PCB Tech - PCB design methods and skills1

How to choose a PCB board

1. The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material issue is usually more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency.

2. How to avoid high-frequency interference?

The basic idea of avoiding high-frequency interference is to minimize the interference of the electromagnetic field of high-frequency signals, which is the so-called crosstalk (Crosstalk). Can increase the distance between high-speed signal and analog signal, or add ground guard/shunt traces besides the analog signal. Also, pay attention to the noise interference from the digital ground to the analog ground.

3. How to solve the problem of signal integrity in high-speed design?

Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to terminate (termination) and adjust the topology of the wiring.

4. How is the differential wiring method realized?

There are two points to pay attention to in the layout of the differential pair. One is that the length of the two wires should be as long as possible, and the other is that the distance between the two wires (this distance is determined by the differential impedance) has to be kept constant, that is, to keep parallel. There are two parallel ways, one is that the two wires run on the same side-by-side, and the other is that the two wires run on two adjacent layers above and below (over-under). Generally, the former has more side-by-side implementations.

5. How to implement differential wiring for a clock signal line with only one output terminal?

To use differential wiring, it makes sense that both the signal source and the receiving end are differential signals. Therefore, it is impossible to use differential wiring for a clock signal with only one output terminal.

6. Can a matching resistor be added between the differential line pairs at the receiving end?

The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. This way the signal quality will be better.

7. Why should the wiring of the differential pair be closest and parallel?

The wiring method of the differential pair should be appropriately close and parallel. The so-called appropriate distance is because the distance will affect the value of the differential impedance, which is an important parameter for designing a differential pair. The need for parallelism is also to maintain the consistency of the differential impedance. If the two lines are suddenly far and near, the differential impedance will be inconsistent, which will affect the signal integrity and timing delay.

8. How to deal with some theoretical conflicts in the actual wiring

1. Basically, it is right to divide and isolate the analog/digital ground. It should be noted that the signal trace should not cross the divided place (moat) as much as possible, and the return current path of the power supply and signal should not be too large.

2. The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the loop gain and phase specifications. The oscillation specifications of this analog signal are easily disturbed. Even if ground guard traces are added, it may not be able to completely isolate the interference. . And if it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible.

3. It is true that there are many conflicts between high-speed wiring and EMI requirements. But the basic principle is that the resistance and capacitance or ferrite bead added by EMI cannot cause some electrical characteristics of the signal to fail to meet the specifications. Therefore, it is best to use the skills of arranging traces and PCB stacking to solve or reduce EMI problems, such as high-speed signals going to the inner layer. Finally, the resistance capacitor or ferrite bead method is used to reduce the damage to the signal.

9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?

Most of the automatic routers of strong wiring software now have set constraints to control the winding method and the number of vias. The winding engine capabilities and constraint setting items of various EDA companies sometimes differ greatly. For example, whether there are enough constraints to control the way of serpentine winding, whether it is possible to control the trace spacing of the differential pair, etc. This will affect whether the routing method of the automatic routing can meet the designer's idea. In addition, the difficulty of manually adjusting the wiring is also absolutely related to the ability of the winding engine. For example, the pushing ability of the trace, the pushing ability of the via, and even the pushing ability of the trace to the copper coating and so on. Therefore, choosing a router with strong winding engine capability is the solution.

10. About the test coupon.

The test coupon is used to measure whether the characteristic impedance of the produced PCB board meets the design requirements with TDR (Time Domain Reflectometer). Generally, the impedance to be controlled has two cases: a single line and a differential pair. Therefore, the line width and line spacing on the test coupon (when there is a differential pair) should be the same as the line to be controlled. The most important thing is the location of the grounding point during measurement. In order to reduce the inductance of the ground lead, the grounding place of the TDR probe is usually very close to the probe tip. Therefore, the distance and method between the signal measurement point and the ground point on the test coupon Must match the probe used.

11. In high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper coating of multiple signal layers be distributed on the ground and power supply?

