Short Answers to Big Questions about PCB Design

Q1: How to choose PCB (Printed Circuit Board) material?


A1: PCB material has to be selected totally based on the balance between design demand, volume production and cost. Design demand involves electrical elements that should be taken into serious consideration during high-speed PCB design. In addition, dielectric constant and dielectric loss should be considered whether they go with the frequency.

Q2: How to avoid high-frequency interference?


A2: The leading principle to overcome high-frequency interference is to reduce crosstalk as much as possible, which can be achieved by enlarging the distance between high-speed signals and analog signals or equipping ground guard or shunt traces beside analog signals. In addition, the noise interference caused by digital ground on analog ground should be carefully considered.

Q3: How to arrange traces carrying differential signals?


A3: Two points should be focused in terms of traces carrying differential signals design. On one hand, the length of two lines should be the same; on the other hand, the spacing between two lines should maintain parallel.

Q4: How to arrange traces carrying differential signals when there’s only one clock signal line at output terminal?


A4: The premise of traces carrying differential signals arrangement is that both signal sources and receiving end should be differential signals. Therefore, differential routing can never work on clock signals containing only one output end.

Q5: Can matched resistance be added between differential pairs at receiving end?


A5: Matched resistance is usually added between differential pairs at receiving end and its value is equal to that of differential impedance. As a result, signal quality will be better.

Q6: Why should differential pair traces be close to each other and parallel?


A6: Differential pair traces should be properly close and parallel. The distance between differential pair traces is determined by differential impedance that is a key reference parameter in terms of differential pair design.

Q7: How to resolve the conflicts between manual routing and auto-routing on high-speed signals?


A7: Now most automatic routers are able to control wire running method and number of through holes by setting constraint conditions. All EDA companies differ a lot from each other in terms of wire running methods and constraint condition setting. The difficulty of automatic routing is closely related with wire running capability. Therefore, this problem can be resolved by picking up a router with high capability of wire running.

Q8: In high-speed PCB design, the blank area of signal layers can be coated with copper. How should copper be distributed on multiple signal layers on grounding and powering?


A8: Generally, copper coating is mostly connected with the ground in blank area. The distance between copper coating and signal lines should be strictly designed because coated copper will reduce characteristic impedance a little. Meanwhile, characteristic impedance of other layers should not be influenced.

Q9: Can characteristic impedance on power plane be figured out by micro strip line model? Can micro strip line model be used on signals between power plane and ground plane?


A9: Yes. During the procedure of characteristic impedance calculation, both power plane and ground plane can be regarded as a reference plane.

Q10: Can the test points generated through automation on high-density PCB meet the testing demands of massive production?


A10: It all depends on the case whether regulations on test points are compatible with the requirement laid by test machines. In addition, if routing is run too densely and regulations on test points are very strict, there may be no ways to put test points on each segment of line. Of course, manual methods can be used to complement test points.

Q11: Can test point adding influence the quality of high-speed signals?


A11: It all depends on the case whether test point adding method and the running speed of signals. Basically, adding test points are obtained through adding them to lines or pulling a segment out. Both methods can more or less affect high-speed signals and the effect extent is related with frequency speed and edge rate of signals.

Q12: When a couple of PCBs are connected into a system, how should ground lines of each PCB be connected?


A12: Based on Kirchoff current law, when power or signals are sent from Board A to Board B, equivalent amount of current will be returned to Board A from ground plane and the current on ground plane will flow back at the path where the impedance is the lowest. Therefore, the number of pins contributed to the ground plane should never be too small at each interface of power or signal interconnection so that both impedance and noise on the ground can be reduced. Additionally, the whole current loop should be analyzed, especially the portion where current is the largest and connection of ground plane or ground lines should be adjusted to control the current running and decrease the influence on other sensitive signals.

Q13: Can ground lines be added to the middle of differential signal lines?


A13: Basically, ground lines cannot be added among differential signal lines because the biggest significance of differential signal line principle lies in the advantage led by mutual coupling between differential signal lines, such as flux cancellation, noise immunity etc. Coupling effect will be destroyed if ground lines are added among them.

