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For electronic products, PCB design is a design process necessary for it to change from an electrical schematic diagram to a specific product. The rationality of its design is closely related to product production and product quality. For many people who are just engaged in electronic design, It is said that there is less experience in this area. Although PCB board design software has been learned, the designed PCB board often has such problems. The engineer recommended by the editor has been engaged in PCB board design for many years, and will share some experience in printed circuit board design with everyone, hoping to play a role in attracting ideas. The engineer's printed circuit board design software recommended by the editor was TANGO a few years ago, and now it uses PROTEL2.7 FOR WINDOWS.
PCB board layout
The usual order of placing the components on the PCB board:
1. Place the components in a fixed position that closely match the structure, such as power sockets, indicator lights, switches, connectors, etc. After these components are placed, use the LOCK function of the software to lock them, so that they will not be mistaken in the future move;
2. Place special components and large components on the circuit, such as heating components, transformers, ICs, etc.;
3. Place small devices.
The distance between the component and the edge of the PCB board
If possible, all components are placed within 3mm from the edge of the PCB or at least greater than the thickness of the PCB. This is due to the fact that the assembly line plug-in and wave soldering in mass production must be provided to the guide groove for use, and also to prevent Due to the defect of the edge part caused by the shape processing, if there are too many components on the PCB board, if it is necessary to exceed the range of 3mm, you can add a 3mm auxiliary edge to the edge of the PCB board, and open the V-shaped groove on the auxiliary edge. Just break it.
Isolation between high and low voltage
There are high-voltage circuits and low-voltage circuits on many PCB boards at the same time. The components of the high-voltage circuit parts and the low-voltage parts should be placed separately. The isolation distance is related to the withstand voltage to be withstand. Normally, the distance between the PCB board and the PCB board is 2mm at 2000kV., If you want to withstand the 3000V withstand voltage test, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, it is still on the PCB to avoid creeping. Slotting between the high and low pressure.
Wiring arrangement between PCB board components:
(1) Cross circuits are not allowed in printed circuits. For lines that may cross, you can use "drilling" and "winding" two methods to solve them. That is, let a lead "drill" through the gap under other resistors, capacitors, and triode pins, or "wind" from one end of a lead that may cross. In special circumstances, how complex the circuit is, it is also necessary to simplify the design. It is allowed to connect with wires to solve the problem of cross circuit.
(2) Components such as resistors, diodes, and tubular capacitors can be installed in "vertical" and "horizontal" installation methods. The vertical type refers to the installation and welding of the component body perpendicular to the circuit board, which has the advantage of saving space. Horizontal type refers to the installation and soldering of the component body in parallel and close to the circuit board. Its advantage is that the mechanical strength of the component installation is better. For these two different mounting components, the component hole pitch on the printed circuit board is different.
(3) The grounding point of the same level circuit should be as close as possible, and the power filter capacitor of this level circuit should also be connected to the grounding point of this level. In particular, the grounding points of the base and emitter of the transistor of this level should not be too far apart, otherwise the copper foil between the two grounding points will be too long, which will cause interference and self-excitation. Using such a "one-point grounding method" circuit will work better. Stable and not easily self-excited.
(4) The main ground wire must be arranged strictly according to the principle of high frequency-intermediate frequency-low frequency in the order of weak current to strong current. It must not be turned over and over randomly. To comply with this requirement. In particular, the grounding wire arrangement requirements of the frequency conversion head, regeneration head, and frequency modulation head are more stringent. If improperly, it will self-excite and make it unable to work. High-frequency circuits such as FM heads often use large-area surrounding ground wires to ensure a good shielding effect.
(5) Strong current leads (common ground, power amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, and reduce self-excitation caused by parasitic coupling.
(6) The PCB traces with high impedance should be as short as possible, and the traces with low impedance can be longer, because the traces with high impedance are easy to whistle and absorb signals and cause the circuit to be unstable. The power cord, ground wire, base trace without feedback components, emitter lead, etc. are all low impedance traces. The base trace of the emitter follower and the ground traces of the two channels of the radio must be separated, each forming one path., Until the end of the function is combined again, if two ground wires are connected back and forth, it is easy to produce crosstalk and reduce the degree of separation.
