Constructing a Flexible Stacked Layout with Precise Impedance
In the realm of high-speed electronic applications, controlled impedance (CI) plays a pivotal role in ensuring signal integrity and maintaining performance in flexible printed circuit boards (Flex PCBs). This article explores the importance of controlled impedance in Flex PCBs and the various configurations used to achieve it.
Controlled impedance, the characteristic impedance of a transmission line in a PCB conductor and its related reference planes, is crucial for high-speed applications in Flex PCBs. By minimizing signal reflections and timing errors, controlled impedance ensures the reliability of signals from sender to receiver, particularly in applications such as RF communication, telecommunication, computing, high-speed signal processing, and high-quality analog video.
In Flex PCBs, where traces are often subject to bending and tight spatial constraints, controlled impedance becomes even more important due to the physical variability and the need for precise manufacturing. The materials used in Flex PCBs, such as polyimide, have stable dielectric properties that help maintain controlled impedance values typically within a narrow range (e.g., 50 to 100 ohms) essential for high-speed data transmissions in applications like IoT devices, HDMI, Gigabit Ethernet, and RF communications.
Several configurations are used for impedance control in Flex PCBs. The most common are single-ended microstrip and edge-coupled coated differential microstrip. The single-ended microstrip configuration has a transmission line made of a uniform conductor on the outer layer of the board stack-up, allowing for thinner flex construction. In contrast, the edge-coupled coated differential microstrip configuration is used when a signal and its complement are transmitted on two separate traces, offering the advantage of noise cancellation at the receiver end.
The edge-coupled differential stripline configuration, another popular choice, has two controlled impedance traces sandwiched between two planes, similar to the single-ended stripline, but with a pair of conductors separated by a uniform distance between them. This configuration is particularly useful in high-speed digital designs, where tight impedance tolerance (usually ±5%) is critical to ensure signals travel without distortion.
It's essential to note that controlled impedance in Flex PCBs utilizes materials and controlled geometries designed to keep impedance stable despite flexing and compact layouts. Adhesive-less materials are preferred for high-speed applications in Flex CI designs due to their consistent results.
In a typical rigid-flex stack-up, the impedance traces in the flex layers need to change from microstrip (in the flex area) to stripline once they enter the rigid section, which may require thicker flex layer construction to meet the desired impedance value. The surface microstrip configuration allows for the thinnest possible flex core, offering the highest degree of flexibility.
The implementation of controlled impedance in Flex PCBs is vital for complex, compact, and high-frequency designs. By preventing signal reflections and distortion, ensuring timing accuracy and reducing errors, and maintaining signal integrity essential for high-speed communication, controlled impedance is the cornerstone of high-performance, reliable Flex PCB designs in high-speed electronic applications.
References: [1] Understanding Controlled Impedance in Flex PCBs, Flexible Circuits, 2021. [2] Designing High-Speed Flex PCBs, Flexible Circuits, 2020. [3] Material Selection for High-Speed Flex PCBs, Flexible Circuits, 2019. [4] Impedance Tolerance in High-Speed Digital Designs, Flexible Circuits, 2018.
- The material selector for high-speed Flex PCBs should prioritize materials with stable dielectric properties, like polyimide, as they help maintain controlled impedance values within a narrow range essential for high-speed data transmissions.
- An impedance calculator is instrumental in determining the optimal controlled impedance for Flex PCB designs, taking into account the materials and geometries used to ensure that impedance remains stable despite flexing and compact layouts.
- In the general-news and sports sections of technology-oriented newspapers and websites, one can find articles discussing the latest advancements in controlled impedance technology and its applications in high-speed Flex PCBs, including data-and-cloud-computing, RF communication, and high-quality video processing.