Generally, the copper plating in the blank area is mostly 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 the dual stripline.

12. 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?

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.

13. Can test points be automatically generated by software on high-density printed boards under normal circumstances to meet the test requirements of mass production?

Generally, whether the software automatically generates test points to meet the test requirements depends on whether the specifications for adding test points meet the requirements of the test equipment. In addition, if the wiring is too dense and the specifications for adding test points are strict, it may not be possible to automatically add test points to each segment of the line. Of course, you need to manually fill in the places to be tested.

14. 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 (not using the existing via or DIP pin as test points) may be added to the line or pulled out a short line from the line. The former is equivalent to adding a small capacitor on the line, while the latter is an extra branch. Both of these conditions 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. In principle, the smaller the test point, the better (of course, it must meet the requirements of the test tool) the shorter the branch, the better.

15. Several PCBs form a system, how should the ground wires between the boards be connected?

When the signal or power supply between each PCB board is connected to each other, for example, when board A has a power supply or a signal is sent to board B, there must be an equal amount of current flowing from the ground back to board A (this is Kirchoff current law). The current on this ground will find the place with the least impedance to flow back. Therefore, at each interface, whether it is power or signal interconnection, 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, you can also analyze the entire current loop, especially the part with a large current, and 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 Go somewhere) to reduce the impact on other more sensitive signals.

16. Can you introduce some foreign technical books and materials on high-speed PCB design?

Nowadays, high-speed digital circuits are used in related fields such as communication networks and computers. In terms of communication network, the working frequency of the PCB board has reached up to GHz, and the number of layers is as high as 40 layers as far as I know. Computer-related applications are also due to the advancement of chips, whether it is a general PC or a server (Server), the highest operating frequency on the board has also reached over 400MHz (such as Rambus). In response to this demand for high-speed and high-density wiring, the requirements for blind/buried vias, microbial, and build-up processes have gradually increased. These design requirements are available for mass production by manufacturers.

Here are a few good technical books:

1. Howard W. Johnson, "High-Speed Digital Design – A Handbook of Black Magic";

2. Stephen H. Hall, "High-Speed Digital System Design";

3. Brian Yang, "Digital Signal Integrity";

4. Douglas Brook, "Integrity Issues and Printed Circuit Board Design".

17. Two characteristic impedance formulas that are often referred to:

a. Microstrip

Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] where W is the line width, T is the copper thickness of the trace, and H is the trace to the reference plane Distance, Er is the dielectric constant of the PCB material. This formula must be applied when 0.1<(W/H)<2.0 and 1<(Er)<15.

b. 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 located in the middle of the two reference planes. This formula must be applied when W/H<0.35 and T/H<0.25.

18. Can a ground wire be added to the middle of the differential signal line?

It is generally not possible to add a ground wire in the middle of the differential signal. Because the most important point of the application principle of differential signals is to use the benefits of coupling between differential signals, such as flux cancellation and noise immunity. If you add a ground wire in the middle, it will destroy the coupling effect.

19. Does rigid-flex board design require special design software and specifications? Where can we undertake such circuit board processing in China?

You can use general PCB design software to design a flexible printed circuit (Flexible Printed Circuit). It is also produced by FPC manufacturers in Gerber format. Since the manufacturing process is different from that of general PCBs, various manufacturers will have their limitations on the minimum line width, minimum line spacing, and minimum vias based on their manufacturing capabilities. In addition, it can be reinforced by laying some copper skin at the turning point of the flexible circuit board. As for the manufacturer, you can find it on the Internet "FPC" as a keyword query.

20. What is the principle of properly selecting the grounding point between the PCB and the case?

The principle of selecting the PCB and shell grounding points is to use the chassis ground to provide a low-impedance path for the returning current and to control the path of the returning current. For example, usually near high-frequency devices or clock generators, fixed screws can be used to connect the ground layer of the PCB to the chassis ground to minimize the area of the entire current loop and reduce electromagnetic radiation.