1. Consider the whole
Whether a product is successful or not 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 is required to be balanced, dense and orderly, not top-heavy or heavy.
Will the PCB be deformed?
Is there a process edge reserved?
Do you reserve MARK points?
Do you need jigsaw panels?
How many layers of board can guarantee impedance control, signal shielding, signal integrity, economy, and achievability?
2. Eliminate low-level errors
Is the size of the printed board consistent with the size of the processing drawing? Can it meet the requirements of the PCB manufacturing process? Are there positioning marks? Are there any conflicts between components in two-dimensional and three-dimensional spaces?
Is the layout of the components dense and orderly, neatly arranged? Is it all finished?
Can the components that need to be replaced be replaced easily? Is it convenient to insert the plug-in board into the equipment? Is there an appropriate distance between the thermal element and the heating element?
Is it easy to adjust the adjustable elements?
Where heat dissipation is required, is there a radiator installed? Is the air flow unobstructed?
Is the signal flow smooth with minimal interconnections?
Are the plugs, sockets, etc. inconsistent with the mechanical design? Is the interference of the line considered?
3. Bypass or decoupling capacitors
When wiring, both analo and digital devices need these types of capacitors, and both need to connect a bypass capacitor close to their power supply pins. This capacitor value is usually 0.1uF. The pins should be as short as possible to reduce the inductance of The traces, and should be as close as possible to the device.
Adding bypass or decoupling capacitors on the circuit board, and the arrangement of these capacitors on the board, are common sense for digital and analo design, but their functions are different.
In analo 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 analo chips through power pins. exceed the ability of analo devices to reject high-frequency signals.
If bypass capacitors are not used in analo circuits, noise may be introduced in the signal path, and in severe cases, even vibration may be caused. For digital devices such as controllers and processors, decoupling capacitors are also required, but for different reasons.
One function of these capacitors is to serve as a "miniature" charge bank, because in digital circuits, performing the switching of the gate state (ie, switching) usually requires a large current, and when switching, the switching transient current is generated on The chip and It is advantageous to have this extra "spare" charge flowing through the board.
If there is not enough charge when the switching action is performed, it will cause a large change in the power supply voltage. Too much voltage variation can cause the digital signal level to go into an indeterminate state and likely cause the state machine in the digital device to misbehave.
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
Where V=voltage change, L=circuit board trace inductance, dl=change of current flowing through the trace, dt=time of current change.
Therefore, it is good practice to apply bypass (or decoupling) capacitors at the power supply or at the supply pins of active devices for several reasons.
4. If the current of the input power supply is relatively large, it is recommended to reduce the length and area of the wiring, and do not run all over the field
Switching noise on the input is coupled to the plane of the output of the power supply. The switching noise of the MOS tube of the output power affects the input power of the previous stage.
If there are a large number of high-current DCDcs on the circuit board, there will be different frequencies, high-current and high-voltage jump interference.
So we need to reduce the area of the input power supply, just enough to meet the current flow. Therefore, when laying out the power supply, it is necessary to consider avoiding the full board operation of the input power supply.
The location of the power and ground wires is well matched to reduce the possibility of electromagnetic interference (EMI). If the power and ground lines are not mated properly, system loops will be designed in and noise will likely be introduced.
An example of a PCB design with improperly mated power and ground lines is shown in the figure. On this circuit board, different routes are used to lay out the power line and the ground wire. Due to this improper coordination, the electronic components and lines of the circuit board are more likely to be subject to electromagnetic interference (EMI).
5. Digital-analo separation
In every PCB design, the noisy portion of the circuit is separated from the “quiet” (non-noisy) portion.
Generally speaking, digital circuits can tolerate noise interference and are not sensitive to noise (because digital circuits have a larger voltage noise tolerance); on the contrary, analo circuits have much smaller voltage noise tolerance. Signal systems, these two circuits are separated.
6. Heat dissipation considerations
During the layout process, it is necessary to consider the heat dissipation air duct and the heat dissipation dead angle; the heat sensitive components should not be placed behind the heat source air. Priority should be given to the layout location of users with heat dissipation difficulties such as DDR. Avoid repeated adjustments due to failure of thermal simulation.