Q14: What is the principle of picking up suitable PCB and cover grounding point?


A14: The principle is to take advantage of chassis ground to provide a path with low impedance to returning current and to control the path of this returning current. For example, screw can be normally used near high-frequency component or clock generator to connect the ground plane of PCB with chassis ground to reduce the whole current loop area as much as possible, that is, to reduce electromagnetic interference.

Q15: Where should PCB debug start?


A15: As far as digital circuit is concerned, the following things should be done in order. First, all power values should be confirmed to averagely achieve design requirement. Second, all the clock signal frequencies should be confirmed to normally work and there’s no non-monotonic problem on the edge. Third, reset signals should be confirmed to achieve standard requirement. If the above things have been confirmed, chip should send signals in the first cycle. Then, debug will be carried out based on system running protocol and bus protocol.

Q16: What is the best way to the design of high-speed and high-density PCB with board area fixed?


A16: In the process of high-speed and high-density PCB design, crosstalk interference should be especially focused since it greatly affects timing and signal integrity. Some design solutions are given. First, the continuity and matching of the routing characteristic impedance should be controlled. Second, spacing should be noticed and spacing is normally twice line width. Third, proper termination methods should be picked up. Fourth, routing in adjacent layers should be implemented in different directions. Fifth, blind/buried vias can be used to increase routing area. In addition, differential termination and common-mode termination should be maintained so as to reduce the influence on timing and signal integrity.

Q17: LC circuit is usually applied to filter wave at analog power. Why does LC sometimes perform better then RC?


A17: The comparison between LC and RC should be based on the assumption whether frequency band and inductance are suitably selected. Because reactance of inductance is correlated with inductance and frequency, if the noise frequency of power is too low and inductance isn’t high enough, LC performs worse than RC. However, one of the disadvantages of RC lies in the fact that resistor itself will consume energy with low efficiency.

Q18: What is the optimal way to achieve EMC requirement without cost pressure?


A18: PCB board suffers from higher cost due to EMC usually because layer count goes up to strengthen shielding stress and some components are prepared such as ferrite bead or choke that are used to stop high-frequency harmonic wave components. Besides, other shielding structure on other systems should be used to meet the demands of EMC. First, components with low slew rate should be applied as many as possible so as to decrease high-frequency portions generated by signals. Second, high-frequency components should never be placed too near exterior connectors. Third, impedance matching, routing layer and return current path of high-speed signals should be carefully designed to cut down high-frequency reflection and radiation. Fourth, sufficient decoupling capacitors should be placed at power pins in order to reduce the noise at power plane and ground plane. Fifth, the ground near exterior connector can be cut away from ground plane and connector ground should be near chassis ground.

Q19: When a PCB board features multiple digital/analog modules, the ordinary solution is to divide digital/analog modules. Why?


A19: The reason for dividing digital and analog modules is that noise is usually generated at power and ground at the switching of high and low potential and the extent of noise is related with signal speed and current amount. If analog and digital modules are not divided and the noise generated by digital module is larger and circuit at analog area is similar, even if analog and digital signals don’t come across, analog signals will still be affected by noise.

Q20: When it comes to high-speed PCB design, how should impedance matching be implemented?


A20: As far as high-speed PCB design is concerned, impedance matching is one of the leading considerations. Impedance features absolute relationship with routing. For example, characteristic impedance is determined by a couple of elements including spacing between microstrip or stripline/double stripline layer and reference layer, routing width, PCB material etc. Differently speaking, characteristic impedance can never be determined until routing. The essential solution to this problem is to stop impedance discontinuity from occurring as much as possible.

Q21: In the process of high-speed PCB design, which measures should be taken in consideration of EMC/EMI?


