The design of high speed PCB is very important to support practical application. When PCB processes high-speed signal, the problem of signal transmission is prominent. The excellent high-speed board is a board that integrates various components and wiring, while avoiding signal integrity problems.
When will high speed PCB design rules become important?
A good way to check if you need to pay attention to high-speed PCB design guidelines is to place new devices on the breadboard or development board. If you can arrange the new equipment on the breadboard and the effect is good, you can arrange the circuit board in any way you like, and the finished PCB can still be used. However, high-speed devices usually do not work on breadboards. This is one of the reasons component manufacturers release their component development boards, especially FPGA, MCU, RF components and many other components.
If the peripherals can be arranged on the development board, and the device can work normally, you can arrange the new board according to your preference, as long as you follow the stacking design on the development board. In this case, you should also pay attention to the signaling standards (SPI, UART, I2C, etc.) used on the board to ensure that the signal will not be seriously degraded during wiring. In many cases, you won’t be able to take this simple approach, and everything needs to be integrated into a single PCB. In this case, you need to pay attention to the following design points to ensure that the high-speed signal will not be seriously distorted during wiring.
High speed PCB needs attention
In high-speed PCB, the rise time is fast enough that the bandwidth of digital signal can be extended to high MHz or GHz frequency. When this happens, if the circuit board is not designed with high-speed PCB design rules, some signal problems will be noticed. In particular, you may notice:
Too much transient ringing. This usually happens when the routing is not wide enough, although you should be careful when making the routing wider (see the “impedance control” section in PCB design below). If the transient ringing is large, there will be large overshoot or undershoot in signal conversion.
Strong crosstalk. As the signal speed increases (i.e., the rise time decreases), the capacitive crosstalk may become very large as the induced current experiences the capacitive impedance.
Reflection of driver and receiver components. Whenever the impedance does not match, your signal may be reflected from other components. If impedance mismatch becomes important, it is necessary to check the input impedance, load impedance and characteristic impedance of transmission line. You can read more about this in the next section.
Power integrity issues (transient PDN ripple, ground bounce, etc.). In any design, this is another set of unavoidable problems. However, with proper stack design and decoupling measures, the transient PDN ripple and any resulting EMI can be greatly reduced. You can read more about high-speed PCB stack design later in this guide.
Conducted and radiated EMI is strong. In the IC level and high-speed PCB design level, the research of solving EMI problem is very extensive. EMI is essentially a process of equivalence. If you design a circuit board to have strong EMI immunity, it will emit less EMI. Thirdly, most of them come down to the correct PCB stack design.
With these issues in mind, let’s take a look at the important high-speed board design guidelines that designers should use to address these signal integrity and power integrity issues.
Three problems in high speed PCB design
In high-speed PCB design, there are many problems in the process of signal interaction from point a to point B. But in all these problems, three main problems need to be paid attention to are:
opportunity. In other words, do all signals on the PCB layout arrive at an appropriate time relative to other signals? All high-speed signals on the circuit board are controlled by the clock. If the timing is not correct, it is likely to receive damaged data.
sincerity. In other words, when you arrive at your destination, does your signal look like it should? If not, it means that your signal may be disturbed in the process of destroying its integrity.
Noise. In other words, is your signal subject to any interference from the transmitter to the receiver? Every PCB will emit some kind of noise, but when there is too much noise, it will increase the chance of data corruption.
Now, the good news is that the following three major problems you can encounter in high-speed PCB design can be corrected through the following three solutions:
- Impedance. Proper impedance between transmitter and receiver will directly affect the quality and integrity of signal. This will also affect the sensitivity of the signal to noise.
- Matching. Matching the lengths of the two coupled traces ensures that your traces arrive at the same time and are synchronized with the clock rate. For DDR, SATA, PCI Express, HDMI and USB applications, matching is an essential solution.
- Spacing. The closer your wires are, the more vulnerable they are to noise and other forms of signal interference. The noise on the board can be reduced by not placing the routing closer than needed.