1, PCB layout/wiring, the impact on electrical performance is often seen in books about electronics, saying “digital ground wire should be separated from analog ground wire”. Anyone who has laid the board knows that it is difficult in practice.
To lay out a better board, first of all, you have to have an electrical understanding of the IC you are using. Which pins will generate higher harmonics (rising/falling edges of digital signals or switching square wave signals) and which pins are susceptible to electromagnetic interference. The signal block diagram (signal processing unit block diagram) inside the IC will help us understand.
The layout of the whole machine is the primary condition to determine the electrical performance, while the layout between boards is more concerned with the direction or flow of signals/data between IC’s, and the major principle is the part close to the power supply that is prone to electromagnetic radiation; The weak signal processing part is mostly determined by the overall structure of the equipment (that is, the overall planning of the previous equipment), and it is as close as possible to the signal input or the detection head (probe), which can better improve the signal-to-noise ratio and provide purer signals/accurate data for subsequent signal processing and data identification.
2. Treatment of PCB copper and platinum
Now, the working clock of IC (digital IC) is getting higher and higher, and its signal puts forward certain requirements for the line width. It is good for low-frequency and strong current to take the line width (copper and platinum), but it is not the case for high-frequency signals and data line signals. Data signals are more synchronous, and high-frequency signals are mostly affected by skin effect, so they should be separated.
High-frequency signal traces should be thin rather than wide, short rather than long, which in turn involves layout problems (signal coupling between devices), which can reduce induced electromagnetic interference.
However, the data signal appears on the circuit in the form of pulse, and its higher harmonic component is the decisive factor to ensure the correctness of the signal. The same wide copper and platinum will have skin effect (increased distributed capacitance/inductance) on high-speed data signals, which will lead to signal deterioration and incorrect data identification. Moreover, if the line widths of data bus channels are inconsistent, it will even affect data synchronization (resulting in inconsistent delay). In order to better control the synchronization of data signals, serpentine lines appear in data bus routing, which is to make the signals in data channels more consistent in delay. Large-area copper plating is aimed at shielding interference and inductive interference, and double panels can make the ground as a copper plating layer; However, there is no problem of laying copper on the multilayer board, because the power layer between them plays a very good role in shielding and isolating.
3. Interlayer layout of multilayer board
Taking a four-layer board as an example, the power supply (positive/negative) layer should be placed in the middle, and the signal layer should be routed on the outer two layers. Pay attention to the fact that there should be no signal layer between the positive and negative power supply layers. The advantage of this approach is to make the power supply layer play the role of filtering/shielding/isolation as much as possible, and at the same time facilitate the production of PCB manufacturers to improve the yield.
4. via hole
The design of via holes should be reduced as much as possible, because while the via holes will generate capacitance, they are also prone to burr and electromagnetic radiation.
The aperture of the via hole should be small rather than large (this is for electrical performance; However, too small an aperture will increase the difficulty of PCB production. Generally, 0.5mm/0.8mm is commonly used, and 0.3mm is used as little as possible. The probability of burr in small aperture after copper deposition process is smaller than that in large aperture, which is due to the drilling process.
5. Software application
Every software has its ease of use, just your familiarity with the software. I have used PADS(POWER PCB)/PROTEL. When making a simple circuit (the one I am familiar with), I will use PADS to directly Layout；; When making complex and new device circuits, it is better to draw the schematic diagram first and use the form of network list to make it more correct and convenient.
When laying out PCB, there are some non-circular holes, which are not described by the software. My usual practice is: open a layer dedicated to expressing holes, and then draw the desired hole shape on this layer. Of course, the drawn wireframe should be filled in. This is done to better enable PCB manufacturers to recognize their own expressions, and explain them in the sample making documentation.
6. PCB is sent to the manufacturer for sample.
1. PCB computer files
2. Layering scheme of PCB files (every electronic engineer has different drawing habits, and the application of PCB files after Layout will be different, so you should attach a white oil diagram/green oil diagram/circuit diagram/mechanical structure diagram/auxiliary opening diagram of your files, and make a correct list document to explain your wishes)
3. PCB production process requirements, you should attach a document to explain the process of making the board: gold plating/copper plating/tin spraying/rosin sweeping, board thickness specification, PCB board material (flame retardant/non-flame retardant).
4. Number of samples to be made
5. Of course, the contact information and the person in charge must be signed.
