PCB News - Basic principles of PCB drawing board

Starting from determining the size of the PCB board, the size of the PCB is limited by the size of the chassis shell, so it is advisable to fit it into the shell. Secondly, consider the PCB and external components (mainly potentiometers, sockets or other PCBs) Connection method. The PCB and external components are generally connected by plastic wires or metal isolation wires. But sometimes it is also designed as a socket. That is: to install a plug-in PCB in the device, leave a contact position as a socket. For larger components installed on the PCB, metal accessories should be added to fix them to improve vibration and impact resistance.

2. The basic method of PCB drawing board

First of all, it is necessary to have a complete understanding of the specifications, sizes, areas, etc. of the selected components and various sockets; reasonable and careful consideration of the location of each component, mainly from the perspective of electromagnetic field compatibility and anti-interference. Short line, less crossover, power supply, ground path and decoupling are considered. After the position of each component is determined, it is the connection of each component. Connect the relevant pins according to the circuit diagram. There are many ways to complete it. The design of printed circuit diagrams has two methods: computer-aided design and manual design.

The most primitive is to arrange the layout by hand. This is more laborious, and it often takes several iterations to complete it. This is also possible when there is no other drawing equipment. This manual arrangement of layout methods is also very helpful for those who are just learning the printing plate layout. Computer-aided drawing, now there are many kinds of drawing software with different functions, but generally speaking, drawing and modification are more convenient, and they can be saved and printed.

Next, determine the required size of the PCB board, and initially determine the position of each component according to the schematic diagram, and then adjust the layout to make the layout more reasonable. The wiring arrangement between the components in the PCB is as follows:

(1) 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 cannot 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.

(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, and the horizontal type refers to the installation and welding of the component body in parallel and close to the circuit board, and 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 PCB is different.

(3) 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 PCBs such as FM heads often use large-area surrounding ground wires to ensure a good shielding effect.

(4) The 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, causing 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.

(5) The strong current lead (common ground wire, power amplifier power lead, etc.) should be as wide as possible to reduce wiring resistance and its voltage drop, and reduce self-excitation caused by parasitic coupling.

(6) 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.