Design Method and Key Points Analysis of PCB Circuit Board

Basic Principles and Requirements of PCB Board Design

1. The design of printed circuit board begins with the determination of the size of the board. The size of the printed circuit board is limited by the size of the enclosure, so that it can be placed in the enclosure.

Secondly, the connection mode between PCB and external components (mainly potentiometers, sockets or other PCB) should be considered. Printed circuit boards (PCBs) are usually connected with external components through plastic wires or metal isolation wires. But sometimes they are also designed as sockets. That is, to install a plug-in printed circuit board in the equipment, the contact position that serves as the outlet should be set aside. For larger components mounted on printed circuit boards, metal accessories should be added to fix them to improve their vibration and impact resistance.

2. Basic methods of wiring design

First of all, we need to have a complete understanding of the specifications, dimensions and areas of the selected components and sockets.

Reasonable and careful consideration is given to the location of each component, mainly from the perspective of electromagnetic compatibility and anti-interference, short routing, less cross, power supply, ground path and decoupling.

After the location of each component is determined, it is the on-line connection of each component. There are many ways to connect the pins according to the circuit diagram. There are two ways to design the printed circuit diagram: computer-aided design and manual design.

The most primitive is the manual layout. It's rather laborious, and it often takes several iterations before it can be finished. It can also be done without other drawing equipment. This manual layout method is also very helpful for those who have just learned the printing board drawing design. There are many kinds of computer-aided drawing software with different functions, but in general, drawing and modifying are more convenient, and can be stored and printed on the disk.

Secondly, the size of printed circuit board is determined, and the position of each component is determined preliminarily according to the schematic diagram. Then, the layout is more reasonable through continuous adjustment. The wiring arrangement between components in printed circuit board is as follows:

(1) There is no crossing circuit in printed circuit. For possible crossing lines, we can use "drilling" and "winding" to solve the problem. That is to say, let a lead "drill" through the gap at the foot of other resistors, capacitors and transistors, or "circle" from one end of a lead that may intersect. In special cases, if the circuit is very complex, in order to simplify the design, it is also allowed to use the lead to cross-connect, so as to solve the cross-circuit problem.

(2) Resistors, diodes, tubular capacitors and other components have "vertical" and "horizontal" installation.

Vertical means that the assembly body is vertical to the circuit board installation and welding. Its advantages are space saving. Horizontal means that the assembly body is parallel and close to the circuit board installation and welding. Its advantages are that the mechanical strength of the assembly installation is better. These two different installation components, PCB components on the hole spacing is different.

(3) The grounding point of the same stage circuit should be as close as possible, and the power filter capacitor of this stage circuit should also be connected to the grounding point of this stage. Especially the base and emitter of the transistor should not be far away from each other. Otherwise, the copper foil between the two grounding points is too long, which will cause interference and self-excitation. Using this kind of "one point grounding" circuit, the work is more stable and it is not easy to self-excitation.

(4) Ground wires must be arranged strictly according to the principle of high-frequency-intermediate-low-frequency first-level in order from weak to strong electricity. They must not be randomly connected over and over. The long connection points between levels are preferable, and this requirement must be observed.

Especially the grounding wire arrangement of frequency converter, regenerator and FM head is more strict. If there is any improper arrangement, it will cause self-excitation and can not work.

Frequency modulation head and other high frequency circuits often use large area surrounded ground wire to ensure good shielding effect.

(5) Strong current leads (common ground wires, power supply leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, and to reduce self-excitation caused by parasitic coupling.

(6) The line with high impedance is as short as possible, and the line with low impedance can be longer, because the line with high impedance is easy to flute and absorb signals, resulting in circuit instability.

The base line of power supply, ground line, non-feedback component and emitter lead are all low impedance lines. The base line of emitter follower and the ground line of recorder must be separated and separated into one line until the end of the effect is combined again. If the two ground lines are connected, it is easy to produce crosstalk, which reduces the separation degree.

The following points should be noted in PCB design

1. Wiring direction: Seen from the welding surface, the arrangement orientation of components should be as consistent as possible with the schematic diagram, and the wiring direction should be in accordance with the route direction of the circuit diagram.

It is easy to check, debug and overhaul in production because it is usually necessary to test various parameters on the welding surface in the production process.

(Note: Refers to the premise of meeting the requirements of circuit performance, installation and panel layout of the whole machine).

2. Arrangement and distribution of each component should be reasonable and uniform, striving to be neat, beautiful and structurally rigorous process requirements.

3. Resistors and diodes are placed in two ways: horizontal and vertical.

(1) Leveling: When the number of circuit components is small and the size of the circuit board is large, it is generally better to use the level.

When the resistance is below 1/4W, the distance between the two pads is generally 4/10 inches. When the resistance is below 1/2W, the distance between the two pads is generally 5/10 inches.

When the diode is leveled, the 1N400X series rectifier is usually 3/10 inches; the 1N540X series rectifier is usually 4-5/10 inches.

(2) Vertical placement: When the number of circuit components is large and the size of the circuit board is small, it is generally used to vertical placement. When vertical placement, the distance between the two pads is generally 1-2/10 inches.

4. Potentiometers: The Placement Principle of IC Seat

(1) Potentiometer: It is used to regulate the output voltage in the regulator, so the output voltage of the designed potentiometer should rise when it is full of clockwise regulation, while the output voltage of the counter-clockwise regulator decreases when it is full of clockwise regulation.

The potentiometer in the adjustable constant current charger is used to adjust the charging current fold. When the potentiometer is designed to adjust clockwise, the current increases.

Potentiometer placement boom should meet the requirements of the whole machine structure installation and panel layout, so the boom should be placed at the edge of the board as far as possible, with the rotating handle facing outward.

(2) IC pedestal: When designing printed circuit board drawings, when using IC pedestal, special attention should be paid to the correct position of positioning groove on IC pedestal, and to the correct position of each IC pedestal, such as the first foot can only be located in the lower right or upper left corner of IC pedestal, and close to the positioning groove (from the welding surface).

5. Arrangement of access terminals

(1) The two connected leads should not be too far apart, generally about 2-3/10 inches is more appropriate.

(2) As far as possible, the end of the line should be concentrated on one to two sides, not too discrete.

6. Pay attention to the order of pins when designing wiring diagram, and the spacing of component pins should be reasonable.

7. On the premise of guaranteeing the performance requirements of the circuit, the design should strive for reasonable alignment, less use of external crossover lines, and according to certain requirements of smooth charging, in order to be intuitive, easy to install, high and maintenance.

8. When designing the wiring diagram, the routes should be as few as possible and the lines should be simple and clear.

9. Wiring width and line spacing should be moderate, and the spacing between two pads of capacitor should be as close as possible to the spacing of capacitor lead foot.

10. Design should be carried out in a certain order, such as from left to right and from top to bottom.