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PCB Reverse Engineering

Analysis and summary of PCB reverse

PCB reverse size should be moderate, too large when the printed line length, impedance increases, not only anti-noise capacity decline, the cost is high; too small, the heat is not good, and vulnerable to adjacent line interference. In the device layout and other logic circuits, should be related to the device as far as possible close to some, so you can get a better anti-noise effect.
First, the printed circuit board size and device layout
PCB reverse size should be moderate, too large when the printed line length, impedance increases, not only anti-noise capacity decline, the cost is high; too small, the heat is not good, and vulnerable to adjacent line interference. In the device layout and other logic circuits, should be related to the device as far as possible close to some, so you can get a better anti-noise effect. Clock generator, crystal and CPU clock input are easy to produce noise, to be close to each other. Easy to produce noise devices, small current circuits, high current circuits should be kept away from the logic circuit, if possible, should do another circuit board, which is very important.
Second, the decoupling capacitor configuration
In the DC power supply circuit, the load changes will cause power supply noise. For example, in a digital circuit, when the circuit from one state to another state, it will produce a large peak current on the power line to form a transient noise voltage. Configuring the decoupling capacitor to suppress noise due to load changes is a common practice for the reliability design of printed circuit boards.
The configuration principle is as follows
Power input terminal across a 10 ~ 100uF electrolytic capacitors, if the PCB reverse position allows the use of more than 100uF electrolytic capacitor anti-jamming effect will be good.
Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and can not be installed, each 4 to 10 chips can be equipped with a 1 ~ 10uF tantalum electrolytic capacitors, the device's high-frequency impedance is particularly small, in the range of 500kHz ~ 20MHz impedance less than 1Ω, And leakage current is very small (0.5uA below).
For devices with low noise capability, large current changes at shutdown, and memory devices such as ROM and RAM, the decoupling capacitors should be connected directly between the chip's power supply line (Vcc) and ground (GND).
Decoupling capacitors can not lead too long, especially high-frequency bypass capacitors can not lead.
Third, the thermal design
From the point of view of heat dissipation, the printed version is best upright installation, the distance between the board and the board should not be less than 2cm, and the device in the printed version of the arrangement should follow certain rules:
1. For equipment with free convective air cooling, it is advisable to arrange the integrated circuit (or other device) in an elongated manner; for equipment using forced air cooling, it is advisable to use an integrated circuit (or other device) Way row.
2, the same piece of PCB reverse on the device should be as far as possible the size of its heat and heat degree of partition arrangement, heat or heat resistance of poor devices (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) on the cooling The most upstream (inlet) of the airflow, heat-generating or heat-resistant devices (such as power transistors, large scale integrated circuits, etc.) are placed downstream of the cooling airflow.
3, in the horizontal direction, high-power devices as close as possible to the edge of the printed circuit board layout, in order to shorten the heat transfer path; in the vertical direction, high-power devices as close as possible to the top of the printed circuit board in order to reduce these devices work on other device temperature Impact.
4, the temperature sensitive devices are best placed in the lowest temperature areas (such as the bottom of the device), do not put it directly above the heating device, multiple devices are best in the horizontal plane staggered layout.
5, the device inside the PCB reverse the heat mainly rely on air flow, so in the design to study the air flow path, a reasonable configuration device or printed circuit board. Air flow always tends to flow in small areas, so when configuring devices on PCB reverse, avoid having a larger airspace in a certain area.
Fourth, the electromagnetic compatibility design
Electromagnetic compatibility is the ability of electronic equipment to be able to work effectively and effectively in various electromagnetic environments. Electromagnetic compatibility design aims to make electronic equipment can not only inhibit a variety of external interference, so that electronic equipment in a specific electromagnetic environment can work properly, while reducing the electronic equipment itself on other electronic equipment, electromagnetic interference.
1. Select a reasonable wire width
As the transient current on the printed line generated by the impact of interference is mainly caused by the inductance of the printed wire, and therefore should minimize the inductance of the printed wire. The inductance of the printed wire is proportional to its length, inversely proportional to its width, so short and fine wires are advantageous for suppressing interference. Clock lines, line drivers, or bus drivers' signal lines often carry large transients, and the printed leads are as short as possible. For discrete components circuit, the printed wire width of 1.5mm or so, you can fully meet the requirements; for integrated circuits, printed wire width can be selected between 0.2 ~ 1.0mm.
2. Use the correct routing strategy
The use of equal alignment can reduce the wire inductance, but the wire between the mutual inductance and distributed capacitance increases, if the layout allows, it is best to use well-shaped mesh fabric structure, the specific approach is printed on the side of the horizontal wiring, the other side of vertical wiring, And then connected at the cross hole with a metal hole. In order to suppress the crosstalk between the printed circuit board wire, in the design of wiring should try to avoid long distance equal alignment.
Fifth, ground design
In electronic equipment, grounding is an important way to control interference. Such as grounding and shielding the correct combination of use, can solve most of the interference problem. Electronic equipment in the ground structure roughly systematically, chassis (shield), digital (logical) and analog ground. In the ground design should pay attention to the following:
1. Select the single point of grounding and multi-point grounding
In the low-frequency circuit, the signal frequency is less than 1MHz, its wiring and the inductance between the device less affected, and the formation of the ground circuit on the interference of a larger impact, and therefore should be a little ground. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large, this time should try to reduce the ground impedance, should be used near the multi-point grounding. When the operating frequency of 1 ~ 10MHz, if the use of a little ground, the ground length should not exceed the wavelength of 1/20, or should be used multi-point grounding method.
2. Separate the digital circuit from the analog circuit
PCB reverse both high-speed logic circuit, there are linear circuits, so that they should be separated as far as possible, and the two ground do not mix, respectively, connected with the power supply side ground. To maximize the grounding of the linear circuit area.
3. As much as possible to increase the ground wire
If the ground wire is very thin, the ground potential changes with the current changes, resulting in electronic equipment, timing signal level instability, anti-noise performance deteriorated. So the ground wire should be as thick as possible so that it can pass the three allowable current in the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.
4. Make the ground wire form a closed loop
Designed by the digital circuit composed of printed circuit board ground system, the ground wire made closed loop can significantly improve the anti-noise ability. The reason is: printed circuit board has a lot of integrated circuit components, especially in the case of multi-power components, due to the grounding wire thickness limit, will produce a large potential difference in the ground, causing anti-noise capacity , If the grounding structure into a loop, will reduce the potential difference, improve the anti-noise ability of electronic equipment


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