Discussion on layout design of printed circuit components

Discussion on layout design of printed circuit components

Printed circuit board power supply, ground bus wiring structure selection ---- System structure: Analog circuit and digital circuit There are many similarities and differences in the layout and layout of the component layout. In the analog circuit, due to the existence of the amplifier, the extremely small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise margin is 0.4V-0.6V, and the CMOS noise tolerance is 0.3 for Vcc. ~0.45 times, so the digital circuit has a strong anti-jamming capability.

Reasonable choice of good power supply and ground bus mode is an important guarantee for the reliable operation of the instrument. A considerable number of interference sources are generated through the power supply and ground bus, and the noise caused by the ground line is the largest.

First, the basic principles of printed circuit board design requirements

1. The design of the printed circuit board starts with the determination of the size of the board. The size of the printed circuit board is limited by the size of the chassis casing, so that it can be properly placed into the housing. Second, the printed circuit board and external components should be considered (mainly It is the connection of potentiometer, socket or other printed circuit board). Printed circuit boards and external components are generally connected by plastic wires or metal wire. But sometimes it is designed as a socket. That is, installing a plug-in PCB in the device leaves the contact position as a socket.

For larger components mounted on printed circuit boards, add metal attachments to improve vibration and shock 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 component devices and sockets. Reasonable and careful consideration should be given to the arrangement of each component, mainly from the perspective of electromagnetic field compatibility and anti-jamming. Short lines, few crossings, power supply, ground path and decoupling etc. After fixing each component's position, it is the connection of each component. Connect the relevant pin according to the circuit diagram. There are many methods to complete. The design of the printed circuit diagram has two kinds of computer-aided design and manual design methods.

The most primitive is the manual layout. This is more time consuming and often takes a few iterations to complete. This can be done without other drawing equipment. This hand-arranging method is also very helpful for designers who are just learning to print the board. Computer-aided drawing, there are a variety of graphics software, features vary, but in general, draw, modify more convenient, and can save and print.

Then, the required size of the printed circuit board is determined, and the position of each component is preliminarily determined according to the schematic diagram, and then the adjustment is continuously made to make the layout more reasonable. The wiring arrangement among the components in the printed circuit board is as follows:

(1) Cross circuits are not allowed in printed circuits. For lines that may intersect, "drilling" and "winding" can be used. That is, if a certain lead is “drilled” from the gaps under the feet of other resistors, capacitors, and triodes, or “wraps around” from one end of one of the leads that may cross. In a special case, how the circuit is complicated is to simplify the design. Allows cross-connections to solve cross-circuit problems.

(2) Components such as resistors, diodes, and tubular capacitors have two mounting methods, "vertical" and "horizontal". Vertical refers to the assembly body perpendicular to the circuit board installation, welding, its advantage is to save space, horizontal refers to the assembly body parallel and close to the circuit board installation, welding, its advantage is that the assembly of the mechanical strength is better. With these two different mounting components, the pitch of the components on the printed circuit board is different.

(3) The grounding point of the same stage circuit should be as close as possible, and the power supply filter capacitor of the current stage circuit should also be connected to the level grounding point. In particular, the grounding point of the base and emitter of this transistor cannot be too far away. Otherwise, the copper foil between the two grounding points will cause interference and self-excitation. Using such a “one-point grounding” circuit, the operation Stable, not easy to self-excited.

(4) The general grounding line must be in strict accordance with the order of weak currents to strong currents in the high-frequency, medium-frequency and low-frequency levels. It must not be used to re-connect randomly, but it is better to connect long-distance between grades and grades. Obey this rule. In particular, the arrangement of the grounding wires for the frequency conversion head, the regeneration head, and the frequency modulation head is more stringent, and if it is improper, it will cause self-excitation so that it cannot work.

Frequently, high-frequency circuits such as frequency modulation heads use large-area surrounding ground to ensure good shielding effect.

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

(6) The trace with high impedance should be as short as possible, and the trace with low impedance can be longer, because the trace with high impedance is easy to flute and absorb the signal, causing circuit instability. Power supply lines, ground lines, base traces with no feedback components, and emitter leads are all low-impedance traces. The base trace of the emitter follower and the two-channel ground of the recorder must be separated. Until the end of efficiency is combined again, if the two ground lines are connected, the crosstalk can easily occur and the degree of separation can be reduced.

Second, the following points should be noted in the design of printed board

1. Wiring direction: From the soldering surface, the orientation of the components is kept consistent with the schematic diagram as much as possible, and the wiring direction is preferably the same as the wiring direction of the circuit diagram. Because the production process usually requires testing of various parameters at the welding surface, This facilitates inspection, commissioning and maintenance in production (Note: This refers to satisfying the requirements of circuit performance and overall machine installation and panel layout requirements)

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2. The arrangement of the components should be reasonable and uniform in order to be neat, beautiful and structurally rigorous.

3. Resistance, diode placement: divided into two kinds of flat and vertical:

(1) Placement: When the number of circuit components is small, and the circuit board size is large, it is generally better to use a flat place; for a place where the resistance is less than 1/4W, the distance between two lands is generally small. Take 4/10 inch, 1/2W resistance when laid flat, the distance between the two pads is generally 5/10 inch; diode flat, 1N400X series rectifier, generally take 3/10 inch; 1N540X series rectifier, generally Take 4 to 5/10 inches.

(2) Vertical placement: When the number of circuit components is large, and the size of the circuit board is not large, vertical placement is generally adopted. When the vertical placement is performed, the distance between the two pads is generally 1 to 2/10 inches.

4. Potentiometer: Placement principle of IC holder

(1) Potentiometer: used to adjust the output voltage in the regulator, so the design potentiometer should be full clockwise adjustment when the output voltage increases, anti-clock regulator output voltage decreases at the festival; in the adjustable constant current charger The medium potentiometer is used to adjust the charge current fold size. When the potentiometer is designed to adjust clockwise in full clock, the current increases.

Potentiometer mounting position Xuan should be full of the structure of the whole machine installation and panel layout requirements, so should put Xuan on the edge of the board, rotating handle outward.

(2) IC Holder: When designing a printed board drawing, when using an IC Holder, special attention must be paid to whether or not the orientation of the positioning groove on the IC Holder is correct, and whether each IC pin is correct. For example, pin 1 can only be Located in the lower right corner or upper left corner of the IC seat, and close to the positioning slot (see from the welding surface).

5. Access terminal layout

(1) The associated two lead terminals should not be too far away, generally about 2 to 3/10 inches.

(2) Inlet and outlet ends should be concentrated on one or two sides as much as possible. Do not disperse them too much.

6. When designing the wiring diagram, pay attention to the order of the pins, and the distance between the components should be reasonable.

7. Under the premise of guaranteeing the circuit performance requirements, the design should be based on the principle of seeking reasonable cabling, using less external crossover wires, and routing the cabling according to certain requirements. It is intended to be intuitive, easy to install, and highly accessible and highly accessible.

8. When designing the wiring diagram, the wiring should be as small as possible, and the lines should be simple and straightforward.

9. The width of the wiring strip and the spacing between the lines should be moderate, and the distance df