In the design of high-speed PCB circuit boards, the blank areas of the signal layers can be coated with copper. How should the copper coating on multiple signal layers be distributed between the grounding and power supply?
1. Generally, copper plating in the blank areas is primarily grounded. When applying copper next to high-speed signal lines, it is important to maintain a sufficient distance between the copper and the signal line, as the copper can slightly reduce the characteristic impedance of the trace. Additionally, ensure that this copper coating does not affect the characteristic impedance of other layers, such as in the structure of a dual strip line.
2. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line on the power plane? Can the signal between the power supply and the ground plane be calculated using the stripline model?
3. Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be considered as reference planes. For instance, in a four-layer board consisting of a top layer, a power layer, a ground layer, and a bottom layer, the characteristic impedance model for the top layer would use the power plane as the reference plane, modeled as a microstrip line.
4. Can test points be automatically generated by software on high-density printed boards to meet the test requirements of mass production?
5. Generally, whether the software can automatically generate test points depends on whether the test point specifications align with the requirements of the test equipment. If the wiring is very dense and the specifications for test points are strict, it might not be possible to automatically place test points on every segment of the line. In such cases, manual addition of test points may be necessary.
6. Will adding test points affect the quality of high-speed signals?
1. Whether signal quality is affected depends on the method of adding test points and the signal speed. Typically, additional test points (avoid using existing vias or DIP pins) may be added to the line or a short line may be pulled from the line. The former acts like adding a small capacitor, while the latter introduces an extra branch. Both conditions impact high-speed signals to varying degrees, with the effect related to the signal’s frequency and edge rate. The impact magnitude can be determined through simulation. Generally, smaller test points and shorter branches are preferable (as long as they meet test tool requirements).
2. When multiple PCBs form a system, how should the ground connections between the boards be handled? When connecting signals or power supplies between PCBs, for example, if board A sends power or a signal to board B, an equal current must flow from the ground back to board A (according to Kirchhoff’s current law). The ground current will seek the path of least impedance. Thus, at each interface, whether power or signal, ensure a sufficient number of ground pins to minimize impedance and reduce noise on the ground layer. Additionally, analyze the entire current loop, especially where current is high, and adjust ground layer or wire connections to manage current flow, possibly creating low-impedance paths to guide most of the current and mitigate impact on sensitive signals.
3. Can you recommend any foreign technical books and data on high-speed PCB design? High-speed digital circuits are now prevalent in communication networks and computing devices. For communication networks, PCB frequencies can reach around GHz, with layer counts up to 40. In computing, advancements in chips mean that boards can reach frequencies up to 400 MHz (e.g., Rambus). This demand for high-speed, high-density wiring has increased requirements for blind/buried vias, microvias, and build-up processes. These design requirements are available for mass production by manufacturers.
4. Two commonly referenced characteristic impedance formulas are:
– Microstrip Z = {87 / [sqrt(Er + 1.41)]} ln[5.98H / (0.8W + T)] where W is the line width, T is the copper thickness, H is the distance from the trace to the reference plane, and Er is the dielectric constant of the PCB material. This formula applies when W/H < 0.35 and T/H < 0.25.
– Stripline Z = [60 / sqrt(Er)] ln{4H / [0.67π(T + 0.8W)]} where H is the distance between the two reference planes, and the trace is centered between them. This formula is used when W/H < 0.35 and T/H < 0.25.
5. Can a ground wire be added to a differential signal line? Typically, adding a ground wire in the middle of a differential signal is not advisable. The key advantage of differential signals lies in their coupling benefits, such as flux cancellation and noise immunity. Adding a ground wire would disrupt this coupling effect.
6. Does rigid-flex board design require special software and specifications? Where can such circuit board processing be undertaken in China? General PCB design software can be used for flexible printed circuit (FPC) design, which is produced in Gerber format. Due to different manufacturing processes, each FPC manufacturer has specific minimum line widths, spacing, and apertures. Reinforcement with copper at flexible circuit board turn points is also possible. For manufacturers, searching online with the keyword “FPC” will yield relevant results.
7. What is the principle for selecting grounding points between the PCB and the case? The principle is to use the chassis ground to provide a low-impedance path for returning current and control its path. For instance, near high-frequency devices or clock generators, connecting the PCB ground to the chassis ground with fixing screws minimizes the current loop area and reduces electromagnetic radiation.
