What does PCB Layout mean?
PCB is the abbreviation of Printed Circuit Board. It is the support of electronic components and the carrier of electrical connection of electronic components. It is known as the "mother of electronic products". LAYOUT means layout planning. Combined: PCBLayout is the meaning of printed circuit board layout and wiring. PCB design, also commonly known as PCB Layout.
PCB design needs to be realized with the help of computer-aided design. Commonly used design software in the industry includes: Cadence Allegro, PADS, AltiumDesigner, etc.
Conventional PCB design includes several steps such as library building, netlist adjustment, layout, wiring, and file output, but the conventional PCB design process is far from meeting the increasingly complex high-speed PCB design requirements. Since SI simulation, PI E MC design, single board process, etc. need to be closely integrated into the design process, and in order to achieve quality control, it is necessary to add review links at each node, the actual PCB design process is much more complicated.
PCB layout
On the basis of comprehensively considering the requirements of signal quality, EMC, thermal design, DFM, DFT, structure, safety regulations, etc., the devices are placed on the board reasonably. --layout
PCB layout design is the first important design link in the entire PCB design process. The more complex the PCB board, the better the layout can directly affect the difficulty of later wiring. The layout should meet the following requirements as far as possible: the total connection line is as short as possible, and the key signal line is the shortest; high voltage, high current signals are completely separated from weak signals of low voltage, small current signals; Separate low-frequency uff signals; spaced. Under the premise of meeting the requirements of simulation and timing analysis, local adjustments are made.
PCB layout
Under the requirements of signal quality, DFM, EMC and other rules, the physical connection design between device pins is realized. --wiring
PCB layout design is the process with the largest workload in the entire PCB design, which directly affects the performance of the PCB board. Some basic requirements for wiring processing are as follows:
1) Avoid using limit values for vias, line widths, and safety spacing.
2) The distance from the wiring to the edge of the board usually needs to be ≥ 2mm. If the conditions cannot be met, at least ensure that it is not less than 20mil.
3) Vias and surface wiring are not allowed under the metal shell device.
4) Try to provide a dedicated wiring layer for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure the smallest loop area. Use methods such as shielding and increasing the safety distance to ensure signal quality.
5) The EMC environment between the power layer and the ground layer is poor, and signal lines that are sensitive to interference should be avoided.
6) Route as close to a plane as possible and avoid cross splits. If it is necessary to cross the split or cannot be close to the power ground plane, these conditions are only allowed to exist in the low-speed signal line.
7) The distribution of the plane layer and the wiring layer is symmetrical, the distribution of the dielectric thickness is symmetrical, and the vias are kept symmetrical across layers.
8) All signal lines must be chamfered, and the chamfering angle is 45 degrees, except for special cases.
Handling of power and ground wires
Even if the wiring in the entire PCB board is done well, the interference caused by the power and ground wires will degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the treatment of power supply and ground The wire should be taken seriously, and the noise and interference generated by power supply and ground wire should be minimized to ensure the quality of the product.
1) Try to widen the width of the power and ground wires, preferably the ground wire is wider than the power wire, and their relationship is: ground wire-power wire-signal wire.
2) For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of the circuit cannot be used in this way)
3) Use a large area of copper layer as the ground wire, and connect the unused places to the ground on the printed board as the ground wire. Or a multi-layer board, the power supply and the ground wire each
occupy one layer.
Common ground processing of digital circuit and circuit
Nowadays, many PCBs are no longer single-function circuits, but are composed of a mixture of digital circuits and circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground.
The frequency of the digital circuit is high, and the sensitivity of the circuit is strong. For the signal line, the high-frequency signal line should be as far away from the sensitive circuit device as possible. For the ground line, the entire PCB has only one node to the outside world, so it must be The problem of digital and common ground is dealt with inside the PCB, and the digital ground and ground are actually separated inside the board, and they are not connected to each other, but only at the interface between the PCB and the outside world (such as plugs, etc.) ). There is a short circuit between the digital ground and the ground (please note that there is only one connection point), and there are also non -common grounds on the PCB, which is determined by the system settings.
Design rule checking (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. At the same time, it is also necessary to confirm whether the established rules meet the requirements of the printed board production process .The general inspection includes the following aspects:
1) Whether the distance between the line and the line, the line and the component pad, the line and the hole, the component pad and the hole, the hole and the hole is reasonable, and whether it meets the production requirements.
2) Whether the width of the power line and the ground line is appropriate, whether there is coupling between the power supply line and the ground line, and whether there is any place in the PCB where the ground line can be widened.
3) Whether the best measures have been taken for the key signal lines (such as the shortest length, protection lines, input lines and output lines are clearly separated)
4) Whether the circuit and the digital circuit part have their own independent ground wires.
5) Whether the graphics (such as icons and labels) added to the PCB will cause a short circuit of the signal.
6) Whether there is a process line on the PCB, whether the solder mask meets the requirements of the production process, whether the size of the solder mask is appropriate, and whether the character mark is pressed against the device pad.
