What Should Be Paid Attention To In High Quality PCB Design?

Reasonable component layout is the basic premise of designing a high-quality PCB diagram. The requirements for component layout mainly include six aspects: installation, force, heat, signal and aesthetics.

1. Component layout

Reasonable component layout is the basic premise of designing a high-quality PCB diagram. The requirements for component layout mainly include six aspects: installation, force, heat, signal and aesthetics.

Installation

It refers to a series of basic requirements put forward in order to smoothly install the circuit board into the chassis, shell, and slot in a specific application, without space interference, short circuit and other accidents, and make the specified connector in the designated position on the chassis or shell. I won't go into details here.

Force

The circuit board should be able to withstand various external forces and vibrations during installation and work. Therefore, the circuit board should have a reasonable shape, and the positions of various holes (screw holes, special-shaped holes) on the board should be reasonably arranged. Generally, the distance between the hole and the edge of the plate should be at least greater than the diameter of the hole. At the same time, it should also be noted that the weakest section of the plate caused by the special-shaped hole should also have sufficient bending strength. In particular, the connectors that "extend" directly out of the equipment housing on the board should be reasonably fixed to ensure long-term reliability.

Exposure To Heat

For high-power, serious heating devices, in addition to ensuring heat dissipation conditions, but also pay attention to placing in the appropriate position. Especially in sophisticated analog systems, special attention should be paid to the adverse effects of the temperature field generated by these devices on the fragile pre-amplifier circuit. Generally, the part with very large power should be made into a module separately, and certain thermal isolation measures should be taken between the signal processing circuit.

Signals

Signal interference is the most important factor to consider in PCB layout design. The most basic aspects are: the weak signal circuit and the strong signal circuit are separated or even isolated; The AC part is separated from the DC part; The high frequency part is separated from the low frequency part; Note the direction of the signal line; Layout of ground wire; Appropriate shielding, filtering and other measures. All these have been repeatedly emphasized in a large number of discussions, and will not be repeated here.

Aesthetics

It is not only necessary to consider the orderly placement of components, but also to consider the beautiful flow of the line. Because the general layman sometimes more emphasis on the former, in order to one-sided evaluation of the advantages and disadvantages of circuit design, in order to the image of the product, the former should be given priority when the performance requirements are not harsh. However, in high-performance occasions, if you have to use dual panels, and the circuit board is also packaged inside, usually can not see, you should prioritize the beauty of the line. The "aesthetics" of wiring will be discussed in detail in the next section.

2. Wiring Principles

Some anti-interference measures that are uncommon in the literature are described in detail below. Considering that in practical applications, especially in the trial production of products, dual panels are still widely used, the following content is mainly for dual panels.

Wiring "aesthetics"

When turning, avoid right angles and try to use oblique or circular transitions.

The wiring should be neat and orderly, and the classification and centralized arrangement can not only avoid the mutual interference of different signals, but also facilitate inspection and modification. For digital systems, there is no need to worry about interference between signal lines in the same camp (such as data lines, address lines), but control signals such as reading, writing, and clocks should be independent, and it is best to protect the ground line.

When a large area of ground is paved (further discussed below), a reasonable equal distance between the ground wire (in fact, it should be the ground "surface ") and the signal line should be kept as close as possible under the premise of preventing short circuit and leakage.

For weak current systems, the ground and power lines should be as close as possible.

In a system with a table mount component, the signal lines should be as front as possible.

Ground Cable Layout

There are many discussions on the importance of ground wires and layout principles in the literature, but there is still a lack of detailed and accurate introduction to the ground wire layout in actual PCB. My experience is that in order to improve the reliability of the system (and not just make an experimental prototype), you can't overemphasize the ground wire, especially in weak signal processing. To this end, we must spare no effort to implement the principle of "large area paving".

When paving, it must generally be in a grid pattern, except for those scattered sites that are separated by other lines. The thermal performance and high-frequency conductivity of the grid ground are much better than that of the whole ground wire. In dual-panel wiring, sometimes in order to take the signal line, the ground wire has to be divided, which is extremely unfavorable for maintaining a sufficiently low ground resistance. Therefore, a series of "clever" means must be adopted to ensure the "smooth" of the earth current. These tips include:

The extensive use of surface mount components eliminates the space occupied by welding holes that should belong to the ground wire.