PCB frame design

1. The design of the physical board frame must pay attention to the precise size, avoid installation troubles, and ensure that the circuit board can be smoothly installed into the chassis, shell, slot, etc.

2. It is better to use rounded corners (such as the four corners of the rectangular plate). On the one hand, it avoids right angles and sharp corners from scratching people. On the other hand, rounded corners can reduce stress and reduce PCB board breakage due to various reasons.

3. Various mounting holes (such as screw holes) and various openings and slots should be determined before layout. Generally speaking, the distance between the hole and the edge of the PCB is at least greater than the diameter of the hole.

4. When the area of the circuit board is larger than 200x150mm, attention should be paid to the mechanical strength of the board. From an aesthetic point of view, the best shape for a board is a rectangle. The ratio of width to length is preferably the golden ratio 0, 618 (the application of the golden ratio is also very wide). In actual application, the width and length may be 2:3 or 3:4, etc.

5. Combined with product design requirements (especially mass production), comprehensively consider the size of the PCB board. If the size is too large, the printed copper wire is too long, the impedance increases, and the anti-noise ability decreases; if the size is too small, the heat dissipation is not good, the line spacing is not easy to control, and adjacent wires are easy to interfere.

6. Generally speaking, the planning of the board and frame is carried out at the KeepOutLayer layer.

2. PCB board layout design

Whether the component layout is reasonable has a great impact on the life, stability, ease of use and wiring of the entire board, which is the prerequisite for designing an excellent PCB board. Different board layouts have their own requirements and characteristics, but there are some general rules and techniques among them. .

1. Placement order of components

①Generally speaking, first place the components that are closely related to the structure of the entire board and have fixed positions. For example, common power sockets, switches, indicator lights, various interfaces with special position requirements (connectors and the like), relays, etc., and do not conflict with the openings and slots in the PCB board, and the positions must be correct. Once placed, it’s best to secure it with the software’s locking feature. ② Then place bulky components, core components and some special components. For example, large components such as transformers, integrated circuits, core IC components such as processors, heating components, etc. These components move with routing considerations, so placement is approximate, not locked.

③ Finally place the small components. Such as RC components, auxiliary small IC, etc.

2. Notes

①In principle, all components should be placed more than 3mm away from the edge of the board. Especially in the assembly line plug-in and wave soldering in mass production, this is to be used for the guide rail groove, and at the same time, it can prevent the edge part from being damaged during the shape cutting process. ②Pay attention to heat dissipation.

For some high-power circuits, components with serious heat generation (such as power tubes, high-power transformers, etc.) should be distributed on the edge of the board as much as possible to facilitate heat dissipation, and should not be too concentrated in one place. In short, it should be appropriate, especially in some sophisticated analog systems, the temperature field generated by the heating device has a serious impact on some amplifier circuits. In addition to ensuring sufficient heat dissipation measures, it is recommended to make some super-powerful parts into a separate module and take heat insulation measures to avoid affecting the subsequent signal processing circuit. Another point is that the electrolytic capacitor should not be too close to the heat source, so as to avoid premature aging of the electrolyte and a sharp reduction in life. Do not keep thermal components close to heat sources! ③Pay attention to the weight of components. For some heavier components, it is recommended to design it to be fixed with brackets and then welded. Some large, heavy and heat-generating components should not be directly installed on the PCB board, but should be considered to be installed on the bottom plate of the chassis.

④ Pay attention to the spacing of high-voltage components or wires on the PCB.

If there are both high-voltage circuits and low-voltage circuits on the circuit board to be designed, there may be a high potential difference between devices or wires. At this time, they should be placed separately and the distance between the wires should be increased to avoid accidental short circuit caused by discharge. It should also be noted that devices with high voltage should be arranged in places that are not easily accessible by hands.

⑤ When placing components, be careful not to overlap or touch the pads to avoid short circuits. In addition, the pads are placed overlappingly, and holes will be drilled multiple times in one place during drilling, which will easily cause the drill bit to break, and the pads and wires will be damaged.

⑥ Pay attention to the placement of components so that there is no space conflict with the positioning holes, fixing brackets, etc. Components should keep an appropriate distance and space from positioning holes, fixing brackets, etc., to avoid installation conflicts. ⑦ Pay attention to the devices used for adjustment in the circuit (such as potentiometers, adjustable capacitors, micro switches, toggle switches, etc.). The layout should be fully combined with the structural requirements of the whole machine: if it is only adjusted inside the machine, it should be placed in a convenient place for adjustment; if it is adjusted by the panel outside the machine, it should be arranged according to the position of the panel knob.

3. Layout skills

① Comparing and combining with the schematic diagram, the core component (usually IC chip) of each functional circuit is centered, and other resistance-capacitance components are arranged around it. Components should be uniform, neat,

The compact layout not only needs to consider neatness and order, but also pays attention to the smoothness of the wiring later. ②Rationally arrange each sub-function circuit according to the flow of the circuit to make the signal smooth and keep the signal in the same direction as possible.

③ Try to shorten the connection distance between related components, especially the connection distance between high-frequency components, and reduce their distribution parameters. For example, oscillator circuit components should be placed as close together as possible. ④ Generally, the components should be aligned in rows as much as possible to avoid chaos. This is not only beautiful, but also convenient for installation, welding and mass production.

⑤The input and output components should be as far away as possible. Components that are likely to interfere with each other should not be too close together.

⑥ Reasonably distinguish the analog circuit part, the digital circuit part, and the part with serious noise (such as relay sparks, high current and high voltage switches). Try to optimize and adjust their positions to minimize mutual signal coupling and reduce electromagnetic interference. For example, keep motors, relays and sensitive microcontrollers as far away as possible.

⑦ Strong signals and weak signals, AC signals and DC signals should be isolated separately.

 ⑧ Check and determine the pad size of various components before wiring. If the size of the pad is modified after the wiring is completed, it      is  very easy to cause the problem of the distance between the pad and the wire or between the pad and the pad, and in serious cases, it will cause a short circuit!

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