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Basic knowledge of PCB design

time :2020-07-15 author : from: scanning : classify :Technical articles

Basic knowledge of PCB design


1. If FPGA devices are included in the designed circuit system, QuartusII software must be used to verify the pin assignment before drawing the schematic diagram. (some special pins in FPGA cannot be used as ordinary IO).

2. From the upper layer to the lower layer, the signal layer is from the upper layer to the lower layer, and the signal layer is from the upper layer to the lower layer. For boards with more than 6 layers (advantage: anti-interference radiation), internal electric layer wiring shall be preferred, plane layer shall be selected if it cannot run, and wiring from ground or power layer is prohibited (reason: power layer will be divided and parasitic effect will be generated).



3. Wiring of multi power system: if FPGA + DSP system is made of 6-layer board, there will be at least 3.3V + 1.2V + 1.8V + 5V.


3.3V is generally the main power supply, and the power layer is directly laid, and the global power network is easily connected through the via hole;


5V may be the power input generally, only need to lay copper in a small area. And as thick as possible.


1.2V and 1.8V are the core power supply (if the wire connection method is adopted directly, it will encounter great difficulties when facing BGA devices). During the layout, 1.2V and 1.8V components should be separated as far as possible, and the components connected in 1.2V or 1.8V should be arranged in a compact area and connected by copper sheet


In a word, because the power supply network is all over the PCB board, it will be very complicated and winding far if the wiring method is adopted. The copper covering method is a good choice!


4. The wiring between adjacent layers adopts cross mode, which can reduce the electromagnetic interference between parallel wires and facilitate wiring.


5. Analog digital isolation, how to isolate? Layout will be used for analog signal devices and digital signal devices separated, and then from the ad chip in the middle!


The analog signal is laid on the analog ground, and the analog ground / analog power supply is connected with the digital power supply through a single point inductance / magnetic bead.


6. PCB design based on PCB design software can also be regarded as a software development process. Software engineering pays most attention to the idea of "iterative development" to reduce the probability of PCB errors.


(1) Check the schematic diagram, pay special attention to the power supply and ground of the device (the power supply and ground are the blood vessels of the system, and no negligence is allowed);


(2) PCB circuit board packaging drawing (confirm whether the pins in the schematic diagram are wrong);


(3) After the PCB package dimensions are confirmed one by one, the verification label is added to the packaging Library of this design;


(4) Import the netlist and adjust the signal sequence in the schematic diagram while layout (the automatic numbering function of OrCAD components cannot be used after layout);


(5) Manual wiring (check the power supply ground network while cloth is being used. As mentioned above: the power network uses copper laying method, so less wiring is used);


In a word, the guiding ideology of PCB design is to draw the packaging layout and wiring while feedback correction schematic diagram (considering the correctness of signal connection and the convenience of signal wiring).


7. The crystal oscillator should be close to the chip as far as possible, and no wiring should be conducted under the crystal oscillator, and the network copper skin should be paved. The clocks used in many places are wired in the form of tree clock tree.


8. The signal arrangement on the connector has a great influence on the difficulty of wiring. Therefore, it is necessary to adjust the signal on the schematic diagram while wiring (but never re number the components).


9. Design of multi board connector:


(1) Use flat cable connection: the upper and lower interfaces are consistent;


(2) Straight socket: upper and lower interface mirror symmetry


10. Design of module connection signal:


(1) If two modules are placed on the same side of the PCB, the number of the training serial is connected with the small one (mirror image connection signal);


(2) If the two modules are placed on different sides of PCB, the small number is connected with the small one and the large one is connected.


By doing this, the signals can be placed to cross as shown above on the right. Of course, the above method is not the rule. I always say that everything changes according to needs (this can only be understood by ourselves), but in many cases, it is very useful to design in this way.


11. Design of power supply ground circuit:


The ground circuit of power supply in the figure above has large area and is easy to be affected by electromagnetic interference.


In the figure above, the circuit area is reduced and the electromagnetic interference (EMI) is reduced (679 / 12.8, about 54 times) through the improvement that the power supply and ground wire are close to each other. Therefore, the power supply and ground should be close to the wiring as far as possible, and the parallel wiring between signal lines should be avoided as far as possible, so as to reduce the mutual inductance effect between signals.