PCB Tech - Engineer's summary: PROTEL Design Encyclopedia (Three)

One. Discussion on layout structure designof printed circuit components

An instrument with excellent performance, in addition to selecting high-quality components and reasonable circuits, the correct structural design of the component layout of the printed circuitboard and the direction of electrical wiring is a key issue that determines whether the instrument can work reliably. Circuits with components and parameters will produce different results due to component layout design and electrical wiring directions, and the results may vary greatly. Therefore, how to correctly design the structure of the printed circuit board component layout, the correct selection of the wiring direction and the process structure of the overall instrument must be considered together. A reasonable process structure can eliminate noise interference caused by improper wiring and facilitate Installation, debugging and maintenance in production.

Below we will discuss the above-mentioned issues. Since there is no strict “definition” and “model” for a good “structure”, the following discussion will only serve as an inspiration for reference and is for reference only. The structure of each instrument must be based on specific requirements (electrical performance, overall structure installation and panel layout requirements), adopt corresponding structural design schemes, and compare and repeatedly modify several feasible design schemes. The choice of wiring structure for printed board power and ground bus-system structure: analog circuits and digital circuits have many similarities and differences in the design and wiring methods of component layouts. In the analog circuit, due to the existence of the amplifier, the very small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise tolerance is 0.4V ~ 0.6V, and the CMOS noise tolerance is 0.3 of Vcc. ~ 0.45 times, so the digital circuit has a strong anti-interference ability. The reasonable choice of a good power and ground bus mode is an important guarantee for the reliable operation of the instrument. Quite a lot of interference sources are generated through the power and ground bus, and the ground wire causes the greatest noise interference.

Two, the basic principle requirements of printed circuit board diagram 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 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 mounted 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 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 printed circuit board, and initially determine the position of each component according to the schematic diagram, and then make the layout more reasonable after continuous adjustment. 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, 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 printed circuit board is different. :

(3) The grounding point of the circuit of the same level should be as close as possible, and the power filter capacitor of the circuit of this level 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.

(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. 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) The strong current lead (common ground wire, power amplifier power lead, 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 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.

Three. The following points should be paid attention to in the design of printed board diagrams

1. Wiring direction: From the perspective of the soldering 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 circuit diagram. Because various parameters are usually tested on the soldering surface 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 resistors 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 not large, 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 on the position of the unit to meet the requirements of the whole machine structure installation and panel layout, so 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, the wiring should be as few as possible to bend, and the lines should be simple and clear.

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

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