In today’s digital and electronic world, with higher frequency electronics and the miniaturization of devices, it is becoming increasingly important to effectively control the electrical signals in the high-speed printed circuit boards used.
PCB designs and components are increasingly becoming smaller, faster, and more complex. It has become essential to have a good comprehension of important connections, circuit characteristics, and how the PCB influences signal performance. Circuit speeds are continually on the rise, with signals in the gigahertz range becoming very common. Maintaining precise circuit impedance is a critical factor to ensure good signal integrity and optimize the performance of printed circuit boards.
The importance of controlled impedance in circuit boards involves several reasons including signal integrity, reduction of cross-talk, prevention of signal loss, reduction of EMI (electromagnetic interference), and synchronization/timing of higher speed data rates.
What is Impedance?
In printed circuit boards, “impedance” refers to the total opposition a circuit trace offers to the flow of alternating current (AC) signals. This is essentially a combination of its resistance, inductance, and capacitance, which affects how signals propagate through the circuit board. Impedance is measured in Ohms, similar to resistance. This becomes more critical at high frequencies.
Controlling impedance on a printed circuit board is critical for maintaining signal integrity and minimizing signal reflections when working with higher-speed frequencies and high-speed signals. High-speed PCBs are essential components in electronic devices and systems that require rapid data processing and signal transmission, such as telecommunications equipment, data centers, networking devices, high-performance computers, aerospace and defense systems, and advanced consumer electronics.
What Controls the Impedance?
There are many variables that require consideration when implementing controlled impedance into printed circuit board design, and these play crucial roles in achieving impedance control.
The circuits in purple are examples of differential pair-controlled impedance circuit structures on the outer copper layer of the PCB (two sets).
Some main factors include:
- Circuit width and spacing: Depending on other variables, impedance geometries are one of the main design factors.
- Layer stack-up: How layers are arranged affects impedance control, properly layer arrangement can help impedance by providing reference planes and reducing EMI.
- Dielectric material: This significantly influences impedance, as high-frequency signals require dielectric materials with low dissipation factor to minimize signal loss.
- Transmission line length: The length of the circuit affects the signal’s propagation time, and therefore impedance control. For high-speed designs, keeping the transmission line lengths as short as possible is essential.
- Differential pair routing: For high-speed designs utilizing differential pair impedances, maintaining precise spacings helps to ensure signal balance and minimizes “noise”.
- Via design: Using “stitching” vias to connect ground planes or reference planes helps maintain uniform impedance through the PCB.
- Reference planes: Ensuring a continuous reference plane beneath the impedance control signal traces helps by providing a consistent return path for the signals. Using cross-hatched ground planes with voids can help minimize parasitic capacitance between the signal trace and the ground plane.
How is Impedance Tested?
Impedance-controlled printed circuit boards are tested by using tools such as Time-Domain Reflectometry (TDR) or Vector Network Analysis (VNA). PCB manufacturers typically use the TDR method for testing the raw bare unassembled printed circuit board. As it is impractical to test each individual impedance circuit, or each individual unassembled PCB for that matter, the general practice for manufacturing is to add a “coupon” (or “coupons”) to each production panel used during processing.
These coupons are designed to mimic each required impedance structure for the PCB (i.e., the geometries on the coupon are exactly what is in the PCB itself). The coupons are kept together with their corresponding parts from the same production panel and are then tested at final processing to determine if parts meet all the impedance requirements.
How is Impedance Best Communicated to Epec?
When impedance control is required for a printed circuit board design, it is desired to have a drawing or notations that define all of the required impedance parameters. These should include:
- A layer stack-up defining the printed circuit board layer order, if possible. This information should include copper layer definition (signal layers vs. plane layers), copper weights for each copper layer, and dielectric spacings between the copper layers.
- Desired material to be used, standard FR4, high-temperature FR4, or other more exotic material (ceramics, polyimides).
