PCB Technical - PCB drawing summary

PCB drawing summary

1. Software application

Each software has its ease of use, but only your familiarity with the software, PADS (POWER PCB)/PROTEL when making simple circuits, use PADS to lay out directly; when making complex and new device circuits, it is better to go first Draw the schematic diagram and do it in the form of a netlist, which should be correct and convenient.

When Layout PCB, there are some non-circular holes, there is no corresponding function to describe in the software, the usual way is: open a layer dedicated to express the holes, and then draw the desired holes on this layer The shape, of course, should be filled with the drawn wire frame. This is to better allow the PCB manufacturer to recognize its own expression and explain it in the sample documentation.

2. Interlayer layout of multi-layer boards

Take a four-layer board as an example. The power positive/negative layer should be placed in the middle, and the signal layer should be routed on the outer two layers. Note that there should be no signal layer between the positive and negative power layers. The advantage of this method is to maximize It is possible for the power layer to play the role of filtering/shielding/isolation, while facilitating the production of PCB manufacturers to improve the yield rate.

3. PCB copper and platinum treatment

As the current IC working clock (digital IC) is getting higher and higher, its signal puts forward certain requirements on the width of the line. The width of the trace (copper platinum) is good for low-frequency and strong current, but for high-frequency signals and data For line signals, this is not the case. Data signals are more about synchronization. High-frequency signals are mostly affected by the skin effect. Therefore, high-frequency signal traces should be thin rather than wide, short rather than long, which involves layout issues. (Coupling of signals between devices), which can reduce induced electromagnetic interference.

The data signal appears on the circuit in the form of pulses, and its high-order harmonic content is the decisive factor to ensure the correctness of the signal; the same wide copper platinum will produce a skin effect (distribution) for the high-speed data signal. Capacitance/inductance becomes larger), this will cause the signal to deteriorate, the data recognition is incorrect, and if the line width of the data bus channel is inconsistent, it will affect the synchronization problem of the data (causing inconsistent delay), in order to better control the data signal Therefore, the snake-shaped routing appears in the data bus routing, which is to make the signals in the data channel more consistent in delay.

4. Via

Engineering design should minimize the design of vias, because vias will generate capacitance, but also burrs and electromagnetic radiation. The aperture of the via hole should be small rather than large (this is for electrical performance; but too small aperture will increase the difficulty of PCB production, generally 0.5mm/0.8mm, 0.3mm is used as little as possible), small aperture is used in copper sinking process The probability of subsequent burrs is smaller than that of large apertures. This is due to the drilling process.

5. Layout/wiring, influence on electrical performance

The digital ground wire should be separated from the analog ground wire. This is a certain degree of difficulty in actual operation. To lay out a better board, you must first understand the electrical aspects of the IC you are using, which pins will produce high-order harmonics (the rising/falling edges of digital signals or switching square wave signals), and which lead The feet are easy to induce electromagnetic interference, and the signal block diagram (signal processing unit block diagram) inside the IC helps us understand.

The layout of the whole machine is the primary condition for determining the electrical performance, and the layout between the circuit boards is more concerned with the direction or flow of the signal/data between the ICs. The general principle is to be as close as possible to the power supply part that is prone to electromagnetic radiation; weak signal The processing part is mostly determined by the overall structure of the equipment (that is, the overall planning of the equipment in the early stage). The weak signal processing part is as close as possible to the input end of the signal or the detection head (probe), which can better improve the signal-to-noise ratio for subsequent signals. Processing and data recognition provide a purer signal/accurate number