How Rigid-Flex PCB Design Configurations are Advancing
Rigid flex PCB technology is found across all commercial, industrial, and military applications. The ubiquitous boards are used in cell phones, smart devices, wearables, and digital capers. They are especially useful in the medical industry for the development of pacemakers due to their lower weight, space, and flexibility.
These tiny, multi-layered systems are improving consistently, especially thanks to new innovations. In this article, we will discuss how rigid flex PCB board design configurations are advancing.
Advancements in Rigid Flex PCB Boards
New designs in rigid-flexible circuits are being manufactured with much higher layer counts of the flexible sections. Some of these designs include buried and blind via structures, zero insertion force connections, and electrical components mounted on both rigid and flexible sections of the circuit.
Certain rigid flex board designs also feature designs that account for EMI. Some designs have asymmetrical constructions and variable thicknesses between rigid areas of the circuit.
Standard Rigid Flex PCB Designs
A standard rigid flex PCB design features a symmetrical construction because it allows for impedance control. Ideally, the flex layers should be located in the center of the design, with even layer counts in both flex and rigid areas.
Odd Layer Counts in Rigid-Flex PCB Designs
Although most fabricators are familiar with even layer count, the odd layer count is worth getting into. It has various benefits such as RF and EMI considerations and a large number of interconnects between rigid sections.
Stripline impedance control requires the use of the odd layer construction to provide two sided shielding in the flex areas. The three layers of flex contain the ground - signal - ground construction. This arrangement allows for a large number of interconnects.
It is possible to have an even layer count on one side and odd on the other; if needed. The main benefit is that it reduces the flex area thickness and increases flexibility as well as mechanical bending properties, which in turn, improves mechanical bend reliability.
This design strategy allows manufacturers to comply with IPC 2223C and ensures both parts have reliability in the short- and long-term. Finally, odd layer designs are more affordable because they minimize the total number of flex layers required in any design.
Asymmetrical Rigid Flex PCB Designs
Another type of construction that is very popular with manufacturers is asymmetrical design. Applications for asymmetrical rigid-flex PCB designs depend on impedance requirements and variance in the thickness of dielectric material within the design.
With that said, the asymmetrical build of the circuit may lead to warp and twist during assembly. A hold-down fixture can be used to allow the circuit to be transported.
In general, the asymmetrical build has no manufacturing problems besides the warps and twists that may be introduced into the circuit during the assembly process.
Varying Flex Layer Count
The next rigid-flex PCB construction is where each layer of the flex part varies between rigid sections. For instance, if a board has three rigid sections, the second and third sections may have one or two flex layers connecting them with three or four layers connecting the first to the second. This design has lots of variations in terms of configuration possibilities.
Blind and Buried Vias in Rigid Flex PCB
Rigid flex PCB boards can also use blind and buried vias. Their applications overlap those of rigid circuit boards. Fabricators use blind vias and buried vias in high-density applications, such as fine pitch component mounting and BGA that may require via-in-pad designs.
Circuits that require blind or buried vias may also utilize an asymmetrical construction to interconnect the flexible circuits.
The number of sequential laminations cycles required by the fabricator may impose limitations on the configuration. In most cases, rigid flex PCBs with multiple layers only facilitate a limited number of cycles of lamination before running into physical limitations that prevent the layers from effectively registering to one another.
Integrated ZIF Tail Boards
A ZIF tail integration into the rigid flex board is a very common stack up. This construction reduces the need for a flex circuit with a ZIF connector mounted onto the rigid board. In turn, the board reduces requirements of real estate in the rigid part. The design is particularly useful with high-density PCBs where space is hard to find or the small form factor makes it impossible to install a ZIF connector.
The integrated ZIF tail construction also improves the durability of the board because it eliminates the need for a rigid section, connector, and interconnect points. In some cases, an additional stiffener using polyimide may be required to achieve the required thickness requirements of the contact area of the ZIF connector.
The combination can make use of multiple layers of flex, in twos, threes, and fours. Anything beyond four layers imposes a practical limit as it becomes difficult to meet the thickness requirements of the ZIF connector. It is recommended to only use one or two-layer flex configurations.
Shielded Flex Layers
Applications that require EMI or FR shielding will use this configuration. The flex area coverlays feature multiple selective openings that expose the ground circuit. Using this configuration saves manufacturers the expense of applying copper layers and also allows the flex construction to be thinner.
Overall, this is a cost-effective solution that increases the mechanical bend capabilities and flexibility of the flex layer.
Air Gap Construction for Rigid Flex PCB
Rigid Flex PCBs with Air Gap construction utilize isolated independent pairs of flex layers to substantially improve flexibility. Hemeixin PCB recommend recommends air gap constructions in boards that utilize two or more layers of flex. This method is especially effective when used with four or more layers and allows the circuit to align with IPC 2223C requirements.
Using air-gap construction eliminates the need for using flexible adhesives within rigid areas. This can provide a high degree of reliability.
Multiple Rigid Area Thickness - Rigid Flex PCB Design
Although this construction is complex., some designs may need rigid flex PCB designs in the case of multiple rigid areas with varying thicknesses. Although they can be used in various applications, It is strongly recommended to ask your manufacturer for alternatives, if possible.
Besides the practical limitations of a maximum of two rigid area thicknesses, the stackup is expensive. However, some applications may demand multiple rigid area thickness constructions.
Using rigid PCB to flex circuits unlocks new possibilities for manufacturers in terms of design integration, functionality, and weight reduction. Many PCB designs that we have reviewed in this article can be combined to create an endless combination of flex and rigid-flex circuit configurations.