1. Generally, copper plating in the blank areas is primarily grounded. When applying copper next to high-speed signal lines, it is important to maintain a sufficient distance between the copper and the signal line, as the copper can slightly reduce the characteristic impedance of the trace. Additionally, ensure that this copper coating does not affect the characteristic impedance of other layers, such as in the structure of a dual strip line.
2. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line on the power plane? Can the signal between the power supply and the ground plane be calculated using the stripline model?
3. Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be considered as reference planes. For instance, in a four-layer board consisting of a top layer, a power layer, a ground layer, and a bottom layer, the characteristic impedance model for the top layer would use the power plane as the reference plane, modeled as a microstrip line.
4. Can test points be automatically generated by software on high-density printed boards to meet the test requirements of mass production?
5. Generally, whether the software can automatically generate test points depends on whether the test point specifications align with the requirements of the test equipment. If the wiring is very dense and the specifications for test points are strict, it might not be possible to automatically place test points on every segment of the line. In such cases, manual addition of test points may be necessary.
6. Will adding test points affect the quality of high-speed signals?
1. Whether signal quality is affected depends on the method of adding test points and the signal speed. Typically, additional test points (avoid using existing vias or DIP pins) may be added to the line or a short line may be pulled from the line. The former acts like adding a small capacitor, while the latter introduces an extra branch. Both conditions impact high-speed signals to varying degrees, with the effect related to the signal’s frequency and edge rate. The impact magnitude can be determined through simulation. Generally, smaller test points and shorter branches are preferable (as long as they meet test tool requirements).
2. When multiple PCBs form a system, how should the ground connections between the boards be handled? When connecting signals or power supplies between PCBs, for example, if board A sends power or a signal to board B, an equal current must flow from the ground back to board A (according to Kirchhoff’s current law). The ground current will seek the path of least impedance. Thus, at each interface, whether power or signal, ensure a sufficient number of ground pins to minimize impedance and reduce noise on the ground layer. Additionally, analyze the entire current loop, especially where current is high, and adjust ground layer or wire connections to manage current flow, possibly creating low-impedance paths to guide most of the current and mitigate impact on sensitive signals.
3. Can you recommend any foreign technical books and data on high-speed PCB design? High-speed digital circuits are now prevalent in communication networks and computing devices. For communication networks, PCB frequencies can reach around GHz, with layer counts up to 40. In computing, advancements in chips mean that boards can reach frequencies up to 400 MHz (e.g., Rambus). This demand for high-speed, high-density wiring has increased requirements for blind/buried vias, microvias, and build-up processes. These design requirements are available for mass production by manufacturers.
4. Two commonly referenced characteristic impedance formulas are:
– Microstrip Z = {87 / [sqrt(Er + 1.41)]} ln[5.98H / (0.8W + T)] where W is the line width, T is the copper thickness, H is the distance from the trace to the reference plane, and Er is the dielectric constant of the PCB material. This formula applies when W/H < 0.35 and T/H < 0.25.
– Stripline Z = [60 / sqrt(Er)] ln{4H / [0.67π(T + 0.8W)]} where H is the distance between the two reference planes, and the trace is centered between them. This formula is used when W/H < 0.35 and T/H < 0.25.
5. Can a ground wire be added to a differential signal line? Typically, adding a ground wire in the middle of a differential signal is not advisable. The key advantage of differential signals lies in their coupling benefits, such as flux cancellation and noise immunity. Adding a ground wire would disrupt this coupling effect.
6. Does rigid-flex board design require special software and specifications? Where can such circuit board processing be undertaken in China? General PCB design software can be used for flexible printed circuit (FPC) design, which is produced in Gerber format. Due to different manufacturing processes, each FPC manufacturer has specific minimum line widths, spacing, and apertures. Reinforcement with copper at flexible circuit board turn points is also possible. For manufacturers, searching online with the keyword “FPC” will yield relevant results.
7. What is the principle for selecting grounding points between the PCB and the case? The principle is to use the chassis ground to provide a low-impedance path for returning current and control its path. For instance, near high-frequency devices or clock generators, connecting the PCB ground to the chassis ground with fixing screws minimizes the current loop area and reduces electromagnetic radiation.
