Category:PCB PCB Layout

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The connections on a PCB should be identical to its corresponding circuit diagram, but while the circuit diagram is arranged to be readable, the PCB layout is arranged to be functional, so there is rarely any visible correlation.

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The Basic Rules of PCB Board Layout Reply 2017-02-23 20:09:25
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David

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Introduction: PCB is known as the printed circuit board which can achieve the line connection and function realization between electronic components. It is also an important part of the power circuit design. Here are the basic rules of PCB board layout.

The basic rules of component layout
1. Layout is in accordance with the circuit module. The relevant circuit achieving the same function is a module; the components in circuit module should be used in the nearby centralized principle, while digital circuits and analog circuits will be separated;
2.3.5mm(For M2.5), 4mm (for M3) around the mounting hole like bolt may not be mounted on the components; 1.27mm around the non-mounting holes like position hole and standard hole should not be mounted with components and devices.
3. The bottom of sleeper resistance, inductance (plug-in), electrolytic capacitors and other components should avoid through-hole, preventing the short circuit between through-hole and component shell after wave soldering;
4. The distance between the outer side of the component and the board edge is 5 mm;
5. The distance between the outer edge of mounting element pad and the outside of the adjacent components is more than 2 mm;
6. Metal shell components and metal parts (shielding boxes, etc.) cannot be collided with other components, cannot be close to the printed line and pad, their spacing should be more than than 2mm. Positioning holes, fastener mounting holes, oval holes and other square holes in the board should be more than 3mm away from the board edge;
7. Heating components cannot be close to the wire and thermal components; high thermal devices should be balanced evenly;
8. Arranging as far as possible around the printed circuit board, the power outlet and the bus bar terminals connected to it should be arranged in the same side. In particular, do not place the power outlet and other solder connectors between the connectors to facilitate the design and bundle of these sockets, connectors and power cables. The layout of power outlet and welding connector should be considered to facilitate the power plug;
9. The layout of other components
All the IC components are aligned side by side; the polarity of the components should be marked clearly but shouldn’t be in more than two directions in the same printed circuit board, or the directions need to be perpendicular to each other;
10. The board wiring should be well-balanced and should be filled with mesh copper when the density difference is too large, and the gridding is more than 8mil (or 0.2mm);
11. The pad cannot have through-hole to avoid the poor solder paste in components. Important signal lines are not allowed to pass through the socket feet;
12. Pay attention to patch unilateral alignment and the character direction and packaging direction should be consistent;
13. The polarity mark of the device in the same board should be as much as possible consistent.
The component wiring rules
1. The distance from the wiring area to the PCB board edge cannot exceed 1mm, and the wiring is prohibited 1mm around installation hole
2. The power wire should be as wide as possible and not be less than 18mil; signal line width should not be less than 12mil; cpu input the output line should not be less than 10mil (or 8mil); line spacing is not less than 10mil;
3. The normal through-hole is not less than 30mil;
4, Dual in-line: pad 60mil, aperture 40mil;
1 / 4W resistance: 51 * 55mil (0805 surface); pad 62mil, aperture 42mil;
Promise capacitance: 51 * 55mil (0805 surface); pad 50mil, aperture 28mil;
5. The power and ground wire should be as radial as possible, and the signal line can not appear loop alignment.
How to improve the anti-jamming capability and electromagnetic compatibility when developing electronic products with processor?
1. the following systems should be paid special attention to anti-electromagnetic interference:
(1) Microcontrollers with particularly high clock frequency and fast bus cycles.
(2) System contains high-power and high current drive circuit, such as the spark production relay, high current switch, etc.
(3) System with weak analog signal circuit and high precision A / D conversion circuit.
2. Measures for increasing the system's anti-electromagnetic interference ability:
Use low frequency microcontrollers
Low external clock frequency microcontrollers can effectively reduce noise and improve the system's anti-jamming capability. The same frequency of square wave and sine wave, square wave in the high-frequency components are much more than the sine wave. Although the wave amplitude of the high frequency component of square wave is smaller than that of the fundamental wave, the higher frequency is easier to emit as a noise source and the most influential high frequency noise generated by the microcontroller is about as three times as the clock frequency.
Reduce the distortion in signal transmission
