Some summaries of PCB wiring

The rapid development of circuits requires our engineers to be more innovative and to constantly sum up experience in practice. PCB wiring is to lay roads for energized signals to connect various devices. This is like building roads to connect various cities to cars. Roads Construction requires two lines, one going and one returning. The same is true for PCB wiring. A two-line loop needs to be formed. For low-frequency circuits, it is a loop. For high-speed electromagnetic fields, it is a transmission line. The most common one is a differential signal line. . Such as USB, network cable, etc.

Differential signal lines are an ideal model for connecting device signals.

The higher the signal requirements, the closer to the differential signal line. There are a lot of components on a board. If they are all laid out according to differential lines, firstly, the area of ​​the PCB will be too large, and secondly, 2N lines will need to be laid out, which will be too much work and very difficult. Therefore, people proposed multi-layer PCB based on actual needs. Concept, the most typical one is double-sided PCB board. Use the bottom layer as a common reference circuit, so that only N+1 wiring is needed, and the PCB layout is greatly reduced.

Each layout engineer has his own way of understanding layout. For the same PCB, different layout engineers will draw different effects. Without affecting the performance of the PCB, whether the placement and layout of the components are beautiful depends on the layout. With the ability of an engineer, it can be said that an excellent layout engineer is an artist!

The picture below shows the TOP silk screen layer of the VPX chassis board:

It can be seen from the device arrangement that the filter decoupling capacitors of some chips are close to the chip. The several DDRs mounted on the FPGA are as close to the FPGA as possible and are neatly arranged. Open the TOP layer: Does it feel very complicated? Does it look high-end?

In the final analysis, no matter how beautiful the PCBlayout is, the functions and performance must be fulfilled, otherwise it will be scum. The hardware cost of a system-level board like this is tens of thousands. It would be great if it can realize the functions. From a veteran perspective, after seeing your components imported, you can basically estimate how long the board layout will take. The more layers a card has, the more powerful it is. Sometimes, more layers are added to take care of sensitive signals. There may only be a few lines on one layer. Why do some people use four layers when laying out their boards, but you use six layers? , have you ever considered whether the wiring length of some signals is a signal line or a transmission line, and whether it will be affected by interference?

Under normal circumstances, the priority of the board is to see whether the layout is modular. The premise of neatness is to ensure that sensitive signals are prioritized and whether the overall devices are neatly placed. It all depends on time. Experienced people will do it in the early stages of the layout. At this time, the direction of the sensitive signal, the layer routing, the direction of the first leg of the device, the direction of resistance and capacitance, the direction of the board passing through the machine during welding, and the impact on surrounding devices during device maintenance when there is a problem with the board are all taken into consideration.

The placement of common silk screens, annotations, test points, etc. also reflects basic skills. For those who use the same board with multiple identical circuits, I just want to say that this can be achieved in minutes by just adjusting the movement distance. Yes, what makes a master better? It’s also a board. It depends on whose performance is stable and how fast the board can be produced.

The general basic PCB design process is as follows: preliminary preparation-----PCB structural design-----PCB layout-----wiring-----wiring optimization and silk screen-----network and DRC inspection and structure Check ----- plate making.

PCB wiring sequence:

The wiring should be neat and uniform, not criss-crossed and unorganized. These must be achieved while ensuring electrical performance and meeting other individual requirements. Otherwise, it will be a waste of time. In the PCB design process, wiring is generally divided into three realms:

(1) The first is routing, which is the most basic requirement in PCB design. If the lines are not laid out and there are flying lines everywhere, it will be an unqualified board, and it can be said that you have not yet started.

(2) The second is the satisfaction of electrical performance, which is the standard to measure whether a printed circuit board is qualified. This is after laying out the wiring, carefully adjusting the wiring so that it can achieve the best electrical performance.

(3) Next is aesthetics. If your wiring is well laid out, there will be nothing that affects the performance of your electrical appliances. However, if it looks messy and colorful at first glance, then no matter how good your electrical appliances are, they will not be seen by others. It's still a piece of garbage in my eyes. This brings great inconvenience to testing and maintenance.

PCB wiring principles:

(1) The key lines should be as short and thick as possible, and add protective ground on both sides.

(2) Test points should be reserved for key signals to facilitate production and maintenance testing.

(3) Any signal line should not form a loop. If it is unavoidable, the loop should be as small as possible; the signal line should have as few vias as possible;

(4) When transmitting sensitive signals and noise field signals through flat cables, use the "ground wire-signal-ground wire" method.

(5) Wire lines with strict requirements (such as high-frequency lines) in advance. The edges of the input end and the output end should avoid being adjacent and parallel to avoid reflection interference. If necessary, ground wire isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur when parallel.

(6) The oscillator case should be grounded, and the clock line should be as short as possible and not lead everywhere. The ground area below the clock oscillation circuit and the special high-speed logic circuit part should be increased, and other signal lines should not be used to make the surrounding electric field approach zero;

(7) Use 45o fold line wiring as much as possible, and do not use 90o fold line to reduce the radiation of high-frequency signals; (double arc lines must be used for lines with high requirements)

(8) After the schematic wiring is completed, the wiring should be optimized; at the same time, after the preliminary network inspection and DRC inspection are correct, the unwired area should be filled with ground wires, and a large area of ​​copper layer should be used as ground wire, and the ground wire should be placed on the printed board. Connect all unused areas to the ground and use them as ground wires. Or it can be made into a multi-layer board, with power supply and ground wires occupying one layer each.

(9) Under normal circumstances, the power wire and ground wire should be routed first to ensure the electrical performance of the circuit board. Within the scope of conditions permitting, try to widen the width of the power and ground wires. It is best to make the ground wire wider than the power wire. Their relationship is: ground wire > power wire > signal wire. Usually the signal wire width is: 0.2 ~ 0.3mm , the thinnest width can reach 0.05~0.07mm, and the power cord is generally 1.2~2.5mm. For digital circuit PCBs, wide ground wires can be used to form a loop, that is, to form a ground network (the ground of analog circuits cannot be used in this way)

The PCB board designed by PCBLayout engineers will be checked to see if it meets the following requirements. The more that meet the requirements, the better the design level is:

1. The function operates normally;

2. Meet EMC test requirements;

3. PCB is cost-effective;

4. The layout and wiring are neat and beautiful;

5. Short design time and high work efficiency;

For high-frequency, high-current PCB wiring, such as switching power supplies, the most taboo is that the drive signal is interfered by the output of high current and high voltage. The drive signal of the MOS tube is easily affected by the strong output current. The two should be kept at a certain distance and not too close. In the era of analog audio, if the amplification factor of the op amp is too high, a self-excitation effect will occur. The reason is the same as that of MOS tubes.

The carrier of PCB wiring is the PCB board. Generally, the reference ground is about 1mm away from the edge of the PCB board, and the signal line is about 1mm away from the edge of the reference ground. In this way, the signals are constrained within the PCB board, which can reduce EMC radiation. When you have no concept of PCB design Yes, just think more about our daily path, the two are completely consistent.

In short, just talking about the number of layers and speed of PCB cannot evaluate whether the technology is powerful or not. When the number of devices is large and the signal speed is high, under the same conditions, the board can be completed with a smaller area, fewer layers, and lower design and production costs. design, and ensure good electrical performance and beautiful layout and wiring. It's relatively powerful to do this. When our society develops more rapidly, we need engineers to constantly sum up experience and innovate.