If you want to design a high-quality PCB, it involves more than just a simple design; several critical factors need to be considered during the design process.
1. If the circuit system being designed includes FPGA devices, it is essential to use Quartus II software to verify pin assignments before drawing the schematic. (Certain pins on the FPGA cannot be used as regular I/O pins.)
2. In a 4-layer PCB, the layer stack-up from top to bottom should be: signal plane, ground, power, and signal plane. For a 6-layer board, the stack-up should be: signal plane, ground, inner signal layer, inner signal layer, power, and signal plane. For PCBs with six or more layers (which provide the advantage of better anti-interference and radiation control), routing should primarily be done on the inner signal layers. Plane layers should not be used for routing. It is also prohibited to route traces on the ground or power layers (the reason being that routing on the power layer could create divisions, leading to parasitic effects).
3. Power distribution in multi-supply systems: In an FPGA + DSP system on a 6-layer PCB, there will typically be multiple voltage rails such as 3.3V, 1.2V, 1.8V, and 5V. The 3.3V supply is generally the main power rail, which should be laid directly on the power layer, making it easier to route the global power network using vias.
5V is typically the power input, and only a small copper area is required, but it should be as thick as possible.
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1.2V and 1.8V serve as the core power supplies (if directly wired, you will face significant challenges when working with BGA devices). It’s recommended to separate the 1.2V and 1.8V regions during PCB layout, keeping the PCB components for these voltages tightly grouped and connected via copper planes, as illustrated in the figure:
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In summary, since the power supply network spans the entire PCB, routing can become very complex and lengthy. Using copper planes for power distribution is an effective solution!
4. The routing between adjacent layers should use a crossing pattern: this not only reduces electromagnetic interference between parallel traces but also simplifies the routing process.
5. How should analog and digital signals be isolated? Keep analog components separate from digital components during layout, and use a crossing technique for the AD chip to minimize interference! The analog signals should be routed with an analog ground, and the analog ground/analog power should be connected to the digital power via a single-point connection, typically using an inductor or ferrite bead.
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6. PCB design, especially when using PCB design software, can be considered part of a software development process. Similar to software engineering, the key concept of “iterative development” is applied to reduce the chances of PCB errors.
(1) Verify the schematic carefully, especially the power and ground connections (power and ground are the system’s lifeblood; they must not be overlooked);
(2) Confirm the accuracy of the PCB package drawings (check for pin errors in the schematic);
(3) After confirming the PCB package size for each component, add a verification label and store it in the design’s package library;
(4) Import the netlist and adjust the signal sequence in the schematic during layout (note that the automatic component numbering function in OrCAD can no longer be used after the layout);
(5) Manual routing (keep checking the power and ground networks during this step, as mentioned earlier: use copper planes for power distribution, and minimize trace routing);
In short, the guiding principle in PCB design is to constantly review and correct the schematic and package layout while drawing, ensuring the correctness of signal connections and the ease of routing.
7. The crystal oscillator should be placed as close as possible to the chip, with no routing under the oscillator. Copper planes should be used for the associated network. Often, clocks are distributed in a tree-like structure.
8. The arrangement of signals on connectors significantly impacts the ease of routing, so adjusting the signals on the schematic is important during layout (but avoid renumbering components).
9. Design considerations for multi-board connectors:
(1) Use flat cable connections: ensure both upper and lower interfaces are identical;
(2) For straight connectors: ensure the upper and lower interfaces are mirrored and symmetrical, as shown below:
10. Design considerations for module connection signals:
(1) If the two modules are placed on the same side of the PCB, the serial number for the supervisor should connect to the smaller module and then to the larger one (mirror the signal connections);
(2) If the two modules are placed on opposite sides of the PCB, the control system’s serial number should connect to the smaller module and then to the larger one. This approach will result in signal crossings, as shown in the figure on the right. Of course, this method is not a strict rule. As I often say, everything should be adjusted based on the specific needs (which is something you’ll learn through experience), but in many cases, this design approach is quite effective.
11. Power ground loop design:
The power supply’s ground loop area is large and susceptible to electromagnetic interference. To mitigate this, improve the layout by keeping the power and ground traces close together, reducing the loop area and, in turn, the electromagnetic interference (679/12.8, about 54 times). Therefore, the power and ground traces should be routed as close as possible. Additionally, signal traces should be kept as far away from the power/ground traces as possible to minimize mutual inductance between signals.