- The impedance structures and geometries (i.e., single-ended/differential microstrip outer layers, single-ended/differential strip line inner layers, coplanar waveguide with or without ground, etc.). These structures should have circuit widths and spacings defined for the controlled impedance geometries, and the reference copper plane layers used for each structure, (for example, 50 Ohms single-ended for 5 mil circuits, layer 1 reference to layer2 plane).
- The targeted impedance values for any impedance structures required in design, and the tolerance required for impedance measurements (for example, 50 Ohms Single Ended +/- 10%).
However, even without complete attributes, Epec can help with the design and structure of impedance-controlled printed circuit boards. We have a robust material library and impedance calculator to help customers with impedance layouts and stack-ups to ensure impedance control requirements.
Summary
Controlled impedance design is not just a matter of technical precision; it's a necessity for achieving consistent signal performance and minimizing signal distortion. Impedance matching ensures that signals propagate with minimal loss, interference, or reflections, allowing electronic devices to function as intended. When designing a controlled impedance printed circuit board, most CAD/CAM software has an internal module to help with stack-ups and layouts for impedance control (for example Altium, Cadence, etc.). Also, there are many free online layout tools and free online impedance calculators that can be used to help with designs.
Whether it's in the telecommunications industry, aerospace, automotive, or even consumer electronics, controlled impedance is vital for reducing signal degradation, enhancing signal quality, and ultimately enabling the seamless and reliable operation of electronic systems. In essence, controlled impedance is the linchpin that holds together the intricate web of high-speed data communication, ensuring that the signals arrive at their destinations intact and on time.
Key Takeaways
- Importance of Impedance Control: Controlled impedance is essential in high-speed PCB designs to maintain signal integrity, reduce electromagnetic interference (EMI), prevent signal loss, and enable reliable high-frequency performance.
- Definition of Impedance in PCBs: Impedance is the total opposition a circuit trace offers to AC signal flow, influenced by resistance, inductance, and capacitance. It becomes increasingly critical in gigahertz-range high-speed designs.
- Applications Requiring Impedance Control: Industries like telecommunications, aerospace, automotive, and advanced consumer electronics rely on controlled impedance to ensure consistent and reliable high-speed data transmission.
- Key Factors Influencing Impedance:
- Circuit Width and Spacing: Critical for achieving target impedance geometries.
- Layer Stack-Up: Proper arrangement reduces EMI and provides consistent reference planes.
- Dielectric Materials: Low-loss materials minimize signal degradation.
- Transmission Line Length: Shorter lines improve propagation time and control.
- Via Design: Stitching vias ensure consistent impedance through layers.
- Reference Planes: Continuous planes improve return paths and minimize parasitic capacitance.
- Impedance Testing Techniques:
- Time-Domain Reflectometry (TDR): Commonly used to test unassembled PCBs.
- Test Coupons: Mimic impedance structures on production panels for reliable testing during manufacturing.
- Communicating Impedance Requirements:
- Provide a layer stack-up with copper layer definitions, weights, and dielectric spacings.
- Specify materials (e.g., FR4, polyimide) and impedance structures (e.g., microstrip, stripline).
- Define target impedance values, geometries, and tolerances.
- Tools for Impedance Design: CAD/CAM software and online calculators can assist with designing stack-ups and layouts for impedance-controlled PCBs, ensuring precision and manufacturability.
- Benefits of Controlled Impedance:
- Minimized Signal Loss: Ensures signals propagate with minimal interference.
- Enhanced Signal Quality: Reduces reflections and distortion.
- Optimized Performance: Enables seamless operation of high-speed and complex electronic systems.
- Epec’s Support for Impedance Control: Epec offers robust material libraries and impedance calculators to assist with PCB designs, ensuring compliance with controlled impedance requirements.
- Essential for High-Speed Communication: Controlled impedance is critical for reducing signal degradation and ensuring accurate, timely data transmission in modern electronic devices.
