Signal fidelity is a key to success in designs employing high speed digital or analog circuits across complex multi-layer printed circuit board (PCB) systems. The transmission line impedance must be accurately maintained across the important signal paths as intended by the design. By working with Macrofab as your PCB partner to identify key traces and sensitive areas of the circuit for matching, the impedance across the system board can be maintained for successful prototyping and manufacturing.
What is Trace Impedance?
Trace impedance is the sum of all the resistance and reactance components of an electrical signal path. An ideal transmission line must have a characteristic impedance that matches both the transmitter and receiver of the intended signal. If the characteristic impedance of the matched transmission line is faithfully maintained, then the full signal sent by the transmitter is seen by the receiver at the end of the trace. There would be no reflection or attenuation of the signal. The full amount if signal power is absorbed by the receiver. For mismatched impedance, a reflection will occur at the far end and the complete signal will not be seen accurately at the receiver destination. Therefore, in practice, the details of the impedance control within the system need to be carefully considered.
For single-ended line transmission of an I/O signal, a 50 ohm impedance is often the required matching characteristic. The characteristic impedance of the signal path is a continuous 50 ohm path relative to the closest ground. The signal will return on the nearest ground path to the trace. Several applications have specific impedance requirements other than 50 ohms, based on legacy standards. Analog CATV video signals require a 75 ohm single-ended characteristic impedance, for example.
Real world parasitic effects and materials that may not be matched cause the ideal transmission line to become a bit more complex. Non-idealities associated with the board layout and assembly can cause signal loss and reflections without careful review of the design. The characteristic impedance of a trace is determined by the stack up geometry and PCB materials used within the transmission line. If this is accurately manufactured, the characteristic impedance will be independent of the trace length. This means that a 50 ohm impedance line could be 1mm, 1cm, 1m or 1 km or more in length.
High-speed differential communication signals are not primarily referenced as signal-ended to ground. Instead, a signal pair is used to transmit and receive high speed digital or RF data. In place of a single-ended specification, maintaining a consistent 100 ohm differential impedance between the + and – signals within the pair is paramount. In this case, while measuring down the + signal of the pair, the trace must maintain close to a 100 ohm impedance relative to the – side of the pair. The tolerance of the 100 ohm requirement across the trace pair is often stated within the IEEE standard, such as USB, HDMI or JESD204B.
Any unique impedance control considerations that need to be recognized within the design must be communicated using additional files to correctly manufacture your PCBs. The file information will identify the impedance controlled traces and their resistance, width, and spacing. An assembly document can show the traces requiring impedance control, provided as either a separate Gerber file or an image with the traces marked. Impedance control requirements are selected at the time of PCB request.
Dielectric PCB Material
The dielectric choice for the board will dictate the signal bandwidth that can be sent down a PCB trace. As signal rates increase for high-speed analog and RF applications, normal FR4 material may not provide adequate bandwidth for the system board. Exotic dielectric materials can be used for high bandwidth applications in lieu of standard FR4 material. Check with Macrofab to determine if your selected premium dielectric materials are in stock or need to be ordered for your design. For more information about the physical properties of the PCB material MacroFab uses and stack up information check the knowledge base.
A special trace for microwave frequency signals is called a coplanar waveguide. A conventional coplanar waveguide consists of a single conducting trace printed onto the dielectric material. A typical conducting material is copper. A return conductor pair is established on either side of the primary trace. All three of these conductors are on the same side or plane of the PCB. They are therefore coplanar. The return conductors are spaced from the central track by a small gap, which has a consistent width along the length of the trace. The return conductors extend a longer distance relative to the primary signal to form a complete ground return. Within a coplanar waveguide, the impedance control of the primary trace relative to each conductor is important in order to maintain the signal integrity.
At higher frequencies, the PCB signal trace impedance will depend on the geometry of the circuit, so it has to be calculated. These calculations are complex. The impedance will depend on 4 parameters:
- Height (H) of the dielectric stack of material used between the signal trace of interest and the signal return plane. Keep in mind that the signal return could be on the same plane as the signal, as in a coplanar waveguide, or may be on a different plane.
- The trace conductor thickness (T)
- Width (W) of the signal trace conductor material. Outer PCB traces are often plated, providing a 20% uncertainty in exterior traces.
