Printed circuit board (PCB) support one of component of the circuit and device in the electronic product. It offers the electric connection between circuit component and device. With the development at full speed in technology of electricity, the density of PCB is higher and higher. The quality that PCB designs opposes interference ability and exerts a tremendous influence. So, while designing PCB. Must observe the general principle that PCB design, should also accord with the requirement designed in anti-interference.
General principle that PCB designs
Want, make electronic circuit obtain best performance, cloth of components and parts and wire it lays to be very important. PCB high quality for design, with low fabrication cost. Should follow the following general principle:
1. Overall arrangement
First of all, should consider PCB size. When PCB is oversized, prints the long lines, impedance increases, the ability to resist noise drops, the cost increases too; Too small, it is not good to dispel the heat, the lines are easy to be interfered with that and neighbouring. After confirming PCB size. Position special component again. Finally, according to the function unit of the circuit, carry on the overall arrangement to all components and parts of the circuit.
Observe the following principles while positioning special component:
(1)Shorten the line between the high-frequency components and parts as much as possible, distribution parameter and mutual electromagnetism of trying to reduce them are interfered with. It is easy to be can’t suffer too near each other by the components and parts interfered with, should try hard to be far away from to input and export the component.
(2)There may be difference of higher electric potential between some components and parts or wire, should strengthen the distance between them, so as not to discharge and draw shorting out in surprise. The components and parts with high voltage should try hard to fix up the difficult place to touch of hand in the debugging.
(3)The weight exceeds the components and parts of 15g, should be fixed with the support, then weld. Those large and heavy components and parts with much caloric value, it is unsuitable to put in printing plate making, and should install on the machine case baseplate of the complete machine, and should consider the heat-dissipating problem. The temperature sensing component should be far away from generating heat the component.
(4)Should consider the structure of the complete machine is required as to the overall arrangement of adjustable components such as the potentiometer, adjustable inductor coil, variable condenser, microswitch,etc.. If regulate in the machine, should put in printing plate making conveniently in the place to regulate; If regulate outside the machine, its position wants and regulates the position on the machine case panel of knob and suit.
(5)Should reserve and print and pull and orientate the hole and position where the fixed support takes up.
According to the function unit of the circuit. While carrying on the overall arrangement to all components and parts of the circuit, should accord with the following principles:
(1)Arrange the position of each function circuit unit according to the procedure of the circuit, make the convenient signal of the overall arrangement circulate, and make the signal keep unanimous direction as much as possible.
(2)Regard key component of each function circuit as the centre, carry on the overall arrangement around it. The components and parts should be even, arrange on PCB neatly, compactly. Try hard to reduce and shorten the lead wire between all components and parts and connect.
(3)The circuit working under high frequency, should consider the distribution parameter between the components and parts. The general circuit should make the components and parts arrange parallelly as much as possible. In this way, not only esthetic. And it is easy to hold and weld. Easy to produce in batches.
(4)The components and parts located in edge of the circuit board, are seldom smaller than 2mm from the edge of the circuit board. The best shape on board of the circuit is the rectangle. The aspect ratio is 3: Twenty percent 4: 3. The circuit board when the size is greater than 200×150mm. Should consider the mechanical intensity that the circuit board receives.
2. Wiring
The principle of connecting up is as follows:
(1)Should try hard to avoid to input the wire used for output end adjacently and parallelly. You had better add the ground wire of space, so as to avoid and feedback the lotus root to shut.
(2)Printing the minimum width of taking a photograph of wire mainly glues and enclose the intensity and electric current value flowing through them and determine when pulled by wire and insulating base. Look on copper as foil thickness When 0.05mm, width are 1 15mm. Through the electric current of 2A, the temperature will not be higher than 3 , so. The wire width is that 1.5mm can meet the demands. As to the integrated circuit, especially digital circuit, usually select 0.02~0.3mm wire width. Certainly, so long as allow, or use the wide thread as much as possible. Especially cable and ground wire. The minimum interval of the wire is mainly decided by insulating resistance of space and puncturing the voltage under the worst situation. As to the integrated circuit, especially digital circuit, so long as the craft is allowed, can enable the interval as little as 5~8mm.
(3)Print the wire corner to generally fetch circularly, and the right angle or contained angle will influence electric performance in the high-frequency circuit. In addition, try hard to avoid using the copper foil of large area, otherwise. When being heated for a long time, the foil of copper incident swells and losses the phenomenon. When must use the copper foil of large area, had better use the bar trellisedly. Help to get rid of among copper foil and base plate like this the binder is heated the volatile gas produced.
3.Pad
The centre bore of pad is slightly larger than the diameter of lead wire of the device. It is welded that the pad is too large and apt to take shape emptily. The external diameter D of the pad is seldom smaller than (d +1.2) Mm, among them d is the aperture of the lead wire. To the digital circuit of the high density, the minimum diameter of the pad is desirable (d +1.0) mm.
PCB and circuit interference-free measure
The anti-interference on board of the printed circuit is designed there are close relations with the concrete circuit, make some explanation on several daily measures designed in PCB anti-interference alone here.
1.The cable is designed
According to printing the size of the board electric current of circuit, the cable width of the added rent that try hard, reduce the cycle resistance. At the same time, make the trends of the cable, ground wire keep the same with direction in which the data transmits, contribute to strengthening the ability to resist noise like this.
2. The ground wire is designed
The principle that the ground wire is designed is:
(1)The figure is separated from simulation. If have both logical circuit and linear circuit on the circuit board, should make them try hard to separate. Circuit of low frequency should try hard to adopt and click and link only, connect up and can link the ground after connecting partly in series while having difficulties actually. High-frequency circuit should adopt some connect earthing in series more, ground wire should short renting, try hard, use the intersection of bar and large the intersection of area and the intersection of place and foil trellised around the high-frequency component.
(2)The earth connection should try hard to add thickly. If the earth connection is with the lines that are threaded very much, ground the electric potential and vary with electric current, make it make an uproar performance lower not to resist. So should add the earth connection thickly, enable it to pass and 3 times as much as that of printing the permission electric current on plate making. If possible, the earth connection should be in above 2~3mm.
(3)The earth connection forms and closes the cycle. The seal plate making only composed of digital circuits, its cycle of group of cloth of earthed circuit can mostly improve the ability to resist noise.
3.Return the electric capacity of the lotus root to dispose
One of the regular methods that PCB designs is printing each key position of plate making and disposing appropriate returning the electric capacity of the lotus root.
The capacitive general disposition principle is to return the lotus root:
(1)The input terminus of the power steps the electrolytic capacitor which connects 10 100uf. If possible, it is better to connect above 100uF.
(2)Each IC chip should assign the porcelain of one 0.01pF a slice of electric capacity in principle, in case of it is not enough to print space of plate making, very every 4~8 chips assign 1 10pF for one but electric capacity.
(3)Make an uproar weak ability resisting and power change old device shut off, such as RAM, ROM store the device, should insert and return the electric capacity of the lotus root directly between cable of the chip and ground wire.
(4)The lead wire of electric capacity can’t be too long, there can’t be lead wire in the electric capacity of especially high-frequency bypass.
In addition, should also pay attention to the following two points:
(1)While there are components such as contactor, relay, button,etc. in plate making to print. Will produce greater spark and discharge, must absorb and discharge the electric current by RC circuit that figures show while operating them. General R fetches 1 2K, C fetches 2.2 47UF.
(2)Input impedance of CMOS very high, and apt to receive reaction, while using to needn’t carry, take earthing or connect straight power.
Thanks for sharing this valuable piece of information with us. I am glad to find this stuff here. Keep up the good work.Printed Circuit Board
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