Microcontrollers are mainly manufactured by high-speed CMOS technology. In the signal input side, the static input current is about 1mA, the input capacitance is roughly 10PF. With high input impedance, high-speed CMOS circuit output has a considerable load capacity, that is a considerable output value. The door output is lead to a very high input impedance input through a very Long line, causing serious reflection problem which will result in signal distortion and increasing system noise. When Tpd>>Tr, a transmission line problem comes into being, so you must consider signal reflection, impedance matching and other issues.
The delay time of signal on the printed circuit board is related to the characteristic impedance of the lead, namely, it is connected with the dielectric constant of the PCB material. It can be roughly assumed that the transmission speed of the signal in the printed circuit board is about 1/3 to 1/2 of the light speed. The Tr (standard delay time) of the commonly used logical telephone elements in the system constituted by the microcontrollers is between 3 and 18 ns.
On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead and the line delay time is approximately between 4 and 20ns. In other words, the shorter lead of signal on the printed circuit is better, and the longest should not exceed 25cm. Besides, the number of vias should be as small as possible, preferably less than two.
When the rise time of the signal is faster than the delay time, it is necessary to follow the rules from fast electronics. At this moment, you should consider the impedance matching of transmission line. For the signal transmission between integrated block on a printed circuit board, you should avoid the emergence of Td>>Trd. Generally, the speed of the printed circuit board system cannot be too fast if the board is greater. In conclusion, when the signal is transmitted on the printed board, the delay time should be less than the nominal delay time of the device used.
Reduce the interference between signal lines:
The rise time of point A is Tr step signal which is passed through the lead AB to B side, while the delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the point A signal, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal with a width of Tr will be induced after the Td time. At point C, as the transmission and reflection of the signal on AB line, it will sense a 2Td positive pulse signal that is twice the width of the delay time of the signal on the AB line. This is the interaction between the signal interference. The intensity of the interfering signal is related to the di/at of the C-point signal, which is related to line spacing. When the two signal lines are not very long, the two pulse superposition can be seen on line AB.
The micro-control of CMOS technology is of high input impedance, noise and noise margin, while the digital circuit is superimposed 100 ~ 200mv noise which does not affect its work. If the AB line is an analog signal, then this interference becomes intolerable. If a four-layer board has
large area of ground or the opposite of signal lines is a large area of the ground in double-sided board, the interference between the signals will become smaller. The reason is that the large area of ground reduces the characteristic impedance of the signal line and the signal reflection at the D-side. The characteristic impedance is inversely proportional to the square of the dielectric constant of medium from the signal line to ground, which is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, for avoiding the interference of digital circuit signal line CD on line AB, the distance between them should be 2 to 3 times greater than the one from line AB and ground. Partially shielding the ground, the lead with junctions will be wired on the both sides.
Reduce the noise from the power supply
The power supply offers energy to the system and adds noise to the powered power supply. In the circuit, microcontroller reset line, interrupt line and some other control lines are most susceptible to external noise interference. The Strong interference on the power grid passes through power supply into the circuit, even if the battery itself also has high-frequency noise in the battery-powered system. Analog signal in circuits cannot withstand the interference from the power supply.
Notice the high-frequency characteristics of PCB and components
In high frequency, the lead, through-hole, resistance, capacitance, distributed inductance and capacitance of connectors cannot be ignored. Besides, distributed inductance of capacitance and distributed capacitance of inductance cannot be neglected. The resistance produces a reflection of the high frequency signal, and the distributed capacitance of the lead will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect is generated and the noise is spreading out through the lead. The through-hole in printed circuit board brings about 0.6pf of capacitance, while the encapsulated material in an integrated circuit causes 2 ~ 6pf.A connector on a circuit board has a 520nH distributed inductance. A double-stranded 24-pin integrated circuit socket leads in distributed inductance of 4 ~ 18nH.These small distribution parameters are negligible for microcontrollers in these lower frequency systems, but special attention must be paid to high speed systems.
Component layout should be in rationalization partition
The anti-electromagnetic interference should be fully considered in components placement on the PCB board, one of the principles is making the lead between the components as short as possible. In layout, the analog signal part, high-speed digital circuit part and the noise source part (such as the relay, high current switch, etc.) are reasonably separated, so that the signal coupling is the smallest.
Handle the ground wire
