11 PCB design experiences that will last you a lifetime

- Dec 25, 2019-

11 PCB design experiences that will last you a lifetime

abstract:

1. If the designed circuit system contains FPGA devices, you must use Quartus II software to verify the pin assignments before drawing the schematic. (Some special pins in FPGA cannot be used as ordinary IO)

From top to bottom, the four-layer board is: signal plane layer, ground, power, and signal plane layer; from top to bottom, the six-layer board is: signal plane layer, ground, signal internal electrical layer, signal internal electrical layer, Power and signal plane layers. Boards with more than 6 layers (advantages: anti-interference radiation), the internal electrical layer wiring is preferred, and the plane layer cannot be selected, and the wiring from the ground or power layer is prohibited (reason: the power layer will be divided and parasitic effects will occur). Zh

3. Wiring of multiple power supply systems: If the FPGA + DSP system is a 6-layer board, there will generally be at least 3.3V + 1.2V + 1.8V + 5V. 3.3V is usually the main power supply, and the power supply layer is directly laid. The global power supply network is easily distributed through the vias.

5V may be a power input, and only copper is needed in a small area. Be as thick as possible (you ask me how thick-as thick as possible, the thicker the better)

1.2V and 1.8V are the core power (if you directly use the wire connection method, you will encounter great difficulties when facing BGA devices), try to separate 1.2V from 1.8V during layout, and connect the 1.2V or 1.8V The components are arranged in a compact area and connected using copper skin, as shown below:

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In short, because the power supply network is spread throughout the PCB, if the wiring method is used, it will be very complicated and it will be far away. Using the copper-clad method is a good choice!

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4. The wiring between adjacent layers adopts a crossover method: it can reduce the electromagnetic interference between parallel wires (high school), and it is convenient for wiring (Reference 1). The following figure shows the traces of two adjacent layers in a PCB, which are roughly one horizontal and one vertical.

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5. Analog and digital are to be isolated. What is the isolation method? During the layout, separate the components used for analog signals from the components for digital signals, and then cut across the AD chip!

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The analog signal is laid on the analog ground, and the analog ground / analog power and the digital power are connected at a single point through an inductor / bead.

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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". I think this idea can also be introduced into PCB design to reduce the probability of PCB errors. Zh

(1) Schematic inspection, pay special attention to the power and ground of the device (the power and ground are the blood of the system, there must be no negligence)

(2) PCB package drawing (confirm whether the pins in the schematic are wrong)

(3) After confirming the PCB package size one by one, add a verification label and add it to the design package library.

(4) Import the netlist and adjust the signal sequence in the schematic while layout (you cannot use OrCAD's automatic numbering function after layout)

(5) Manual wiring (check the power and ground network while cloth, as mentioned earlier: the power network uses copper wiring, so less wiring is required)

In short, the guiding idea in PCB design is to correct the schematic diagram by drawing the package layout and feedback (considering the correctness of the signal connection and the convenience of signal routing). Zh

7. The crystal oscillator should be as close as possible to the chip, and no trace should be run under the crystal oscillator, and the copper network of the ground should be laid. Clocks used in many places are routed using a tree clock tree. Zh

8. The arrangement of the signals on the connector has a great impact on the difficulty of wiring. Therefore, you must adjust the signals on the schematic diagram while wiring (but you must not renumber the components).

9.Design of multi-board connector:

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

(2) Straight socket: the upper and lower interfaces are mirror-symmetrical, as shown in the figure below

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10. The design of the module connection signal:

 

(1) If two modules are placed on the same side of the PCB, as follows: the management serial number is connected to the small one (mirror connection signal)

 (2) If the two modules are placed on different sides of the PCB, the management serial numbers are connected from small to large.

Doing so will place the signal crossover like the right picture above. Of course, the above method is not a rule. I always say that everything changes as needed (this can only be understood by yourself), but in many cases it is very useful to design in this way. Zh

11, the design of the power ground loop:


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The power supply ground loop in the above figure has a large area and is easily susceptible to electromagnetic interference.

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   The above figure has been improved-the power and ground lines are close to the wiring, which reduces the loop area and reduces electromagnetic interference (679 / 12.8, about 54 times). Therefore, the power supply and ground should be as close as possible to the wiring! And the signal lines should be avoided as much as possible and parallel to reduce the mutual inductance effect between the signals.

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