft plate. This is one of the most important design rules in a hard and soft composite board that must be adhered to.

 

Design of pad and Through holes

The pad and the through hole in accordance with the electrical requirements to win the maximum value, the pad and the conductor between the connection between the use of a smooth transition line to avoid right angles. independent pad should be added to the toe to strengthen the supporting action.

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In the design of soft and hard laminates, over-holes or pads are easily damaged. The rules to be followed to reduce this risk: Welding pad or through the hole layer exposed copper ring, the larger the better. Through the hole line as far as possible to add tears, increase mechanical support. Add disc toe, reinforcing action.


Line design

In the flex zone (flex), if there is a different layer of wiring, try to avoid one line in the top layer, the other line in the bottom layer of its same path. So in the bending of the soft plate, the upper and lower layers of the line copper force is not consistent, easy to cause mechanical damage to the line. Instead, the paths should be crossed.  

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The line design in the flex requires a circular arc rather than an angular line. contrary to the recommendation of the hard plate (rgid) zone. This can protect the flexible plate part of the line in bending is not easy to break. Lines should also avoid sudden expansion or shrinkage, thick lines between the use of tear-drop-shaped arc connection.


Copper laying design 02

For flexible bending of flexible panels, copper or flat layers are best meshed. However, for impedance control or other applications, the mesh structure in the electrical quality is unsatisfactory, therefore, the designer in the specific design needs to be based on the design needs to take its light reasonable judgment, whether to use mesh copper skin or solid copper. For the scrap zone, however, as much solid copper as possible. as shown below.


Design of rigid and flexible junction zone 03

In the rigid and flexible bonding zone, the soft plate is best designed to connect with the hard plate in the middle of the layer stack. and the through hole of the soft plate is considered as the buried hole in the rigid and soft junction area. The following points for attention in the rigid-flexible junction area are:

The line should be smooth transition, the direction of the line should be perpendicular to the direction of the bend. The wire shall be evenly distributed throughout the bending zone. The width of the wire should be maximized throughout the bending zone. The rigid deflection transition zone as far as possible without pth design.


Bending radius of bending zone of rigid and flexible plate

The flexible bending zone of the rigid and flexible composite plate shall be capable of resisting the phenomenon of 100,000 flexures without broken circuit, short circuit, low performance or unacceptable delamination. The flexure resistance shall be determined by special equipment or equivalent instruments, and the tested samples shall conform to the requirements of the relevant technical specifications. In design, the bending radius should be as shown in the following figure for reference. The design of bending radius should be related to the thickness of the flexible bending zone and the number of soft plate layers. The simple reference standard is R = WxT. t is the total thickness of the soft plate. Single panel W is 6, double panel 12, multilayer board 24. So the minimum bending radius of the single plate is 6 times the thickness of the plate, the double panel is 12 times the thickness of the plate, and the multilayer plate is 24 times the thickness of the plate. All should not be less than 1.6mm.

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