Often when we consider multilayer circuit board PCB design, we often think of a combination of board racks or gaming platforms in a server environment. But what if our typical rigid board is not suitable for a physical chassis used in multilayer boards? Will we be willing to pay an extra price to use a flexible circuit board? What if we can combine the advantages of both?
In this article, we will introduce the advantages and nature of the rigid-flex combination and how to better meet the PCB design requirements of multilayer boards.
What is a rigid-flex PCB?
In the standard multilayer circuit board PCB design, we use the board concept to divide the different functional circuits into smaller boards and put the system into a single housing using various interconnects.
The problem with this standard approach is that the reliability of the interconnects cannot be guaranteed (especially after considering electromagnetic interference/electromagnetic compatibility issues). Standard card edge connectors with good electrical conductivity that meet our size requirements do not always exist; the best alternative is cable, but the cables are not practical and they are not able to meet the space requirements of the chassis.
If a multilayer board design requires us to interconnect several rigid boards in a compact enclosure with high-level connections and high-speed connections, the combination of rigid-flexible combination is the best solution.
What is a rigid combination? Simply put, two or more rigid boards are electrically connected through a flexible portion.
A single flexible layer typically consists of the following materials:
- Flexible polyimide core;
- Conductive copper layer;
The conductive copper layer is sandwiched between the flexible polyimides on both sides by an adhesive. The polyimide layer and the adhesive layer are generally regarded as one unit (referred to as a cover layer) which can be laminated on the copper layer by heat and pressure. Multiple flexible layers can be used in any given design.
The rigid portion is added to the flexible layer through a rigid layer of standard PCB material:
- Injecting glass fiber prepreg with resin, it will flow and bond when heated;
- Non-conductive glass fiber base layer (usually FR-4);
- traditional green solder mask;
- Screen printing marks and identification information
The flexible polyimide layer and the conductive copper layer are typically continuous throughout the board (including rigid layers and flexible layers). However, some designs limit the amount of flexible polyimide used to fill the rigid layer portion with a prepreg.
In terms of design, rigid-flexible bonding is considered a piece of folded circuit board. This reduces the total number of interconnects required in the system and avoids manual steps such as soldering flat ribbon cables to rigid boards.
Common rigid and soft combination configuration
Standard configuration: A symmetrical structure with a flexible layer on the center of the stack. It typically uses a uniform layer count similar to a standard multilayer PCB design.
Odd Layer Count Configuration: Although not common in traditional PCB designs, odd layer counts provide EMI shielding on both sides of the flexible layer to meet strip impedance control and electromagnetic compatibility requirements.
Asymmetric configuration: If the flexible layer is not in the center of the stack, it is considered an asymmetric configuration. Sometimes the requirements for impedance and dielectric thickness vary widely, resulting in a "top heavy" design. At other times, the blind hole aspect ratio can be reduced by an asymmetric structure. Since it is easy to deform and twist the design, it may be necessary to press the clamp.
Blind and buried vias: rigid-flex circuits support blind vias that connect the outer layer of the PCB to one or more inner layers without passing through the entire board; while buried vias connect one or more inner layers without Pass the outer layer. Complex via structures often require asymmetric structures when processing flexible layers.
Shielded flexible layer: Special shielding film (such as Tatsuta and APlus) laminated on a flexible layer. A special cover opening with a conductive adhesive contacts the shielding film to the ground. These films can shield the flexible regions without significantly increasing the thickness.
There are many different configurations possible for rigid and soft combinations. The number of layers between the rigid and flexible parts does not need to be matched, allowing us to fully customize the PCB design to fit the sealed enclosure; just make sure the design follows the quality standards specified in IPC 2223C.
To sum up
Rigid-flexible bonding helps us meet complex geometric or electromagnetic interference requirements, allowing us to use flexible circuits or rugged rigid boards wherever possible to minimize manufacturing and assembly costs.
Since rigid designs typically handle complex 3D requirements, it is essential to have a robust PCB design software that supports the overall design approach to bridge the gap between electromechanical fields.
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