Since the advancement of technologies in Rigid PCBs and Flexible PCBs, it now necessitates a stable Controlled Impedance that can tolerate high-speed designs without experiencing any issues. In terms of values, it may vary depending on the requirements of the materials used.
Nonetheless, the approximate range of impedance values lies between fifty-ohm to one hundred twenty-ohm per set. Hence, it may still change per the desired parameters.
In line with that, we’d like to thoroughly tackle the definition of Controlled Impedance in this article. Further, we’ll discuss its determinants, configurations, transition, and usage.
What is Controlled Impedance?
As its term indicates, the Controlled Impedance has an equilibrium performance in delivering its responsibility; thus, it can offer excellent and reliable High-Frequency performance. One way of achieving the appropriate Controlled Impedance is through the substrate and traces.
In line with this, to attain the desired value for an application, experts thoroughly match the properties of substrate material and the trace widths. In that way, we can achieve the Controlled Impedance we are highly seeking for noteworthy performance.
In some cases, Controlling the Impedance can be challenging, especially for those manufacturers that don’t have extensive experience in the industry since the PCB’s trace and environment must be comprehensively designed. In a situation wherein it is not properly performed, it can be difficult to attain the desired Controlled Impedance.
Typically, Controlled Impedance is highly in demand in High-Frequency applications since it can ensure that all of the signals exchanged in the platform are stable and reliable. Hence, it offers exceptional performance to the device.
Definition of Controlled Impedance
What Determines the Controlled Impedance in Rigid and Flex PCBs?
In essence, Controlled Impedance can be defined through various aspects; we have capacitive reactance, conductance, and resistance. These factors are affected by the dielectric profundity, the compression ratio of the element, and the dielectric constant at a particular frequency of the PCB base substance. Furthermore, its traces can vary from 25 ohms up to 125 ohms. However, depending on the structure, the impedance values may change. Stiff and Flexible Boards’ resistance levels are determined by a number of different variables.
Detailed Explanation of the Standard 50-Ohm Measurement
Rigid PCBs
- Dimensions of a Signal Trace
- The density of the Prepreg and the Dielectric Core
- Proximity toward other Copper Elements
Flexible PCBs
- Enclosure Pitch and Width
- FCCL Thickness Measured in Polyimide (PI)
- Dimensions of a Signal Trace per the Breadth and Depth
- Bonding Thickness Between FCCLs
- Parameters for the Dielectric Constant of Polyimide (PI), Coverlay, and Adhesive
Setting for Controlled Impedance
Typically, the way of attaining a Controlled Impedance in Flexible PCB is quite similar in Rigid PCBs; it utilizes the four (4) fundamental layer settings.
Embedded Microstrip
In respect of flexible schematic capture, the main distinction between the Microstrip and Stripline designs is the influence on the bendable width, which significantly influences the dynamic curve capacity and dependability of the circuitry. The Microstrip topologies do not have protection on one edge.
Edge Coupled Embedded Microstrip
If planning to utilize the optimum flexural capability, we recommend utilizing a 2-Layer Microstrip to achieve it. Once combined with 0.5 ounces of copper, a 0.002-inch flex foundation, and 0.0005-inch Coverlays after layering, the final flexibility thickness is approximately 0.006 inches. These standards are ten times the thickness criterion converts this to a minimum ductility of 0.060 inches.
Symmetrical Stripline
The Stripline versions hide the electronics on either side of the component but at the expense of seriously restricting the part’s capacity to flex and significantly increasing its curve thickness. The protective barrier improvement and matching dielectric network infrastructure are required, and the width of each core must increase from 0.002 to 0.003 inches. The resulting flexible thickness grows to more than 0.011 inches following lamination for a configuration using 0.5-ounce copper. The twist capacities are thus restricted to 0.200-inch and above.
Edge Coupled Stripline
In a nutshell, the fundamental piece in every configuration mentioned is the presence of 0.5 ounces of copper. When paired with the aforementioned core densities, this enables the sub-0.005″ branch widths required to attain the impedance values. Due to constraints in the solidification process, bars must be at least 0.005 inches wide when 1-ounce copper is utilized.
Along with that, the core widths would rise to 0.004 inches. The Microstrip version increases up to nearly 0.009 inches, and the Stripline variant covers nearly 0.016 inches when 1-ounce copper resistance lines are used. Usually, designs with resistance and greater currents are what fuel the demand for 1-ounce copper. To find the choices that will satisfy the curve criteria, we advise conducting a thorough design evaluation.
Transition Between Flexible to Rigid Impedance
A Rigid-Flex PCB architecture with controlled impedance is distinguished by the necessity of changing the impedance link layout as the circuitry moves from Flexible segments to the substantial community. In greater layer total designs, it is common for the impedance links in the malleable sections to be built as Microstrip. However, the layout may change into a Stripline since electricity and neutral pathways are needed in the rigid sections.
Furthermore, some higher layer densities are unlikely to be capable of supporting the additional thickness, but it is better to expand the rigid area’s fundamental thickness to the adjacent surface in order to maintain the same branch width and orientation in both the flexible and rigid areas. As the electronics transition from a pliable to a rigid section, the line width and spacing should be adjusted. Last but not least, in Stripline configurations, the bendable segment coordinates lines continue through the rigid areas, making this situation irrelevant.
Example Illustration
Higher Currents with Regulated Impedance
Once impedance control and higher current carrying need interplay, a design issue may occur. Once more, for the necessary parameters, the heavier copper joins with the resistance requirements to create an extra thick modular design that might not always meet the specified curvature requirements. In line with this, there are several approaches to address it; some of them will be thoroughly tackled underneath.
- During an Air-Gap layered bendable structure, detach electrical channels to divide the flexible sections.
- We recommend the usage of 0.5-ounce copper and a broader bend to enable bigger current lines.
- Use mobbed current lines on various flexible sections.
- Employ extra open room for corralled current lines and limit the standard planes to the limited regions immediately above, and beneath impedance sections.
- Although there are still other options existing, they can be discussed in future articles.
Overall, in order to consistently satisfy the curve standards, it is always important to reduce the adjustable width as much as necessary.
When Should You Employ Controlled Impedance?
Controlled Impedance must be considered and used when a transfer necessitates a particular impedance at harmonic polarities. In terms of High-Frequency devices, it’s crucial to coordinate the resistance of PCB lines in order to keep data fidelity and transmission intelligibility. Ultimately, the phase shift may expand and the system may encounter sporadic flaws if the impedance is not in accordance with the typical amplitude of the constituent.
Evaluating the Need of Controlled Impedance
Summary
In summary, there is a highly significant necessity to incorporate a Controlled Impedance in every Flexible and Rigid PCB because of their particular mechanical bending parameters. Furthermore, by integrating a Controlled Impedance, the device can achieve optimum performance with exceptionally high stability and reliability.
If you’re curious to learn more about Impedance Control; then, we suggest giving us a call or message so that we can swiftly assist you with any questions you have in mind. We at MV Flex Circuit want our consumers to comprehensively understand the concept of our services and products; hence, we are always here to accommodate all of your questions pertaining to the electronics industry. Further, if you want to avail of Controlled Impedance in your High-Frequency Devices, then feel free to message us.
Please get in touch with us immediately so that we can promptly fulfill your requests. In addition, you’ll have the option to take advantage of our daily specials.
