100 Questions That Will Help You Meet Today’s Top PCB Design Pressures!(chapter 1）

PCB Design Tips 100 Questions

1. How to choose PCB board?

The choice of PCB board must strike a balance between meeting design requirements and mass producibility and cost. Design requirements include both electrical and mechanical components. 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 signal attenuation, so it may not be suitable. As far as electrical is concerned, attention should be paid to whether the dielectric constant (dielectric constant) and the dielectric loss are combined at 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. You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also be aware of the noise interference from the digital ground to the analog ground.

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

Signal integrity is basically a matter 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 rely on termination and adjust the topology of the traces.

4. How is differential wiring implemented?

There are two points to pay attention to in the wiring of the differential pair. One is that the length of the two lines should be as long as possible, and the other is that the distance between the two lines (this distance is determined by the differential impedance) should always remain unchanged, that is, keep it parallel. There are two parallel ways, one is for two lines to go on the same wiring layer (side-by-side), the other is for two lines to go over-under. Generally, there are many ways to implement the former side-by-side.

5. For a clock signal line with only one output, how to implement differential wiring?

To use differential wiring, it only makes sense that both the signal source and the receiving end are differential signals. Therefore, differential routing cannot be used for clock signals with only one output.

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 will make the signal quality better.

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

The differential pairs should be routed appropriately close and parallel. The so-called appropriate proximity is because this distance will affect the differential impedance (differential impedance). which is an important parameter for designing differential pairs. Parallelism is also required 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 actual wiring

1. Basically, it is right to isolate the analog/digital ground division. It should be noted that the signal traces should not cross the moat as far as possible, and the returning current path of the power supply and the signal should not become too large. 2. The crystal oscillator is an analog positive feedback oscillation circuit. In order to have a stable oscillation signal, it must meet the specifications of loop gain and phase, and the oscillation specification of this analog signal is easily disturbed. Even adding ground guard traces may not be able to completely isolate the interference. . And too far away, the noise on the ground plane will also affect the positive feedback oscillator circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible. 3. Indeed, there are many conflicts between high-speed wiring and EMI requirements. But the basic principle is that some electrical characteristics of the signal cannot meet the specifications due to the resistance, capacitance or ferrite bead added by EMI. Therefore, it is best to use the skills of arranging traces and PCB stack-up to solve or reduce EMI problems, such as high-speed signals going to inner layers. Finally, the resistor-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 routing software now have set constraints to control the routing method and the number of vias. The winding engine capabilities and constraints set by the various EDA companies are sometimes quite different. For example, are there enough constraints to control the way the serpentine is meandering, can the trace spacing of the differential pair be controlled, etc. This will affect whether the automatic routing method 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 pushability of the trace, the pushability of the via, and even the pushability of the trace to the copper, and so on. Therefore, choosing a router with a strong winding engine is the solution.

10. About test coupons.

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: single line and differential pair. Therefore, the line width and line spacing (when there are differential pairs) on the test coupon should be the same as the lines to be controlled. The most important thing is the location of the ground point when measuring. In order to reduce the inductance value of the ground lead, the grounding place of the TDR probe is usually very close to the probe tip. to match the probe used.

11. In the high-speed PCB design, copper can be applied to the blank area of the signal layer, and how should the copper applied to the grounding and power connections of multiple signal layers be distributed?

Generally, most of the copper deposits in the blank area are 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. Also be careful not to affect the characteristic impedance of its layers, for example in dual stripline structures.

12. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line above the power plane? Can the signal between the power and ground planes be calculated using the stripline model?

Yes, both the power plane and the ground plane must be considered as reference planes when calculating the characteristic impedance. For example, a four-layer board: top layer - power layer - ground layer - bottom layer. At this time, the model of the characteristic impedance of the top layer trace 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 to meet the test requirements of mass production in general?

Generally, whether the software automatically generates test points to meet the test requirements must see whether the specifications for adding test points meet the requirements of the test equipment. In addition, if the traces are too dense and the specification for adding test points is strict, it may not be possible to automatically add test points to each 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?

As for whether it will affect the signal quality, it 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 pins as test points) may be added to the wire or pulled out a small piece of wire from the wire. The former is equivalent to adding a small capacitor on the line, and the latter is an extra branch. Both of these conditions will affect the high-speed signal more or less, and the degree of influence is related to the frequency speed of the signal and the edge rate of the signal. The magnitude of the effect can be known through simulation. In principle, the smaller the test point, the better (of course, to meet the requirements of the test equipment). 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, the A board has power supply or the 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 the Kirchoff current law). The current in this formation will find the place of least resistance 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, you can also analyze the entire current loop, especially the part with large current, and adjust the connection method of the ground layer or ground wire to control the current path (for example, create a low impedance somewhere, so that most of the current flows from this place) to reduce the impact on other more sensitive signals.

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

The applications of high-speed digital circuits now include communication networks and computers and other related fields. In terms of communication network, the working frequency of PCB board has reached up to GHz, and the number of layers is as many 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 reached 400MHz (such as Rambus) or more. In response to the demand for high-speed and high-density wiring, the demand for blind/buried vias, mircrovias and build-up processes is gradually increasing. These design requirements all have manufacturers that can mass-produce them. 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. Dooglas 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)] Among them, W is the line width, T is the copper thickness of the trace, H is the distance from the trace to the reference plane, and 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 traces are located on the two reference planes in the middle. This formula must be applied when W/H<0.35 and T/H<0.25.

