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PCB Layer Stack Up for Rigid Flex Circuits
Posted on 26/01/2018

Stackup in the design of a multilayer Printed Circuit Board (PCB) is an important factor from the point of view of the Electro-Magnetic Compatibility (EMC) of the product in which the PCB is used. Much of the radiation from the PCB can be reduced with a good stackup, whereas a poor stackup can worsen the radiation considerably.

Basic Factors to Consider

While designing the stack-up of a PCB, a designer must consider the following factors;

·      The number of layers to be used

·      The number of power and ground planes

·      Sequence of the planes and spacing between the planes.

While the spacing between the planes is of importance to the PCB manufacturer, the other factors are important for an optimum design of the PCB and its EMC performance.

Design of Multilayer PCB’s

Signals

In a regular PCB, the designer decides on the number of layers based primarily on the number of signals he/she has to route and their frequency of operation. For a rigid flex PCB, the designer must consider an additional factor—the nature of the bend of the PCB. For instance, a flexible part of the PCB may be joining two of its rigid parts, and the placement of the flexible part in the stackup in relation to the rest of the rigid part helps to minimize stresses and determines the final form and fit of the PCB.

Layers

The total number of layers to be used also depends on whether the PCB will finally reside within an unshielded enclosure or a shielded one, and the type of emission class the product is required to meet. Requirements that are more stringent need higher number of layers, with larger number of ground and power layers, making the stackup design more critical. Usually, the stackup is more of an optimum achievement within the restrictions of time and cost.

Planes

Introduction of multiple power and ground planes in the stackup of multilayer rigid flex circuits leads to a significant reduction of radiated emission. This is because the presence of the planes allows improved signal routing configurations. The designer has proper control over the impedance and is able to reduce the ground noise significantly by using a large ground plane.

Balanced/unbalanced Construction

While designing the stack-up of a rigid flex PCB, it is important that the designer maintain a symmetrical cross-section of the board to enhance the mechanical strength and to prevent warping of the board. Sometimes, designers also use an unbalanced construction as this allows stackup configurations that are more suitable to the design.

Signal Layers

While deciding on the stackup of a multilayer rigid flex PCB, the designer usually keeps the signal layer adjacent to a plane, which couples the signal layers tightly to their respective planes. While this process allows the ground and power planes to couple closely together, it also allows the designer to route high-speed signals within buried layers located between the planes. This helps to reduce cross-coupling and interference.

Stack-up

The stack-up design requires a close coordination between the designer and the PCB fabricator. The spacing between the planes within a stackup is important for maintaining the impedance, and for achieving the required overall PCB thickness.

4,6,8 & 10 Layer Standard stack up examples as per below

Conclusion

The design, construction and purpose for Rigid Flex PCB’s leads to endless possibilities for their use. For more information on Rigid flex PCB’s please visit our blog at http://www.pcbglobal.com/17/blog.htmlor if you would like some advice on how to get the most out of your PCB stack up for your Rigid Flex PCB’s, please feel free to contact the PCB Global team at sales@pcbglobal.com