How to do a good job in PCB design proofing

When I once saw the PCB of the computer motherboard, I thought how great it was to draw it myself. Later, when I came into contact with protel99se, I stepped into the team of drawing boards, and then altium, cadence and so on. With the accumulation of experience in drawing boards, more and more matters need attention. A good PCB board doesn’t just connect the wires. Let me tell you the story in it slowly.
First, most PCB designers are proficient in the working principle of electronic components, know their mutual influence, and better understand the various data transmission standards that constitute the input and output of circuit boards. An excellent electronic product not only needs an excellent schematic diagram, but also needs PCB layout and wiring people, and the latter plays a vital role in the success or failure of the final circuit board. However, the more schematic designers know about excellent layout technology, the more opportunities they have to avoid major problems.
Second, the treatment of noise problems. With the increase of PCB design speed, concurrent switching noise, concurrent switching output, ringing, crosstalk ground rebound and power supply noise also appear. To solve these problems, it is necessary to prescribe the right medicine:
1. Ringing and crosstalk. For the key signal lines, we must pay attention to the crosstalk problem. Differential signals are commonly used and differential pairs are used for routing, which can fundamentally eliminate the inductive influence and help to reduce the “bounce” noise generated by the induced current in the return path.
2. Pay attention to impedance matching. I used to make antenna matching circuit, and impedance matching played a vital role. At present, the characteristic impedance of 100 ω has become the industry standard value of differential connection line. A 100ω differential line can be made of two 50ω single-ended lines of equal length. Because the two traces are close to each other, the field coupling between the traces will reduce the differential mode impedance of the traces. 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 wire in the 100Ω differential pair will be slightly higher than 50 Ω. If you really don’t want to do this, when making PCB, you should agree with the manufacturer what kind of impedance is needed for the wiring.
3. Use of decoupling and bypass capacitors. Generally speaking, decoupling capacitors help to reduce the inductance between the power supply of PCB and the ground plane, and control the impedance of signals and IC at all places on PCB. The bypass capacitor provides a clean power supply (provides a charge pool). Usually, we should arrange decoupling capacitors anywhere convenient for PCB wiring. For the use of capacitors, it should be noted that the wiring of decoupling capacitors should be as short as possible.
Third, the layout problem. As we all know, the shortest and most direct path is the most critical connection in PCB design, so that the simplest way can be used to get the best results. So, why not?
Fourthly, the processing of clock signal. I believe that PCB designers are experiencing or preparing to experience the problem of clock signal interference. Because the clock line is too long or passes through the signal line, etc., it will amplify the jitter and offset for the downstream, especially when the clock speed increases. First of all, PCB design should avoid using multiple layers to transmit the clock, and there should be no via holes on the clock lines, because the via holes will increase the impedance change of the traces and the reflection of signals. Secondly, if the inner layer must be used to lay the clock, the upper and lower layers should use the ground plane to reduce the delay. Thirdly, if unfortunately the clock noise on the power plane will increase the jitter of PLL, a “power island” can be created when the PCB design is modified. This technology can use the thicker etching in the metal plane to realize the isolation of PLL analog power and digital power.
Fifth, refer to the design scheme. Now any MCU will give its corresponding reference design, although these circuit boards are usually designed for multiple purposes, which may not exactly match your design requirements. However, they can still be used as a starting point for creating solutions. From this, we can see the routing and positioning of key parts, which is also a great improvement for the success rate of design. Contact customer service to place an order.