In PCB design, layout is an important part. The result of the layout will directly affect the effect of the wiring, so it can be considered that a reasonable layout is the first step to a successful PCB design. Today, China PCBA factory-SysPCB will learn the design essentials that must be mastered in PCB design with everyone.
1. Consider the whole
The success of a product depends on the internal quality and the overall aesthetics. Only when both are perfect can the product be considered successful. On a PCB board, the layout of components must be balanced and proper.
Will the PCB be deformed? Will there be process edges reserved? Will marl points be reserved? Do you need arrange panel? How many layers of boards can ensure impedance control, signal shielding, signal integrity, economy, and feasibility? All of the above what needs to be considered.
2. Eliminate low-level errors
Including but not limited to the following: Does the size of the printed board match the size of the processing drawings? Can it meet the requirements of the PCB manufacturing process? Are there positioning marks? Are there conflicts in the two-dimensional and three-dimensional space of the components? Is the layout of the components proper and orderly, arranged neatly? Can the components that need to be replaced frequently be easily replaced? Is it convenient to insert the plug-in board into the device? Is there an appropriate distance between the thermal sensitive component and the heating component? Is it convenient to adjust the adjustable components? Is there a radiator installed? Is the air flow unobstructed? Is the signal flow smooth and whether the shortest interconnection? Plugs, sockets etc. are inconsistent with the mechanical design? Is the line interference problem considered?
3. Bypass capacitor or decoupling capacitor?
Adding bypass capacitors or decoupling capacitors to the circuit board and the arrangement of these capacitors on the board are basic common sense for digital and analog designs, but their functions are different. In analog wiring design, bypass capacitors are usually used to bypass high-frequency signals on the power supply. If bypass capacitors are not added, these high-frequency signals may enter sensitive analog chips through the power supply pins. Generally, the frequency of these high-frequency signals exceeds the ability of analog devices to suppress high-frequency signals. If bypass capacitors are not used in the analog circuit, noise may be introduced into the signal path, and more serious cases may even cause vibration. For digital devices such as controllers and processors, decoupling capacitors are required. One function of these capacitors is to be used as a "miniature" charge banks. This is because in digital circuits, switching of the gate state usually requires a large amount of current.
When switching, the chip generates a switching transient current and flowing through the circuit board, it is advantageous to have these extra "spare" charge. If there is not enough charge when performing the switching action, the power supply voltage will change greatly. Too much voltage change will cause the digital signal level to enter an uncertain state, and may cause the state machine in the digital device to operate incorrectly. The switching current flowing through the circuit board trace will cause the voltage to change. Due to the parasitic inductance of the circuit board trace, the following formula can be used to calculate the voltage change: V=Ldl/dt, V=voltage change, L=circuit board trace inductive reactance, dI = change of current flowing through the trace, dt = time of current change. Therefore, for many reasons, it is a good practice to apply bypass capacitor or decoupling capacitors at the power supply or at the power supply pins of active devices.
4. Input power, if the current is relatively large, it is recommended to reduce the length and area of the trace
If there is a large amount of large current DC on the circuit board, there will be different frequencies, large current and high voltage jump interference. So we need to reduce the area of the input power supply to satisfy the current flow. Therefore, when the power supply layout, it is necessary to consider avoiding the input power supply running full board.
5. Power line and ground line
The position of the power line and the ground line are well matched to reduce the possibility of electromagnetic interference (EMI). If the power line and the ground line are not properly matched, a system loop will be designed and noise will likely be generated. An example of a PCB design where the power line and ground line are not properly matched is shown in the figure. On this circuit board, different routes are used to wiring the power lines and ground lines. Due to this improper cooperation, the electronic components and circuits of the circuit board are more likely to be subject to electromagnetic interference(EMI).
6. Digital and analog separation
In each PCB design, the noise and non-noise parts of the circuit should be separated. Generally, digital circuits can tolerate noise interference and are not sensitive to noise (because digital circuits have a larger voltage noise tolerance); on the contrary, the voltage noise tolerance of analog circuits is much smaller. Of the two, analog circuits are the most sensitive to switching noise. In the wiring of a mixed-signal system, these two circuits must be separated.
The basic knowledge of circuit board wiring is applicable to analog circuit also applicable to digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This basic rule can reduce the dI/dt effect in digital circuits, because the dI/dt effect can cause ground potential and cause noise to enter analog circuits. The wiring techniques for digital and analog circuits are basically the same, with one exception. For analog circuits, there is another point to note, that is, keep the digital signal lines and loops in the ground plane as far away from the analog circuits as possible. This can be achieved by connecting the analog ground plane separately to the system ground connection, or placing the analog circuit at the farthest end of the circuit board, which is the end of the line. This is done to keep the external interference on the signal path to a minimum. This is not necessary for digital circuits, which can tolerate a large amount of noise on the ground plane without problems.
7. Consider heat dissipation
In the layout process, you need to consider the heat dissipation air ducts and dead heat dissipation; heat sensitive components should not be placed behind the heat source wind. Priority is given to the layout position of DDR, which is difficult for heat dissipation. Avoid repeated adjustments due to failure of thermal simulation.
