PCB Design Interview Questions

1. What PCB (Printed Circuit Board) material should I use?

The PCB material must be chosen entirely on the basis of a balance of design requirement, volume production, and cost. Electrical elements that need be taken into account during high-speed PCB design are called design demand. In addition, the frequency should be taken into account when determining the dielectric constant and dielectric loss. 

2: How can high-frequency interference be avoided?

The most important approach for overcoming high-frequency interference is to minimise crosstalk, which can be accomplished by increasing the distance between high-speed signals and analogue signals or by using ground guard or shunt traces alongside analogue channels. Furthermore, the noise interference induced by digital ground on analogue ground must be taken into account.

3. What is the best way to arrange traces that convey differential signals?

When designing traces carrying differential signals, two points should be kept in mind. On the one hand, two lines should be the same length; on the other, the spacing between two lines should remain parallel.

4. How can you organise traces conveying differential signals when the output terminal only has one clock signal line?

In order for traces carrying differential signals to work, both the signal sources and the receiving end must be differential signals. As a result, differential routing cannot be used with clock signals with only one output end.

5 .Is it possible to apply matching resistance between differential pairs at the receiving end?

At the receiving end, matched resistance is frequently applied between differential pairs. matching resistance is usually used.

6. Why should differential pair traces be parallel and close to each other?

Differential pair traces should be close and parallel to one other. Differential impedance, a critical reference parameter in differential pair design, determines the distance between differential pair traces.

7. How can conflicts between manual and automatic routing on high-speed signals be resolved?

Most automatic routers may now specify constraint constraints to regulate wire running manner and number of through holes. In terms of wire running methodologies and constraint condition setup, all EDA vendors differ significantly. The ability to run wires is closely connected to the difficulty of autonomous routing. As a result, this issue can be rectified by purchasing a router with a high throughput.

8. The blank space of signal layers can be plated with copper in high-speed PCB design. On grounding and powering, how should copper be divided across many signal layers?

In most blank areas, copper covering is largely attached to the ground. Because coated copper reduces characteristic impedance a little, the distance between copper coating and signal lines should be carefully calculated. Other layers' characteristic impedance should not be altered in the meantime.

9. Can a micro strip line model be used to calculate characteristic impedance on the power plane? Is it possible to utilise the micro strip line model on communications between the power plane and the ground plane?

 Of course. Both the power plane and the ground plane can be used as reference planes in the calculation of characteristic impedance.

10. Can test points created by automation on high-density PCBs match the testing demands of large-scale manufacturing?

It depends on the situation whether test point regulations are consistent with test machine requirements. Furthermore, if routing is done too intensively and test point restrictions are too rigorous, there may be no way to put test points on each line segment. Manual procedures can, of course, be employed to supplement test points.

11. Can the addition of test points affect the quality of high-speed signals?

 It all relies on the situation, such as the test point adding method and the signal running speed. Adding test points is accomplished by attaching them to lines or removing a segment.

12. How should the ground lines of each PCB be connected when a few of PCBs are integrated into a system?

According to Kirchoff's current law, when power or signals are delivered from Board A to Board B, an equal amount of current is returned from the ground plane to Board A, and the current on the ground plane flows back at the path with the lowest impedance. As a result, the number of pins contributing to the ground plane at each interface of power or signal connectivity should never be too small, in order to limit ground impedance and noise. In addition, the entire current loop should be examined, particularly the area where current is the greatest and the ground plane connection.

13: Can ground lines be added to differential signal lines in the middle?

Ground lines cannot be added to differential signal lines because the benefit of mutual coupling between differential signal lines, such as flux cancellation and noise immunity, is the most important aspect of the differential signal line principle. If ground lines are put between them, the coupling effect will be lost.

14. What is the principle behind selecting an appropriate PCB and covering the grounding point?

The idea is to use chassis ground to provide a low-impedance conduit for returning current and to control the path of that returning current. Screws, for example, are commonly used to connect the ground plane to a high-frequency component or clock generator. to limit the total current loop area as much as feasible, i.e. to reduce electromagnetic interference.

15. When it comes to PCB debugging, where should you start?

When it comes to digital circuits, the following steps should be followed in order. To begin, all power levels should be double-checked to ensure that the design requirement is met on average. Second, make sure that all of the clock signal frequencies are working properly and that there are no non-monotonic issues on the edge. Third, in order to meet the standard requirement, reset signals must be confirmed. If all of the above is true, the chip should send signals in the first cycle. Then, using the system operating protocol and the bus protocol, debugging will be carried out. 