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

Generally, the ground wire cannot be added in the middle of the differential signal. Because the most important point of the application principle of differential signals is to take advantage of the benefits brought by the coupling between differential signals, such as flux cancellation, noise immunity, and so on. If you add a ground wire in the middle, it will destroy the coupling effect.

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

Flexible Printed Circuits can be designed with general PCB design software. The same Gerber format is used for FPC manufacturers. Since the manufacturing process is different from that of general PCBs, each manufacturer will have restrictions on the minimum line width, minimum line spacing, and minimum via (via) according to their manufacturing capabilities. In addition, some copper sheets can be laid at the turning points of the flexible circuit board for reinforcement. As for the manufacturer of the production, 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 casing?

The principle of selecting the ground point of the PCB and the case is to use the chassis ground to provide a low-impedance path for the returning current and a path for controlling the return current. For example, usually in the vicinity of high-frequency devices or clock generators, the ground layer of the PCB can be connected to the chassis ground by fixing screws, so as to minimize the area of the entire current loop and reduce electromagnetic radiation.

21. What aspects should the circuit board DEBUG start from?

As far as digital circuits are concerned, first determine three things in order: 1. Confirm that all power supply values are as large as the design requires. Some systems with multiple power supplies may require certain specifications for the order and speed of certain power supplies. 2. Verify that all clock signal frequencies are working properly and that there are no non-monotonic edges on the signal edges

(non-monotonic) problem. 3. Confirm whether the reset signal meets the specification requirements. If these are normal, the chip should send out the first cycle (cycle) signal. Next, debug according to the operating principle of the system and the bus protocol.

22. When the size of the circuit board is fixed, if the design needs to accommodate more functions, it is often necessary to increase the wiring density of the PCB, but this may lead to increased mutual interference of the wiring, and at the same time the wiring is too thin. Can't be reduced, ask experts to introduce the skills in high-speed (>100MHz) high-density PCB design?

When designing high-speed and high-density PCBs, crosstalk interference is indeed something to pay special attention to because it has a great impact on timing and signal integrity. Here are a few points to note:

1. Control the continuity and matching of the characteristic impedance of the trace.

2. The size of the trace spacing. The most commonly seen spacing is twice the line width. The impact of trace spacing on timing and signal integrity can be known through simulation, and the minimum spacing tolerable can be found. Results may vary from chip to chip signal.

3. Select the appropriate termination method.

4. Avoid the routing directions of the upper and lower adjacent layers in the same direction, and even if the traces are just overlapped up and down, because this crosstalk is larger than that of the adjacent routing on the same layer.

5. Use blind/buried vias to increase the trace area. But the production cost of the PCB board will increase. It is indeed difficult to achieve complete parallelism and equal length in actual implementation, but it is still necessary to try to achieve it. In addition, differential and common-mode terminations can be reserved to mitigate the impact on timing and signal integrity.

23. The filtering at the analog power supply often uses LC circuits. But why sometimes LC filtering is less effective than RC?

The comparison of LC and RC filtering effects must consider whether the frequency band to be filtered out and the choice of inductance value are appropriate. Because the inductive reactance (reactance) of the inductor is related to the inductance value and frequency. If the noise frequency of the power supply is low and the inductance value is not large enough, the filtering effect may not be as good as that of the RC. However, the cost of using RC filtering is that the resistor itself dissipates energy, is less efficient, and pays attention to the power that the resistor you choose can handle.

24. What is the method of selecting inductance and capacitance value when filtering?

The selection of the inductance value not only considers the noise frequency to be filtered out, but also considers the response ability of the instantaneous current. If the output terminal of the LC has the opportunity to output a large current instantaneously, the large inductance value will hinder the speed of the large current flowing through the inductor, increasing the ripple noise. The capacitance value is related to the specification value of ripple noise that can be tolerated. The smaller the ripple noise value requirement is, the larger the capacitance value will be. The ESR/ESL of the capacitor will also have an impact. In addition, if the LC is placed at the output of the switching regulation power, pay attention to the influence of the pole/zero generated by the LC on the stability of the negative feedback control loop .

25. How to meet EMC requirements as much as possible without causing too much cost pressure?

The cost increase due to EMC on the PCB board is usually due to the increase of the number of ground layers to enhance the shielding effect and the addition of ferrite bead, choke and other devices to suppress high-frequency harmonics. In addition, it is usually necessary to match the shielding structure on other mechanisms to make the entire system pass the EMC requirements. The following are just a few PCB design tips to reduce the effects of electromagnetic radiation generated by the circuit. 1. Use devices with slow signal slopes as much as possible to reduce the high-frequency components generated by the signal. 2. Pay attention to the placement of high-frequency devices, and do not get too close to the external connectors. 3. Pay attention to the impedance matching of high-speed signals, the trace layer and its return current path to reduce high-frequency reflection and radiation. 4. Place enough and appropriate decoupling capacitors on the power pins of each device to mitigate the noise on the power plane and ground plane. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements. 5. The ground near the external connector can be properly divided with the ground, and the ground of the connector can be connected to the chassis ground nearby. 6. Ground guard/shunt traces can be properly used next to some particularly high-speed signals. But pay attention to the effect of guard/shunt traces on the characteristic impedance of the trace. 7. The power layer is 20H smaller than the ground layer, and H is the distance between the power layer and the ground layer.

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