A21: Generally speaking, EMI/EMC design should be considered from both radiated and conducted aspects. The former belongs to the portion whose frequency is higher (more than 30MHz) while the latter to the portion whose frequency is lower (less than 30MHz). Therefore, both high-frequency portion and low-frequency portion should be noticed. A good EMI/EMC design should start from components’ placement, PCB stack up, routing, component selection etc. Once those aspects leave unconsidered, cost will possibly rise. For example, clock generator should not be close to exterior connector as much as possible. Additionally, connecting points should be properly selected between PCB and chassis.

Q22: What is routing topology?


A22: Routing topology, also called routing order, refers to the order of routing in terms of network with multiple terminators.

Q23: How should routing topology be adjusted to increase signal integrity?


A23: This type of network signals is so complex that topology is different based on different directions, different levels, different kinds of signals. Therefore, it’s difficult to judge which type of signals is beneficial to signal quality.

Q24: What’s the reason for copper coating?


A24: There are usually a couple of reasons for copper coating. First, massive ground or power copper coating will have shielding effect and some special ground, PGND for example, can have a role of protection. Second, to ensure high performance of electroplating or stop lamination from being deformed, copper should be coated on PCB board with less routing. Third, copper coating derives from the requirement on signal integrity. A complete returning path should be provided to high-frequency digital signals and DC network routing should be reduced. In addition, thermal dissipation should be taken into consideration as well.

Q25: What is return current?


A25: As high-speed digital signals are running, signals flow from drivers to carrier along PCB transmission line and then return to driver terminal through the shortest path along ground or power. The returning signals at ground or power are called return current.

Q26: How many types of terminals are there?


A26: Terminal, also called matching, is usually classified into source matching and terminal matching. The former refers to series resistor matching while the latter refers to parallel matching. A lot of methods are available, including pull-up resistor, pull-down resistor, Davenan matching, AC matching, Schottky diode matching etc.

Q27: What elements can determine matching types?


A27: Matching type is usually determined by BUFFER characteristics, topology, level classifications and judgment type. Besides, signal duty cycle and system energy consumption have to be considered as well.

Q28: What inspection should be carried out on PCB before it is released by manufacturing factory?


A28: Most PCB manufacturers implement on-off test on PCBs before they leave factory in order to make sure all the circuits are correctly connected. Up to now, some advanced manufacturers carry out X-ray inspection to find out some obstacles on etching or lamination. When it comes to the products going through SMT assembly, ICT is usually applied, which calls for ICT test points setting during PCB design phase. As soon as problems occur, a special type of X-ray inspection can be used as well.

Q29: For a circuit composed by a couple of PCB boards, should they share the same ground?


A29: A circuit composed by a couple of PCB boards should normally share the same ground because it’s impractical to apply a couple of powers in a single circuit. Of course, if your conditions allow, different powers can be used as well. After all, that’ll help reduce interference.

Q30: How should ESD be considered by a system containing DSP and PLD?


A30: As far as ordinary systems are concerned, the portions should be first considered with direct contact with human and proper protections should be done on circuit and structures. The extent of influence ESD bring towards system is usually determined according to different situations. In dry environment, ESD will become worse, especially on the system that is more sensitive. Even though larger system features unobvious effect on ESD, more attention should also be paid.

Q31: When it comes to a 4-layer PCB design, what side should receive copper coating on both sides?


A31: The following aspects should be taken into consideration for copper coating: shielding, thermal dissipation, reinforcement and PCB manufacturing demand. Therefore, the main reason should be considered. For example, in terms of high-speed PCB design, shielding should be most considered. Surface grounding is beneficial to EMC and copper coating should be completely done in case of lonely island. Generally speaking, if components on the surface receive too much routing, it’ll be difficult to keep copper foil complete. Therefore, it’s suggested that boards with many surface components or much routing aren’t coated with copper.

Q32: In the process of clock routing, is it necessary to add ground shielding on both sides?


A32: It depends on crosstalk or EMI of board. If shielding ground lines are not properly processed, it’ll bring forward bad effects on the contrary.

Q33: What is the strategy of clock routing for signals at different frequencies?


A33: In terms of routing for clock lines, signal integrity analysis should be first carried out and routing principles should be manipulated. Then it’s time to implement routing based on the principles.

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