16. What is the ideal method for designing a high-speed, high-density PCB with a set board area?

Crosstalk interference should be given special attention during the design of high-speed and high-density PCBs since it has a significant impact on timing and signal integrity. There are a few design options presented. First, the routing characteristic impedance should be regulated for continuity and matching. Second, observe the spacing, which is usually twice the line width. Third, the appropriate termination mechanisms should be chosen. Fourth, routing should be done in diverse directions in adjacent levels. Fifth, to expand route space, blind/buried vias might be used. Furthermore, differential and common-mode termination should be preserved to minimise the impact on timing and signal integrity.

17 .At analogue power, the LC circuit is commonly used to filter the wave. Why is it that LC sometimes outperforms RC?

When comparing LC with RC, it's important to consider if the frequency band and inductance are properly chosen. Because inductance reactance is connected with inductance and frequency, LC performs worse than RC if the noise frequency of power is too low and inductance isn't high enough. However, one of the disadvantages of RC is that the resistor consumes a lot of energy and is inefficient.

18. What is the best strategy to meet EMC requirements without breaking the bank?

 The cost of a PCB board increases due to EMC, mainly because the layer count is increased to increase shielding stress and some components, such as ferrite beads or chokes, are prepared to halt high-frequency harmonic wave components. Other shielding structures on other systems should also be employed to meet EMC requirements. To begin, as many components with a low slew rate as possible should be used to reduce high-frequency sections created by signals. Second, high-frequency components should never be installed too close to connectors on the outside. Third, high-speed signals' impedance matching, routing layer, and return current channel should be carefully planned to minimise high-frequency reflection and radiation.

19. When there are many digital/analog modules on a PCB board, the standard solution is to divide them. Why?

The reason for separating digital and analogue modules is that noise is generated at power and ground when high and low potentials are switched, and the amount of noise is proportional to signal speed and current. Even though analogue and digital signals do not come across, analogue signals will be influenced by noise if analogue and digital modules are not split and the noise generated by the digital module is bigger and the circuit at the analogue region is similar.

20. How should impedance matching be implemented when designing high-speed PCBs?

When it comes to high-speed PCB design, impedance matching is crucial. one of the most important considerations The absolute relationship between impedance and routing can be found in impedance. Characteristic impedance, for example, is determined by a number of factors such as the distance between the microstrip or stripline/double stripline layer and the reference layer, routing width, PCB material, and so on. To put it another way, characteristic impedance cannot be determined until the circuit is routed. The most important answer to this problem is to prevent impedance discontinuity as much as feasible.

21. Which EMC/EMI mitigation measures should be taken throughout the high-speed PCB design process?

In general, both radiated and conducted components of EMI/EMC design should be considered. The former belongs to the segment with a higher frequency (greater than 30MHz), while the latter belongs to the portion with a lower frequency (less than 30MHz) (less than 30MHz). As a result, both the high-frequency and low-frequency portions of the signal should be noted. Component placement, PCB stack up, routing, component selection, and other aspects of a good EMI/EMC design should all be considered. Costs are likely to rise if such factors are ignored. The clock generator, for example, should be kept as far away from the external connector as practicable. Additionally, connecting points between the PCB and the chassis should be carefully chosen.

22. What is the topology of a routing network?

 In a network with numerous terminators, routing topology, also known as routing order, refers to the order of routing.

23. What changes should be made to the routing topology to improve signal integrity?

Because this form of network signal is so complicated, the topology varies depending on the direction, level, and type of signal. As a result, determining which types of signals are favourable to signal quality is tough.

24 What is the significance of copper coating?

Copper plating is frequently done for a couple of reasons. To begin with, a huge ground or power copper covering will have a shielding effect, and some special grounds, such as PGND, can serve as a protective ground. Second, to assure superior electroplating or stop lamination performance. Copper should be coated on PCB boards with less routing to prevent deformation. Third, signal integrity necessitates the use of copper covering. High-frequency digital signals should have a complete return path, and DC network routing should be minimised. Thermal dissipation should also be taken into account.

25. What is the definition of return current?

High-speed digital signals move from drivers to carriers along a PCB transmission line, then back to the driver terminal via the quickest path along ground or power. Return current refers to the signals that return to ground or power.