PCB layout tips

In PCB 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, the design process of wiring is the highest, the skills are the finest, and the workload is the largest. PCB wiring includes single-sided wiring, double-sided wiring, and multilayer wiring. There are two ways of wiring:

 

Automatic routing and interactive routing. Before automatic routing, you can use interactive interactive routing in advance to stringently required lines. The edges of the input end and the output end should be adjacent to each other to avoid reflection interference. When necessary, ground wire isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.

 

The routing rate of automatic wiring depends on a good layout. The routing rules can be set in advance, including the number of bends in the traces, the number of vias, and the number of steps. Generally, firstly carry out exploratory warp threads, quickly connect short lines, and then perform labyrinth wiring. First, optimize the global routing path of the wires to be routed. It can disconnect the already routed wires as needed. And try rewiring to improve the overall effect.

 

The current high-density PCB design has felt that through-holes are not well adapted. It wastes many valuable wiring channels. In order to solve this contradiction, blind and buried hole technologies have emerged, which not only complete the role of vias. It also saves many wiring channels to make the wiring process more convenient, smoother and more complete. The PCB board design process is a complex and simple process. To master it well, it also requires extensive electronic engineering design. Only when people experience it can they get the true meaning of it.

 

1. Handling of power and ground

 

Even if the wiring in the entire PCB is completed very well, the interference caused by inadequate 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 electricity and ground wires must be taken seriously to reduce the noise interference caused by electricity and ground wires to a minimum to ensure the quality of the product. For every engineer who is engaged in the design of electronic products, he understands the cause of the noise between the ground wire and the power line. Now only the reduced noise suppression is described:

 

It is well known to add decoupling capacitors between the power and ground lines. Widen the power supply and ground wire width as much as possible. It is better that 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. 0.05 ~ 0.07mm, power line is 1.2 ~ 2.5 mm, for the PCB of digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground network to use (the ground of analog circuits cannot be used in this way) Use a large area copper layer as the ground For wiring, connect unused places to the ground on the printed board as the ground wire. Or make a multilayer board, power supply, and ground each occupy one layer.

 

2. Common ground processing of digital circuit and analog circuit

 

Many PCB are no longer a single function circuit (digital or analog circuit), but a mixture of digital and analog circuits.

 

Therefore, it is necessary to consider the interference between them when wiring, especially the noise interference on the ground. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines are as far away from sensitive analog circuit devices as possible. For ground lines, the whole PCB has only one node to the outside world, so The digital and analog ground must be handled inside the PCB. The digital and analog grounds inside the board are actually separate. They are not connected to each other, but at the interface where the PCB is connected to the outside world (such as plugs). The digital ground is shorted to the analog ground. 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 multi-layer printed circuit boards are wired, because there are not enough wires left in the signal line layer, adding more layers will cause waste and add a certain amount of work to production. The cost will increase accordingly. To resolve this contradiction, you can consider wiring on the electrical (ground) layer. The power plane should be considered first, followed by the ground plane. Because it is best to preserve the integrity of the formation.

 

4. Handling 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 comprehensively considered. In terms of electrical performance, it is better to fully connect the pads of the component legs to the copper surface, There are some hidden dangers in the welding assembly of components such as:

 

① Welding requires a high-power heater. ② Easy to cause false solder joints.

 

So, taking into account the electrical performance and process requirements, a cross-shaped pad is made, which is called heat shield.

 

Commonly known as thermal pad (Thermal), this can make the virtual solder joints

 

The possibility is greatly reduced. The handling of the ground (ground) leg of the multilayer board is the same.

 

5, the role of network systems in wiring

 

In many CAD systems, the wiring is determined by the network system. The grid is too dense, although the path has increased, but the step is too small, and the amount of data in the picture field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the target computer electronics. Great influence. And some paths are invalid, such as occupied by the pads of the component legs or occupied by mounting holes and fixed holes. The grid is too sparse, and too few channels have a great impact on the spread rate. Therefore, there must be a dense and reasonable grid system 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 to 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.

 

6. Design Rule Check (DRC)

 

After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules formulated by the designer. At the same time, it is also necessary to confirm whether the formulated rules meet the requirements of the printed board production process. The general inspection has the following aspects:

 

A. Line to line, line to component pad, line and through hole, component pad and through hole, through hole and through

 

Whether the distance between the through holes is reasonable and whether it meets the production requirements.

 

B. Is the width of the power and ground wires appropriate, and is there a tight coupling between the power and ground wires (low wave impedance)? Is there any place in the PCB that can widen the ground wires.

 

C. Have you taken the best measures for the key signal lines, such as the shortest length, add protection lines, and the input and output lines are clearly separated.

 

D, whether there are separate ground wires for the analog and digital circuits.

 

E, whether the graphics (such as icons and annotations) added to the PCB will cause signal short circuit.

 

F. Modify some undesired line shapes.

 

G. Is there a process line on the PCB? Does the solder mask meet the requirements of the production process, and the solder mask size is

 

If not, whether the character mark is pressed on the device pad, so as not to affect the quality of the electrical assembly.

 

H. Whether the outer frame edge of the power supply ground layer in the multilayer board is reduced. For example, the copper foil of the power supply ground layer is exposed to the outside of the board, which is likely to cause a short circuit.