In the design, the layout is an important part. The result of the layout will directly affect the effect of wiring, so you can think of it this way, a reasonable layout is the first step in the success of PCB design.
In particular, pre-layout is the process of thinking about the entire board, signal flow, heat dissipation, structure and other architecture. If the pre-layout is a failure, the later more effort is also in vain.
1. Consider the whole
The success of a product or not, one is to focus on the internal quality, the second is to take into account the overall aesthetics, both are more perfect to consider the product is successful.
On a PCB board, the layout of components required to be balanced, sparse and orderly, not top-heavy or head heavy.
Will the PCB be deformed?
Are process edges reserved?
Are MARK points reserved?
Is it necessary to put together the board?
How many layers of the board, can ensure impedance control, signal shielding, signal integrity, economy, achievability?
2. Exclude low-level errors
Does the printed board size match with the processing drawing size? Can it meet the PCB manufacturing process requirements? Is there a positioning mark?
Components in two-dimensional, three-dimensional space there is no conflict?
Is the layout of the components in order and neatly arranged? Is all the cloth finished?
Can the components that need to be replaced frequently be replaced easily? Is it convenient to insert the insert board into the equipment?
Is there a proper distance between the thermal element and the heating element?
Is it easy to adjust the adjustable components?
Is a heat sink installed where heat dissipation is required? Is the air flowing smoothly?
Is the signal flow smooth and the shortest interconnection?
Are plugs, sockets, etc. contradictory to the mechanical design?
Is the interference problem of the line considered?
3. Bypass or decoupling capacitor
In the wiring, analog and digital devices need these types of capacitors, need to be close to their power pins connected to a bypass capacitor, the capacitance value is usually 0.1μF. pins as short as possible to reduce the inductive resistance of the alignment, and as close as possible to the device.
Adding bypass or decoupling capacitors to the board, and the placement of these capacitors on the board, is basic knowledge for both digital and analog designs, but their functions are different. Bypass capacitors are often used in analog wiring designs to bypass high-frequency signals from the power supply that might otherwise enter sensitive analog chips through the power supply pins. Generally, the frequency of these high-frequency signals exceeds the analog device’s ability to suppress them. If bypass capacitors are not used in analog circuits, noise and, in more severe cases, vibration can be introduced in the signal path. For digital devices such as controllers and processors, decoupling capacitors are also needed, but for different reasons. One function of these capacitors is to act as a “miniature” charge bank, because in digital circuits, performing gate state switching (i.e., switch switching) usually requires a large amount of current, and when switching transients are generated on the chip and flow through the board, it is advantageous to have this extra “spare” charge. ” charge is advantageous. If there is not enough charge to perform the switching action, it can cause a large change in supply voltage. Too large a change in voltage can cause the digital signal level to go into an indeterminate state and likely cause the state machine in the digital device to operate incorrectly. The switching current flowing through the board alignment will cause the voltage to change, due to the parasitic inductance of the board alignment, the voltage change can be calculated using the following formula: V = Ldl/dt where V = change in voltage L = board alignment inductance dI = change in current flowing through the alignment dt = time of current change Therefore, for a variety of reasons, the power supply at the power supply or active devices at the power pins applied Bypass (or decoupling) capacitors are very good practice.
The input power supply, if the current is relatively large, it is recommended to reduce the length and area of the alignment, do not run all over the field.
The switching noise on the input coupled to the plane of the power supply output. The switching noise of the MOS tube of the output power supply affects the input power supply of the front stage.
If there is a large number of high current DCDC on the board, there are different frequencies, high current and high voltage jump interference.
So we need to reduce the area of the input power supply to meet the through-current on it. So when the power supply layout, consider avoiding input power full board run.
4. Power lines and ground
Power lines and ground lines are well positioned to match, can reduce the possibility of electromagnetic interference (EMl). If the power and ground lines do not fit properly, the system loop will be designed, and is likely to generate noise. An example of an improperly mated power and ground PCB design is shown in the figure. In this board, use different routes to cloth power and ground, due to this improper fit, the board’s electronic components and lines by electromagnetic interference (EMI) is more likely.
5. Digital-analog separation
In each PCB design, the noise part of the circuit and the “quiet” part (non-noise part) to be separated. In general, the digital circuit can tolerate noise interference, and not sensitive to noise (because the digital circuit has a large voltage noise tolerance); on the contrary, the analog circuit voltage noise tolerance is much smaller. Of the two, analog circuits are the most sensitive to switching noise. In wiring mixed-signal systems, these two types of circuits should be separated.
The basics of circuit board wiring apply to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This basic rule reduces the dI/dt (current versus time) effect in digital circuits because the dI/dt effect causes the ground potential and allows noise to enter the analog circuit. Wiring techniques for digital and analog circuits are basically the same, except for one thing. Another thing to keep in mind for analog circuits is to keep the digital signal lines and loops in the ground plane as far away from the analog circuit as possible. This can be accomplished by either connecting the analog ground plane separately to the system ground connection, or by placing the analog circuitry at the far end of the board, at the end of the line. This is done to keep external interference to 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.
6. Thermal considerations
In the layout process, the need to consider heat dissipation air ducts, heat dissipation dead ends.
Heat-sensitive devices should not be placed behind the heat source wind. Give priority to the layout location of such a difficult heat dissipation household as DDR. Avoid repeated adjustments due to thermal simulation does not pass.
Post time: Aug-30-2022