【 Weak current College 】 PCB design principles and anti-jamming measure 】

The General principles of PCB design

To achieve the best performance of the electronic circuits, components and wire cloth and location is very important. In order to design quality
Of good, inexpensive PCB. the following general principles should be followed:

1. layout
First of all, to consider the PCB size. PCB size is too large, printed lines long, impedance, anti-noise reduced capacity, cost also increases; too small, the heat is not good, and the adjacent line susceptible to interference. After determining the PCB size determines the special symbol again. Finally, the based on the circuit of functional units, all components of the circuit to the layout.

In determining the special symbol position to observe the following principles:

(1) as short lines that connect the high frequency components, managed to reduce their distribution parameters and mutual electromagnetic interference. Susceptible to interference of components does not get too close to each other, input and output components should be as far away as possible from.
(2) certain components or wires may have a high potential difference, should increase the distance between them, so as to avoid accidental discharge leads to short circuit. High voltage components should be arranged in the hand is not easy to reach when debugging.
(3) the weight of more than 15g components, should be fixed with bracket, and welding. Those are big and heavy, the calorific value of components to be mounted on the PCB, but should be mounted on the chassis backplane machine and should consider the heat problem. Thermal components should stay away from the heating element.
(4) for potentiometer, adjustable inductance coils, variable capacitor, micro switch, adjustable components layout should consider the structure of the machine. If the tank regulation, should be placed in the printed circuit board easily to adjust; if it is outside the machine, its location to the chassis with the knob in position on the Panel.
(5) should stay out of the printed circuit board orientation hole and fixation takes place.

Depending on the circuit of the functional units of all. circuit to the layout of components, to comply with the following principles:

(1) in accordance with the arrangements for the various circuit processes function circuit unit position which makes the layout for the signal flow and make the signal as a consistent direction.
(2) to each feature to the core components of the circuit to center around it for layout. Components should be uniform, neat, compact arranged on the PCB, minimize and reduce the components and connections between the leader.
(3) in the high-frequency circuit working, consider the components of the distribution parameters. General circuit should, as far as possible, so that the components are arranged in parallel. In this way, not only in appearance and easy to assemble welding.. easy-to-volume production.
(4) is located on the edge of the circuit board, circuit board edge components are generally not less than 2mm. Circuit Board of the best shape of a rectangle. Aspect ratio 3: 2-4: 3. Circuit board surface dimensions greater than 200x150mm when consideration should be given to the Board of the mechanical strength.

2. wiring
Wiring the principle as follows:
(1) input output with wire should avoid adjacent parallel. Best plus line ground wire in order to avoid feedback coupled.
(2) of the printed photo conductor smallest width is mainly composed of wires and insulating matrix adhesion strength between the Board and through their current value. When copper foil thickness 0.05mm, width of 1 ~ 15mm when electrical current through 2A, temperature not be higher than 3 ° c, therefore, conductor width 1.5mm can meet the requirements. For integrated circuits, especially in the digital circuit, usually selected 0.02 ~ 0.3mm wire width. Of course, as long as possible, permitting or wide line, especially the power and ground wire. Conductor smallest space of mainly by the worst case of wire insulation resistance and voltage breakdown. For integrated circuits, especially in the digital circuit, as long as the technology allows to bring spacing as small as 5 ~ 8mm.
(3) printed conductor corner General from arc, and a right angle or angles in the high-frequency circuits affect the electrical properties. In addition, avoid using large copper foil, otherwise £ ® long heated, prone to the phenomenon of expansion and shedding of copper foil. You must use the large copper foil, it is best to use a grid to facilitate removal of. foil and adhesive between the substrate of volatile gas heating.

3. pads
Pads centre hole diameter than devices leader larger. Pad is too large and easy to form the virtual welding. Pad diameter D is generally not less than (d + 1.2) mm, where d is the wire diameter. For high density digital circuit, pads minimum diameter desirable (d + 1.0) mm.

PCB and anti-interference circuit

Anti-jamming design of printed circuit boards and specific circuits are closely related, here only a antijamming design PCB several commonly used measures do some instructions.

1. power cable design
According to the size of the printed circuit current, to increase the power line width, reduce the loop resistance. At the same time, the power cable and the ground of trends and data transfer direction consistent to help strengthen the ability of noise.
2. a ground wire design
Ground wire design principle is:
(1) digital and analog ones. If the circuit board on both logic and linear circuits, should enable them to separate. Low-frequency circuit to try to use a single point and parallel to ground, the actual wiring is difficult to be part of the series and parallel to ground. High-frequency circuit use multipoint-line to ground, ground rent should be short, high frequency component as far as possible with a grid-like around the large area foil.
(2) Earth wire should be as bold. If Earth wire line with a pick, the ground potential with current changes, and enable noise reduction in performance. It should be grounded to the bold, make it through three times on the printed circuit board allows current. If possible, grounding line should be 2 ~ 3mm above.
(3) the grounding line constitutes a closed-loop road. Only by digital circuits consisting of PCB, the earthing circuit Chief made loops are able to improve the ability of noise.
3. exit Lotus capacitance configuration
PCB design is one of the conventional practices in the various PCB key parts configuration adjustmentWhen the capacitance of a Lotus.
Exit Lotus capacitor General configuration principles are:
(1) power input crossover 10 ~ 100uf of electrolytic capacitors. If possible, add 100uF more better.
(2) in principle, each IC chip should lay out a 0.01pF of porcelain capacitor, in case of printed circuit board space is insufficient, to every 4 ~ 8 chip layout a 1 ~ 10pF but capacitance.
(3) for thresholding the weak, the turn-off time power variation devices, such as RAM, ROM storage devices, chip power and ground direct access between the draws back the Lotus capacitance.
(4) the capacitor leads should not be too long, especially high-frequency bypass capacitor can not have a leader.

In addition, you should note the following points:
(1) in the printed circuit board with contactors, relays, buttons and other elements operate them when will produce greater spark discharges, must be shown in the drawings of the RC circuit to absorb discharge current. General to take 1 ~ R, C of 2.2 2K ~ 47UF.
(2) CMOS input impedance is high, and vulnerable to the induction, therefore when used on the end-to-ground or not received as a power source.