PCB Technical - PCB layout design experience new edition

InPCB board design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB board design, the wiring design process is the most restrictive, the skills are the smallest, and the workload is the largest. PCB board wiring is divided into single-sided wiring, double-sided wiring and multi-layer wiring. There are also two ways of PCB board wiring: automatic wiring and interactive wiring. Before automatic routing, you can use interactive pre-routing for stricter requirements. The edge lines of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference, and grounding should be added if necessary. The wiring of two adjacent layers should be perpendicular to each other, and it is easy to cause parasitic coupling in parallel.

The layout rate of the automatic wiring of the PCB board depends on a good PCB board layout. The wiring rules can be preset, including the number of bending times, the number of vias, the number of steps, and so on. Generally, explore the warp wiring first, quickly connect the short wires, and then perform the labyrinth wiring. First, the wiring to be laid is optimized for the global wiring path. It can disconnect the laid wires as needed, and try to retry. Wiring to improve the overall effect.

The current high-density PCB board design has felt that the through-hole is not suitable, and it wastes a lot of valuable wiring channels. In order to solve this problem, blind and buried hole technologies have appeared. It not only completes the function of the vias, but also saves many wiring channels, making the wiring process more smooth and more complete. The PCB board design process is a complex and simple process. If you want to master it well, you still need electronic engineering designers to experience and sum up their experience.

1. Treatment of power supply and ground wire

In the entire PCB board design, even if the wiring is completed well, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power supply and ground wire should be taken seriously, and the noise interference generated by the power supply and ground wire should be minimized to ensure the quality of the product.

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 the reduced noise suppression is described. It is well known to add decoupling capacitors between the power supply and ground. Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the smallest width can be reached 0.05～0.07mm, the power cord 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 net to use (the ground of the analog circuit cannot be used in this way) Use a large area copper layer as the ground For wire use, connect the unused places on the printed board to the ground as a ground wire. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

2. Common ground processing of digital circuit and analog circuit

Nowadays, many PCB boards are no longer single functional 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 designing and wiring the PCB board, especially the noise interference on the ground wire. 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 as possible from the sensitive analog circuit components. For the ground line, the entire PCB board has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB board. In the PCB board, the digital ground and the analog ground are actually separated, and they are not connected to each other, but at the interface (such as a plug, etc.) connecting the PCB board to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common ground on the PCB board, which is determined by the system design.

3. The signal line is laid on the electric (ground) layer

When wiring a multi-layer PCB board, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste, and will also increase a certain amount of work in 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 common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as: 1. Welding needs

High-power heater. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multi-layer PCB board is the same.

5. The role of the network system in PCB board wiring

In many CAD systems, the PCB layout is determined by the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a well-spaced and reasonable grid system to support the wiring.

The distance between the two legs of the standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 inches, etc. .

6. Design rule check (DRC)

After the PCB board wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer, and at the same time, it is necessary to confirm whether the established rules meet the requirements of the PCB board production process. The general inspection has the following aspects:

(1) Whether the distance between line and line, line and component pad, line and through hole, component pad and through hole, through hole and through 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 the power supply and the ground line are tightly coupled (low wave impedance), and whether there is a place to widen the ground line in the PCB board.

(3) Whether the best measures have been taken for the key signal lines, such as the shortest length, the protection line is added, and the input line and output line are clearly separated.

(4) Whether there are separate ground wires for the analog circuit and digital circuit.

(5) Whether the graphics (such as icons, annotations) added to the PCB board will cause signal short circuit. Modify some undesirable line shapes.

(6) Whether there is a process line on the PCB, whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad, so as not to affect the quality of the electrical equipment.

(7) Whether the outer frame edge of the power ground layer in the multi-layer PCB board is reduced. For example, the copper foil of the power ground layer is exposed to the outside of the board and it is easy to cause a short circuit.