- Dielectric constant (Er) or relative permittivity of the material chosen for the PCB design. The dielectric constant is a measure of the electrical characteristics of the dielectric material. It is fixed once the material is chosen for your PCB. A typical value for FR4 may be 4.4. Small variances in Er can have a large impact on the final impedance tolerance. Only certain specialty materials have well-defined dielectrics.
The impedance of traces is defined by more than only the size of the trace. When a trace is defined to require controlled impedance, the accuracy of the impedance itself is of higher importance than the geometry of the layout feature. In order to maintain the impedance accuracy, Macrofab may change a trace width, trace height, or dielectric thickness given in the layout Gerber file. This will ensure that the final impedance is within the tolerance.
PCB Prepregs Materials
Prepregs are semi-cured materials used as bonding materials between two core laminates within a PCB. After the lamination step in the manufacturing process, the final thickness of the prepreg depends on the percentage of copper in adjoining conducting layers, the height of the copper, and the specific type of the prepreg used within the design.
During the lamination process, a high degree of process control and integrity must be maintained. Post-lamination thicknesses of prepregs are reasonably predictable. The resin content of the PCB material is important as the percentage of the resin content has a great impact on the final thickness. The higher the thickness of the dielectric material, the lower the dielectric constant of the PCB material will be.
High-speed materials have lower dielectric constants and are suitable for applications requiring transmission of high-speed signals, usually having signal frequencies above 500 MHz. A layout designer must include the impedance information in the fabrication drawing notes and tables. The information should include the impedance value, the trace width, the spacing for differential pairs and the layer on which the control impedance traces are routed.
For high-speed digital circuits, it is not the signal toggle rate that determines the signal bandwidth. The rise and fall edge times contain the highest frequency components of the signals. In these applications, the bandwidth of the edge rates needs to be computed to properly design for the required dielectric. The bandwidth can be calculated using the measured or required 10-90% rise time of the digital signal. For high-speed GHz signals, we can simplify the equation using the equation below.
BW(GHz) = 0.35/Risetime(nSec)
The width of the trace and height of the dielectric stack-up can be adjusted as needed in order to maintain the tolerance across a signal transmission line for +/- 15% of the ideal impedance value. Achieving this accuracy requires a good understanding of the Er values and experience about how dielectric laminates behave.
Your printed circuit board will likely fall into one of three classes of service for impedance classification:
- No impedance control. In some cases, a circuit design is low-speed and relatively simple. The impedance tolerance is not particularly important and can be loose. No extra precautions need to be taken for the PCB. This is the fastest and least expensive PCB option.
- Impedance watching. The circuit designer must identify a particular impedance control trace(s). Macrofab must adjust the width of the trace as well as the height of the dielectric stack. The designer provides final approval on the proposed specifications before any manufacturing starts.
- Impedance control. Printed circuit boards with controlled impedance throughout require a meticulous level of accuracy in order to function correctly. PCB designers must specify trace impedance and tolerance required. They must work with Macrofab throughout the manufacturing review process to confirm that their specifications are met. These requirements are usually reserved for high-end performance designs or those requiring a very tight tolerance.
Signals that travel across multiple PCB layers using a via will see a change in impedance through the via. The impedance control through a via is harder to manage than through a predefined stack-up. For the highest sensitive traces, prioritize the routing to maintain signals on a single plane if at all possible. This should include moving transmit and receive components to the top or bottom layers, where the impedance control is this highest. Using internal ground planes between PCB signal layers provides a consistent return signal path and is a good design practice to maintain controlled impedance.
Final empirical measurements on PCB test boards can be measured via a Time Domain Reflectometry (TDR) test to determine if impedance specifications are met. Test equipment that measures TDR will send many pulses and observe the responses to show the impedance down the entire transmission line. An open, or unterminated line, will exhibit infinite impedance at the end of the trace.
System impedance control success depends upon planning during the design phase. The design team must identify any critical signal paths and sensitive traces. Those that need to have controlled impedance should be identified early and given special routing priority. A dielectric material should be chosen that permits the maximum signal integrity, given the bandwidth of the signals within the circuit. The information for the key signals must be communicated and outlined to Macrofab in order to prepare the design for fabrication. The resulting success of the system will warrant this attention to detail.
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