PCB Tech - PCB circuit board design methods and key points analysis

The basic principle requirements of PCB board drawing design

1. The design of the printed circuit board starts from determining the size of the board. The size of the printed circuit board is limited by the size of the chassis shell. It is the connection method of potentiometer, socket or other printed circuit board). The printed circuit board and the 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 printed circuit board in the device, leave a contact position as a socket. For larger components installed on printed circuit boards, metal accessories should be added to fix them to improve vibration and impact resistance.

2. The basic method of wiring diagram design

First of all, it is necessary to have a complete understanding of the specifications, dimensions, and areas 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 lines, 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 the printed circuit diagram 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 printed circuit board, and according to the schematic diagram, initially determine the position of each component, and then continuously adjust the layout to make the layout more reasonable. The wiring arrangement between the components in the printed circuit board is as follows:

(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. 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 printed circuit board is different.

(3) The grounding point of the same level of circuit should be as close as possible, and the power filter capacitor of this level of 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 in strict accordance with 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. The connection between the levels is rather long. 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 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, which will 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 wire 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.

Pay attention to the following points in PCB design

1. Wiring direction: From the perspective of the welding surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction is best to be consistent with the wiring direction of the PCB circuit diagram. Because the welding surface usually needs to be tested for various parameters during the production process, so This is convenient for inspection, debugging and maintenance during production (Note: It refers to the premise that the circuit performance and the installation and panel layout requirements of the whole machine are met).

2. The arrangement and distribution of the components should be reasonable and even, and strive to be neat, beautiful, and strict in structure.

3. Resistor and diode placement methods: there are two types: horizontal placement and vertical placement:

(1) Horizontal placement: When the number of circuit components is small and the size of the circuit board is large, it is generally better to use horizontal placement; for a resistor below 1/4W, the distance between the two pads is general Take 4/10 inches, when the 1/2W resistor is placed flat, the distance between the two pads is generally 5/10 inches; when the diode is placed flat, 1N400X series rectifier tubes, generally take 3/10 inches; 1N540X series rectifier tubes, generally Take 4 to 5/10 inches.

(2) Vertical installation: When there are a large number of circuit components and the size of the circuit board is small, the vertical installation is generally adopted, and the distance between the two pads is generally 1 to 2/10 inches in the vertical installation.

4. Potentiometer: the placement principle of the IC holder

(1) Potentiometer: It is used in the regulator to adjust the output voltage, so the design potentiometer should be fully adjusted clockwise when the output voltage rises, and the output voltage of the counterclockwise regulator decreases when the output voltage is reduced; in the adjustable constant current charger The middle potentiometer is used to adjust the size of the charging current. When the potentiometer is designed, the current will increase when the potentiometer is fully adjusted clockwise.

The potentiometer should be placed in a position that meets the requirements of the whole machine structure installation and panel layout. Therefore, it should be placed on the edge of the board as much as possible, with the rotating handle facing outward.

(2) IC holder: When designing the printed board diagram, when using the IC holder, be sure to pay special attention to whether the orientation of the positioning slot on the IC holder is correct, and pay attention to whether each IC pin is correct, for example, the first pin can only be used. It is located at the lower right or upper left corner of the IC socket, and is close to the positioning groove (viewed from the welding surface).

5. Arrangement of in and out terminals

(1) The distance between the two associated lead ends should not be too large, generally about 2 to 3/10 inches is more appropriate.

(2) The inlet and outlet ends should be concentrated on one or two sides as much as possible, and should not be too discrete.

6. Pay attention to the pin arrangement sequence when designing the wiring diagram, and the component pin spacing should be reasonable.

7. Under the premise of ensuring the performance requirements of the circuit, the design should strive for reasonable wiring, use less external jumpers, and route the wires according to certain smooth charging requirements, and strive to be intuitive, easy to install, height and overhaul.

8. When designing the wiring diagram of PCB, there should be as few turns as possible, and the lines should be simple and clear.

9. The width of the PCB wiring strip and the line spacing should be moderate, and the spacing between the two pads of the capacitor should be as consistent with the spacing of the capacitor lead pins as possible;

10. The PCB design should be carried out in a certain order, for example, it can be carried out in the order from left to right and from top to bottom.