Differences between rigid-flex PCB assembly and multi-board system assembly

What is a rigid-flex PCB?


In the standard multi-layer circuit board PCB design, we use the circuit board concept to divide different functional circuits onto smaller circuit boards, and use a variety of interconnections to place the system into a single housing.


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 conductivity that meet our size requirements do not always exist; the best alternative is the cable, but the cables are not practical and they do not meet the space requirements of the case.


If a multilayer circuit board design requires us to interconnect several rigid circuit boards in a compact housing with high-level numbers and high-speed connection requirements, then the combination of rigid-flexible combination is the best solution.


What is rigid-flexible combination? Simply put, it is to connect two or more rigid circuit boards through flexible parts for circuit connection.


A single flexible layer usually consists of the following materials:

● Flexible polyimide core;

● conductive copper layer;

● Adhesive


The conductive copper layer is sandwiched between the flexible polyimides on both sides by an adhesive. The polyimide layer and the adhesive layer are usually considered as a unit (called a cover layer), which can be laminated on the copper layer by heat and pressure. There can be multiple flexible layers in any given design.


The rigid part is added to the flexible layer through a rigid layer of standard PCB material:

● Pre-impregnated material filled with glass fiber with resin will flow and stick 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 conductive copper layer are usually continuous throughout the circuit board (including rigid and flexible layers). However, some designs limit the amount of flexible polyimide, and the rigid layer portion is filled with prepreg material.


In terms of design, the rigid-flex joint is considered a foldable 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 circuit boards.


Common rigid-flex configurations


Standard configuration: symmetrical structure with flexible layers at the center of the stack. It usually uses a uniform layer count similar to a standard multilayer PCB design.


Odd-layer counting configuration: Although it is not common in traditional PCB design, odd-layer counting can provide electromagnetic interference shielding function on both sides of the flexible layer to meet the requirements of line impedance control and electromagnetic compatibility.


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 "heavy top" designs. At other times, the aspect ratio of blind holes can be reduced by asymmetric structures. Since it is easy to deform and twist the design, it may be necessary to compact the clamp.


Blind and buried vias: Rigid-flex circuits support blind vias. Blind vias connect the outer layer of the PCB to one or more inner layers without passing through the entire circuit board; while buried holes connect one or more inner layers without Pass the outer layer. When dealing with flexible layers, complex via structures often require asymmetric structures.


Shielded flexible layer: A special shielding film laminated on a flexible layer (such as Tatsuta and APlus). A special covering opening with a conductive adhesive brings the shielding film into contact with the ground. These films can shield flexible areas without significantly increasing the thickness.


There are many different configurations possible for rigid-flexible combinations. The number of layers between the rigid and flexible sections does not need to be matched, so that we can achieve full customization to make the PCB design suitable for a sealed enclosure; just make sure the design complies with the quality standards specified in IPC 2223C.


to sum up


The combination of rigidity and flexibility can help us meet complex geometric or electromagnetic interference requirements, so that we can use flexible circuits or rigid and reliable rigid circuit boards when necessary to minimize manufacturing and assembly costs.


Because rigid designs often handle complex 3D requirements, it is necessary to have powerful PCB design software that supports the overall design approach to bridge the gap between the mechanical and electrical fields.