PCB News - Common sense of high frequency PCB wiring (2)

Common sense of high frequency PCB wiring (2)

PCB manufacturers explain: 1. The blank area of the signal layer can be coated with copper, and how to distribute the copper coating of multiple signal layers on grounding and power supply

Generally, most of the copper plating in the blank area is grounded. Just pay attention to the distance between the copper and the signal line when applying copper next to the high-speed signal line, because the applied copper will reduce the characteristic impedance of the trace a little. Also be careful not to affect the characteristic impedance of other layers, for example in the structure of 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? Whether the signal between the power supply and the ground plane can be calculated using the strip line model

Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be regarded as reference planes. For example, a four-layer board: top layer-power layer-ground layer-bottom layer. At this time, the characteristic impedance model of the top layer is a microstrip line model with the power plane as the reference plane.

3. Can the test points for mass production be satisfied under normal conditions through the software initiatively generating test points on the high-density printed board?

Generally, whether the software automatically generates test points to satisfy the test requirements depends on whether the standard for adding test points meets the requirements of the test equipment. In addition, if the wiring is too dense and the criteria for adding test points are relatively strict, it may not be possible to actively add test points to each segment of the line. Of course, you need to manually fill in the place you want to test.

4. Will adding test points affect the quality of high-speed signals?
Whether it will affect the signal quality depends on the method of adding test points and how fast the signal is. Basically, additional test points (do not use the existing via or DIP pin as test points) may be added to the line or a short line drawn from the line. The former is appropriate to add a small capacitor on the line, the latter is an extra branch. Both conditions will
It will affect the high-speed signal more or less, and the degree of the effect is related to the frequency speed of the signal and the edge rate of the signal. The magnitude of the impact can be known through simulation. According to the criteria, the smaller the test points, the better (of course, the requirements of the test equipment must be satisfied), and the shorter the branch, the better.

5. Several PCBs constitute a system, and how to connect the ground wires between the boards
When the signal or power supply between each PCB board is connected to each other, for example, when the A board has a power supply or a signal is sent to the B board, there must be an equal amount of current flowing from the ground layer back to the A board (this is Kirchoff current law). The current on this stratum will find the place with the least impedance to flow back. Therefore, at each interface, whether it is a power supply or a signal, the number of pins allocated to the ground layer should not be too small to reduce the impedance, which can reduce the noise on the ground layer. In addition, it can also analyze the entire current loop, especially the part with larger current, adjust the connection of the ground layer or ground wire to control the current flow (for example, make a low impedance somewhere so that most of the current flows from This local walk), decrease the impact on other more sensitive signals.

6. Can you introduce some foreign technical books and data on high-speed PCB planning?
Nowadays, high-speed digital circuits are used in related fields such as communication networks and accounting devices. In terms of communication network, the operating frequency of the PCB board has reached around GHz, and the number of stacked layers is as high as 40 layers as far as I know. The related use of the calculator is also due to the progress of the chip, whether it is a general PC or a server (Server), the highest operating frequency on the board has now reached more than 400MHz (such as Rambus). In response to this demand for high-speed and high-density wiring, the requirements for blind/buried vias, mircrovias, and build-up processes are gradually increasing. These planning requirements have manufacturers that can produce in large quantities.

7. Two characteristic impedance formulas that are often referred to:
Microstrip Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] In the meantime, W is the line width, T is the copper thickness of the trace, and H is The distance between the trace and the reference plane, Er is the dielectric constant of the PCB board material. This formula must be used only when 0.1<(W/H)<2.0 and 1<(Er)<15. Stripline (stripline) Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} In the meantime, H is the interval between the two reference planes, and the trace is located at the center of the two reference planes . This formula must be used only when W/H<0.35 and T/H<0.25.

8. Can a ground wire be added to the center of the differential signal line?
The differential signal center generally cannot add a ground wire. Because the most important point of the principle of using differential signals is the advantages of coupling between differential signals, such as flux cancellation and noise immunity. If a ground wire is added to the center, the coupling effect will be damaged.

9. Does rigid-flex board planning require special planning software and standards? Where can we accept this type of circuit board processing in China?
Can use general PCB planning software to plan Flexible Printed Circuit (Flexible Printed Circuit). The same uses Gerber format to produce FPC manufacturers. Because the manufacturing process is different from the general PCB, each manufacturer will have its own minimum line width, minimum line spacing, and minimum aperture (via) according to their manufacturing capabilities. In addition, the flexible circuit board can be reinforced by laying some copper at the turning point. As for the manufacturer, you can find it on the Internet "FPC" as a keyword query.

10. What are the criteria for properly selecting the point where the PCB and the case are grounded?
The criterion for selecting PCB and case grounding points is to use chassis ground to provide a low-impedance path to the returning current and to control this return current. For example, usually in the vicinity of high-frequency equipment or clock generators, fixing screws can be used to connect the PCB ground to the chassis ground to minimize the entire current loop area and reduce electromagnetic radiation.