1. Handling of power supply and ground wire
Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsiderate consideration of the power and ground wires will degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wires should be taken seriously, and the noise interference generated by electricity and ground wires should be minimized to ensure the quality of products. Every engineer who is engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only expresses the reduced noise suppression:
(1) It is well known to add a decoupling capacitor between the power supply and the ground.
(2) Widen the width of the power supply and ground wires as much as possible. It is better that the ground wire is wider than the power wire. The width can reach 0.05~0.07mm, and the power line is 1.2~2.5mm. 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 (the ground of the analog 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 make a multi-layer board, the power supply and the ground wire each occupy one layer.
2. Co-location of digital circuits and analog circuits
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog 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 analog circuit is strong. For the signal line, the high-frequency signal line should be as far away from the sensitive analog circuit device as possible. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated, and they are not connected to each other, but only at the interface between the PCB and the outside world (such as plugs, etc.). The digital ground and the analog ground are shorted a bit, please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.
3. The signal line is laid on the electrical (ground) layer
When wiring multi-layer printed boards, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.
4. Treatment of connecting legs in large-area conductors
In large-area grounding (electricity), the legs of commonly used components are connected to it, and the treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but for There are some unfavorable hidden dangers in the welding assembly of components, such as: ①Welding requires high-power heaters. ② It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process requirements, a criss-cross pad is made, which is called a heat shield, commonly known as a thermal pad (Thermal), so that the possibility of virtual solder joints due to excessive cross-section heat dissipation during soldering can be avoided. Sex is greatly reduced. The treatment of the connecting (ground) layer legs of the multilayer board is the same.
5. The role of the network system in wiring
In many CAD systems, wiring is determined according to the network system. If the grid is too dense, although the number of channels increases, the step size is too small, and the data volume of the map field is too large. This will inevitably have higher requirements for the storage space of the device, and at the same time, it will also affect the computing speed of computer electronic products. huge impact. And some paths are invalid, such as those occupied by the pads of component legs or by mounting holes and fixed holes. Too sparse a grid and too few channels have a great impact on the routing rate. Therefore, there must be a grid system with reasonable density to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches etc.
High-frequency PCB design skills and methods
1. The corner of the transmission line should adopt a 45° angle to reduce the return loss
2. It is necessary to adopt high-performance insulating circuit boards whose insulation constant values are strictly controlled according to the level. This approach facilitates efficient management of electromagnetic fields between the insulating material and adjacent wiring.
3. It is necessary to improve the PCB design specifications for high-precision etching. Consider specifying a total line width error of +/-0.0007 inches, managing the undercut and cross-section of wiring shapes, and specifying wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surfaces is critical to address skin effect issues associated with microwave frequencies and to achieve these specifications.
4. There is tap inductance in the protruding leads, so avoid using components with leads. In high frequency environments, it is best to use surface mount components.
5. For signal vias, avoid using the via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias.
6. Provide a rich ground plane. Molded holes should be used to connect these ground planes to prevent the influence of 3D electromagnetic fields on the circuit board.
7. To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating.
8. The solder mask can prevent the flow of solder paste. However, covering the entire board surface with solder mask will result in large variations in electromagnetic energy in microstrip designs due to thickness uncertainty and unknown insulation performance. Solder dams are generally used as the electromagnetic field of the solder resist layer.
In this case, we manage the conversion between microstrip and coax. In coaxial cable, the ground planes are circularly interwoven and evenly spaced. In microstrip, the ground plane is below the active lines. This introduces certain edge effects that need to be understood, anticipated and accounted for at design time. Of course, this mismatch also causes return loss, which must be minimized to avoid noise and signal interference.
Electromagnetic Compatibility Design
Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
1. Choose a reasonable wire width
Since the impact interference generated by the transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be reduced as much as possible. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the traces should be kept as short as possible. For discrete component circuits, the printed wire width can fully meet the requirements when it is about 1.5 mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0 mm.
2. Adopt the correct wiring strategy
The use of equal wiring can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires will increase. If the layout allows, it is best to use a well-shaped mesh wiring structure. The specific method is to wire one side of the printed board horizontally and the other side vertically. Then use metallized holes to connect at the intersection holes.
3. Effectively suppress crosstalk
In order to suppress crosstalk between printed circuit board wires, long-distance equal wiring should be avoided as far as possible when designing wiring, and the distance between lines should be opened as much as possible, and the signal lines and ground lines and power lines should not cross as much as possible. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
4. In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be paid attention to when wiring printed circuit boards:
(1) Minimize the discontinuity of the printed wires, for example, the width of the wires should not change abruptly, the corners of the wires should be greater than 90 degrees, and ring routing is prohibited.
(2) The clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they should be close to the ground loop, and the driver should be close to the connector.
(3) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be in close proximity to the connector
(4) The wiring of the data bus should sandwich a signal ground wire between every two signal wires. It is best to place the ground return next to the least critical address lead, which often carries high frequency currents.
(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.
5. Suppress reflection interference
In order to suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. If necessary, terminal matching can be added, that is, a matching resistor of the same resistance value is added to the end of the transmission line to the ground and the power supply end. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be used when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output driving current and the sinking current of the integrated circuit.
6. The differential signal line layout strategy is adopted in the circuit board design process
Differential signal pairs that are routed very close to each other will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal line very carefully to ensure that the characteristic impedance of the signal line is continuous and constant throughout the signal line.
During the layout and routing process of the differential line pair, we hope that the two PCB lines in the differential line pair are exactly the same. This means that in practical applications, we should try our best to ensure that the PCB lines in the differential line pair have exactly the same impedance and the length of the wiring is exactly the same. Differential PCB lines are usually always routed in pairs, and the distance between them remains constant at any position along the direction of the pair. Typically, the layout and routing of differential pairs is always as close as possible.
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