Following the previous article “ What issues should be considered in the design of printed circuit boards”， we continue to understand the course theory of PCB design.
To make the designed circuit board achieve the expected purpose, the overall layout of the printed circuit board and the placement of the components play a key role, which directly affects the installation, reliability, ventilation, and heat dissipation of the entire printed circuit board and the direct connection of wiring rate.
The outer side of the printed circuit board is given priority. When the PCB size is too large, the printed lines will be long and the impedance will increase.
The anti-noise ability decreases and the cost increases. If it is too small, the heat dissipation will be poor, and the adjacent lines will be easily disturbed. Therefore, first of all, give a reasonable positioning for the size and shape of the PCB. Then determine the location of special components and unit circuits, etc. The whole circuit should be divided into several unit circuits or modules according to the circuit flow, and the core component (such as an integrated circuit) of each unit circuit should be the center. Other components should be Arrange on the PCB evenly, neatly, and compactly in a certain order, but don’t get too close to these large components. There must be a certain distance, especially for larger and taller components, which will help to solder and rework. For integrated circuits with a higher power, a color heat sink should be considered, and sufficient space should be left for it, and it should be placed in a well-ventilated and heat-dissipated position of the printed board.
At the same time, don’t be too concentrated. Several large components must be on the same board with a certain distance, and they must be in the direction of 45 angles. Smaller integrated circuits such as (SOP) must be arranged in the axial direction. The components are arranged in a vertical axis, and all these directions are relative to the transfer direction of the PCB production process. In this way, the components are arranged regularly, thereby reducing defects in welding. Light-emitting diodes used for display, etc., should be placed on the edge of the printed board because they are used for observation during the application process.
Some switches, fine-tuning components, etc. should be placed in an easy-to-operate place.
In the same frequency circuit, the distribution parameters between components should be considered. In general, the distribution parameters between components should be considered in high-frequency circuits. Generally, the components should be arranged in parallel as much as possible in the circuit. This is not only beautiful but also easy to install and solder. It is easy to mass-produce, the components located on the edge of the circuit board must have a distance of 3-5 cm from the edge. While considering the location of the components, the thermal expansion coefficient, thermal conductivity, heat resistance, and bending strength of the PCB board must be fully considered to avoid adverse effects on the components or PCB during production.
After determining the location and shape of the components on the PCB, consider the PCB layout.
With the position of the component, wiring is performed according to the position of the component, and the trace on the printed board is as short as possible.
then. The wiring is short, and the occupied channel and area are small, so the pass-through rate will be higher. The input and output wires on the PCB board should be avoided as far as possible, and it is better to place a ground wire between the two wires. In order to avoid circuit feedback coupling. If the printed board is a multi-layer board, the signal line routing direction of each layer should be different from the routing direction of adjacent board layers.
For some important signal lines, you should reach an agreement with the line designer. In particular, differential signal lines should be routed in pairs, and try to make them parallel and close, and the length is not much different. All components on the PCB board minimize and shorten the leads and connections between the components. The minimum width of the wires in the PCB board is mainly determined by the adhesion strength between the wires and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1-1.5mm, the temperature will not be higher than 3 degrees through a current of 2A. Wire width 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, 0.02-0.03mm is usually used.
Of course, as long as it allows, we use as wide a line as possible, especially the power line and ground line on the PCB. The minimum spacing of the wires is mainly determined by the worst-case insulation resistance and breakdown voltage between the lines. For some integrated circuits (IC), the pitch can be less than 5-8mm from the perspective of the process.
The bend of the printed conductor is generally the smallest arc, and avoid the use of fewer than 90-degree bends. The right angle and the included angle will affect the electrical performance in the high-frequency circuit. In short, the wiring of the printed board should be even, dense, and consistent.
Avoid using large-area copper foil as much as possible in the circuit, otherwise, the copper foil will expand and fall off when heat is generated for too long during use. If large-area copper foil must be used, grid-shaped wires can be used.
The port of the wire is the pad. The center hole of the pad is larger than the diameter of the device lead. If the pad is too large, it is easy to form a virtual solder during soldering. The outer diameter D of the pad is generally not less than (d + 1.2) mm, where d is the aperture. For some components with relatively high density, the minimum diameter of the pad is acceptable (d + 1.0) mm, after the pad design is completed, the outline frame of the device should be drawn around the pad of the printed board, and text and characters should be marked at the same time.
Generally, the height of the text or frame should be around 0.9mm, and the line width should be around 0.2mm. And do not press the marked text and characters on the pad. If it is a double-layer board, the characters on the bottom layer should be mirrored.