Napoleon said, “A soldier who doesn’t want to be a general is not a good soldier.” Similarly, “A PCB engineer who doesn’t want to be a leader in the industry is not a good PCB engineer.” So, as a PCB design beginner or a new PCB engineer, how can you become a PCB design expert? Don’t worry, let’s see if you have mastered the following skills first!
1. How to choose PCB board material?
The choice of PCB board material must strike a balance between meeting design requirements, mass production, and cost. The 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 dielectric loss of the commonly used FR-4 material at frequencies of several GHz will have a great impact on signal attenuation and may not be suitable for use. In electrical terms, attention should be paid to whether the dielectric constant (dielectric constant) and dielectric loss are appropriate at the designed frequency.
2. How to avoid high-frequency interference?
The basic idea to avoid high-frequency interference is to minimize the interference of high-frequency signal electromagnetic fields, 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 pay attention to the noise interference from digital ground to analog ground.
3. How to solve the problem of signal integrity in high-speed PCB 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 wiring, the characteristics of the load end, the topology structure of the wiring, etc. The solution is to rely on termination and adjusting the wiring topology.
4. Can a ground wire be added in the middle of the differential signal line?
Generally, a ground wire cannot be added in the middle of differential signals. Because the most important point in the application principle of differential signals is to take advantage of the benefits brought by mutual coupling between differential signals, such as flux cancellation, noise immunity, etc. If a ground wire is added in the middle, the coupling effect will be destroyed.
5. When laying out the clock, is it necessary to add ground wire shielding on both sides?
Whether to add a shielded ground wire depends on the crosstalk/EMI situation on the board, and if the shielded ground wire is not handled well, it may make the situation worse.
6. How to deal with the line segments (with small boxes) that appear during allegro wiring?
The reason for this is that after the module is reused, an automatically named group is automatically generated, so the key to solving this problem is to break up the group again. Select the group in the placement edit state and then break it up.
After completing this command, move the traces of all small boxes and hit the ix 0 0 coordinate.
7. How to meet EMC requirements as much as possible without causing too much cost pressure?
The increased cost due to EMC on the PCB is usually due to increasing the number of ground layers to enhance the shielding effect and adding ferrite beads, chokes and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure of other institutions to make the entire system pass EMC requirements. The following are just a few PCB board design tips to reduce the electromagnetic radiation effects generated by circuits:
1) Try to use devices with a slower signal slope (slew rate) to reduce the high-frequency components generated by the signal.
2) Pay attention to the placement of high-frequency components and do not place them too close to external connectors.
3) Pay attention to the impedance matching of high-speed signals, wiring layers and their return current paths to reduce high-frequency reflection and radiation.
4) Place sufficient and appropriate decoupling capacitors on the power pins of each device to mitigate noise on the power and ground layers. 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 separated from the ground layer, and the ground of the connector can be connected to the chassis ground nearby.
6) Ground guard/shunt traces can be appropriately used next to some particularly high-speed signals. But pay attention to the impact of guard/shunt traces on the characteristic impedance of the wiring.
7) The power layer is 20H smaller than the ground layer. H is the distance between the power layer and the ground layer.
8. For high-frequency PCB design above 2G, what rules should be followed for microstrip design?
RF microstrip line design requires the use of three-dimensional field analysis tools to extract transmission line parameters. All rules should be specified in this field extraction tool.
9. Which end of the PCB board has the best AC coupling effect for high-speed signals? We often see different processing methods, some are close to the receiving end, and some are close to the transmitting end.
Let’s first look at the role of AC coupling capacitors, which are nothing more than three points: ① The source and sink DCs are different, so DC is blocked; ② DC components may be crosstalked during signal transmission, so DC blocking makes the signal eye diagram better; ③ AC coupling Capacitors can also provide DC bias and overcurrent protection. After all, the function of the AC coupling capacitor is to provide DC bias, filter out the DC component of the signal, and make the signal symmetrical about the 0 axis. So why add this AC coupling capacitor? Of course it is beneficial. Adding the AC coupling capacitor will definitely enable better communication between the two stages and improve the noise margin. It is important to know that the AC coupling capacitor is generally a discontinuous point in the impedance of high-speed signals and will cause the signal edges to become slower.
So, the answer is also clear:
1) Some agreements or manuals will provide design requirements, and we place them according to the design guideline requirements.
2) There is no requirement for the first item. If it is IC to IC, please place it close to the receiving end.
3) If it is IC to connector, please place it close to the connector.
10. How to check whether the PCB meets the design process requirements when it leaves the factory?
Many PCB manufacturers must go through a power-on network continuity test before the PCB is processed and shipped out of the factory to ensure that all connections are correct. At the same time, more and more manufacturers are also using X-ray testing to check some faults during etching or lamination. For finished boards after patch processing, ICT testing is generally used for inspection, which requires adding ICT test points during PCB design. If a problem occurs, a special X-ray inspection equipment can also be used to rule out whether the fault is caused by processing reasons.
