A brief discussion on PCB design from shallow to deep design

When I saw the PCB of a computer motherboard, I thought it would be great to be able to draw it myself. Later, I came into contact with protel99se and joined the team of board drawing, and then altium, cadence, etc. As the experience of drawing boards accumulates, I find that there are more and more things to pay attention to. A good PCB board is not just about connecting the wires. Let me tell you the story of putting it in it slowly.

First, most PCB designers are proficient in the working principles of electronic components, know their mutual influence,

and understand the various data transmission standards that constitute the input and output of the circuit board. An excellent electronic product not only needs an excellent schematic diagram, but also needs people who layout and route the PCB, and the latter plays a vital role in the success or failure of the final circuit board. However, the more the schematic designer knows about excellent layout technology, the more opportunities to avoid major problems.

Second, the handling of noise problems

. With the increase in the speed of PCB design, concurrent switching noise, concurrent switching output, ringing, crosstalk ground bounce and power supply noise also appear. To solve these problems, we must also prescribe the right medicine:

A. Ringing and crosstalk

. For key signal lines, we must pay attention to the crosstalk problem. The common method is to use differential signals and use differential pairs for routing. This can fundamentally eliminate the inductive effect and help reduce the “rebound” noise generated by the inductive current in the return path.

B. Pay attention to impedance matching.

I have made antenna matching circuits before, and impedance matching plays a vital role. Now 100Ω characteristic impedance has become the industry standard value for differential connection lines. 100Ω differential lines can be made with two 50Ω single-ended lines of equal length. Because the two traces are close to each other, the field coupling between the lines will reduce the differential mode impedance of the lines. In order to maintain the impedance of 100Ω, the width of the trace must be reduced a little. As a result, the common mode impedance of each line in the 100Ω differential line pair will be slightly higher than 50 ohms. If you really don’t want to toss so much, when making PCBs, discuss with the manufacturer what impedance is required for what traces.

C. Use of decoupling and bypass capacitors.

In general, decoupling capacitors help reduce the inductance between the power supply and ground planes of the PCB and control the impedance of signals and ICs everywhere on the PCB. Bypass capacitors provide a clean power supply (provide a charge reservoir). Usually we should arrange decoupling capacitors anywhere that is convenient for PCB wiring. One thing that should be noted about the use of capacitors is that the wiring of decoupling capacitors should be as short as possible.

Third, layout issues.

We all know that the most critical connection design in PCB design is the shortest and most direct path, so that the best effect can be achieved with the simplest approach. In this way, why not do it?

Fourth, clock signal processing.

I believe that those who do PCB design are experiencing or preparing to experience clock signal interference. Because the clock line is too long or passes through the signal line, etc., it will amplify jitter and offset for the downstream, especially when the clock speed increases. First, when designing PCBs, you should avoid using multiple layers to transmit clocks, and do not have vias on the clock line, because vias will increase the impedance change of the line and the reflection of the signal. Secondly, if the inner layer must be used to lay out the clock, the upper and lower layers should use ground planes to reduce delays. Thirdly, if the clock noise introduced on the power plane unfortunately increases the PLL jitter, then a “power island” can be created when modifying the PCB design. This technology can use thicker etching in the metal plane to achieve isolation of the PLL analog power supply and digital power supply.

Fifth, reference design solutions.

Now any MCU will give its corresponding reference design. Although these circuit boards are usually designed for multiple purposes and may not match your design requirements exactly. However, they can still be used as a starting point for creating solutions. From this we can see the routing and placement of key parts, which is also a great improvement for the success rate of the design.

The above is a summary of some of my experiences and lessons in PCB design. I hope it will be helpful to everyone. I also hope that everyone can leave me a message to discuss various technologies of PCB design together.