The power and ground wire are the most important parts for printed circuit boards. To overcome the electromagnetic interference, the main method is to ground. For double-sided board, the ground layout is special. By single-point grounding, power and ground are connected to the board from both ends of the power supply, and each of them has a junction.
The printed circuit board should have many returned ground wires, which will be gathered the junction back to the power. This is the so-called single point of grounding. The separation of analog ground, digital ground and high-power device, refers to the wiring separating and finally coming together to the ground junction. When connecting to a signal other than a printed circuit board, a shielded cable is usually used. For high frequency and digital signals, both ends of the shielded cable are grounded. The shielded cable used in low-frequency analog signal should be grounded on one end. The Circuits that are very sensitive to noise and interference or circuits or the ones with severe high frequency noise should be shielded with metal covers.
Use a good decoupling capacitor
A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have high frequency characteristics. When designing printed circuit board, a decoupling capacitor should be added between each integrated circuit power supply and ground. Decoupling capacitors have two effects: On the one hand, it provides and absorbs the charge and discharge energy of integrated circuit; on the other hand reducing high-frequency noise. The typical decoupling capacitor with 0.1uf capacitance in the digital circuit has a 5nH distributed inductor, whose parallel resonant frequency is about 7MHz, this means that the noise below 10MHz has a better decoupling effect and is almost ineffective to the noise above40MHz. 1uf, 10uf capacitor, whose parallel resonance frequency is above 20MHz, effect of removing the high frequency noise is better.
It is often advantageous to place the power supply into the printed circuit board with a 1uf or 10uf high-frequency capacitor, even if the battery-powered system also needs this capacitor.
About 10 integrated circuits should be added a storage capacitor, whose capacitor size is 10uf. You’d better quit electrolytic capacitors, as electrolytic capacitors are rolled up by two layers of thin film. This structure at high frequencies performs as an inductor, and it is best to use the gallium capacitor or polycarbonate capacitor.
Decoupling capacitor value selection is not strict, according to C = 1 / f calculation; namely, 10MHz (0.1uf), for the system composed of micro-controller, you can select between 0.1 ~ 0.01uf.
Reduce the noise and electromagnetic interference
(1) Prefer low-speed chip to high-speed; high-speed chip is used in key places.
(2) By a string of resistance, reducing and controlling the rate of change when circuits go up and down.
(3) Provide relay with some form of damping.
(4) Use the lowest frequency clock that meets the system requirements.
(5) The clock generator is as close as possible to the device using the clock. The shell of Quartz crystal oscillator should be grounded.
(6) Circle up clock with the ground wire and the clock line is as short as possible.
(7) I/O drive circuit should leave the edge of the printing plate instantly. The signal goes into the printed circuit board and from the high noise zone should be added to filter, reducing the signal reflection by stringing termination resistor.
(8) MCD useless end should be high, or ground, or defined as the output, and the end of power supply in integrated circuit need to be grounded, do not vacant.
(9) Do not make the input end of idle gate circuit vacant; positive input termination of idle op amp is grounded and negative input termination connects output.
(10) Try to use 45 fold lines instead of 90 fold lines in PCB to reduce the external launch and coupling from high-frequency signal.
(11) Printed circuit board is distinguished by the frequency and current switching characteristics, the noise components and non-noise ones should be far.
(12) single-sided and double-sided board are adopting single-point power supply and single-point grounding; power and ground wires is as thick as possible, using multi-layer board to reduce the power supply and capacitive inductance if you are not sensitive to money.
(13) Clock, bus and chip select should be away from I/O lines and connectors.
(14) Analog voltage input line, notice the voltage side stays away from the digital circuit signal lines, especially to the clock.
(15) To the A/D class devices, digital and analog part would rather unite rather than cross.
(16) The interference of clock line which is perpendicular to the I/O line is smaller than the parallel and the clock component pin is far from I/O cable.
(17) Component and decoupling capacitor pins are as short as possible.
(18) The key line should to be as thick as possible with protection ground on both sides. High-speed line is short and straight.
(19) Noise-sensitive lines do not parallel to high-current, high-speed switching lines.
(20) Do not route below quartz crystal and noise-sensitive devices.
(21) Do not form a current loop around weak signal circuit and low frequency circuit.
(22) Do not form a loop for any signal, if unavoidable, make loop area as small as possible.
(23) Make a complementary capacitor for each integrated circuit. Each electrolytic capacitor edge should add a small high frequency bypass capacitor.
(24) Using tantalum or cool capacitors of a large capacity instead of electrolytic capacitors for the circuit charge and discharge. The housing needs to be grounded when using a tubular capacitor.
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Statement: This post is only the personal view of the author and does not represent the opinions of ALLPCB.com.

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