PCB wiring has a trick, old engineers say

- Dec 05, 2019-

PCB wiring has a trick, old engineers say

Summary of content: PCB is also called printed circuit board (Printed Circuit Board), it can realize the circuit connection and function realization between electronic components, and it is also an important part of power circuit design. Today, this article will introduce the basic rules of PCB board layout.

Basic rules for component layout

1. Layout according to the circuit module, related circuits that achieve the same function are called a module, and the components in the circuit module should adopt the principle of close proximity, while the digital circuit and the analog circuit are separated;

2. It is not allowed to mount elements and components within 1.27mm around non-installation holes such as positioning holes and standard holes, and components such as screws must not be placed within 3.5mm (for M2.5) and 4mm (for M3);

3. Avoid laying vias under components such as resistances, inductors (plug-ins), electrolytic capacitors, etc., to avoid short-circuiting the vias with the component housing after wave soldering;

4. The distance from the outside of the component to the edge of the board is 5mm;

5. The distance between the outside of the pad of the mounting component and the outside of the adjacent insertion component is greater than 2mm;

6. Metal housing components and metal parts (shielding boxes, etc.) must not collide with other components, and they should not be close to printed wires and pads, and their spacing should be greater than 2mm. The size of the positioning hole, fastener installation hole, oval hole and other square holes in the board is greater than 3mm from the side of the board;

7. The heating element should not be close to the wire and the heat-sensitive element; the high-heat device should be distributed in a balanced manner;

8. The power socket should be arranged around the printed board as much as possible, and the power socket and the bus terminal connected to it should be arranged on the same side. Special care should be taken not to place power sockets and other soldered connectors between the connectors, in order to facilitate the welding of these sockets, connectors, and the design and tying of power cables. The spacing between power sockets and soldered connectors should be considered to facilitate the insertion and removal of power plugs;

9. Arrangement of other components: All IC components are aligned on one side, and the polar components have clear polarity indications. The polarity indications on the same printed board should not be more than two directions. When two directions appear, the two directions are perpendicular to each other ;

10. The wiring on the board should be properly dense. When the difference in density is too large, it should be filled with mesh copper foil, and the grid is larger than 8mil (or 0.2mm);

11. There must be no through-holes on the chip pads, so as to avoid solder paste loss caused by component soldering. Important signal cables are not allowed to pass between the feet of the socket;

12. The single side of the patch is aligned, the character direction is the same, and the packaging direction is the same;

13. Polarized devices should be as consistent as possible with the polarity marking directions on the same board.

Second, the component wiring rules

1. Draw wiring in the area 1mm from the edge of the PCB board, and within 1mm around the mounting hole, wiring is prohibited;

2. The power line should be as wide as possible and should not be less than 18mil; the signal line width should not be less than 12mil; the CPU input and output lines should not be less than 10mil (or 8mil);

3, the normal via hole is not less than 30mil;

4 Dual inline: 60mil pad, 40mil aperture;

1 / 4W resistance: 51 * 55mil (0805 surface mount); pad 62mil when inserting directly, aperture 42mil;

Capacitance: 51 * 55mil (0805 surface mount); 50mil pad, 28mil aperture when plugged directly;

5. Pay attention to the power line and ground line should be as radial as possible, and the signal line should not be looped.

How to improve anti-interference ability and electromagnetic compatibility?

How to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processors?

1. The following systems should pay special attention to anti-electromagnetic interference:

(1) Microcontroller clock frequency is particularly high and the bus cycle is extremely fast.

(2) The system contains high-power, high-current drive circuits, such as spark relays and high-current switches.

(3) A system containing a weak analog signal circuit and a high-precision A / D conversion circuit.

Take the following measures to increase the anti-electromagnetic interference ability of the system:

(1) Use a low-frequency microcontroller:

Selecting a microcontroller with a low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. For square and sine waves of the same frequency, the high-frequency components in a square wave are much more than those of a sine wave. Although the amplitude of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to be emitted as a noise source. The most influential high-frequency noise generated by the microcontroller is about three times the clock frequency.

(2) Reduce distortion in signal transmission

Microcontrollers are mainly manufactured using high-speed CMOS technology. The static input current of the signal input end is about 1mA, the input capacitance is about 10PF, the input impedance is quite high, and the output end of the high-speed CMOS circuit has a considerable load capacity, that is, a relatively large output value. If the long line is led to the input end with relatively high input impedance, the reflection problem is very serious, it will cause signal distortion and increase system noise. When Tpd> Tr, it becomes a transmission line problem, and the problems of signal reflection and impedance matching must be considered.

The delay time of the signal on the printed board is related to the characteristic impedance of the leads, that is, it is related to the dielectric constant of the printed circuit board material. It can be roughly considered that the transmission speed of signals on the printed circuit board leads is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of commonly used logic telephone components in a system composed of a microcontroller is between 3 and 18 ns.

On a printed circuit board, the signal passes a 7W resistor and a 25cm long lead, and the delay time on the line is approximately 4-20ns. In other words, the shorter the lead of the signal on the printed circuit, the better, and the longest should not exceed 25cm. And the number of vias should be as small as possible, preferably not more than two.

When the rise time of the signal is faster than the signal delay time, it must be processed according to fast electronics. At this time, it is necessary to consider the impedance matching of the transmission line. For the signal transmission between the integrated blocks on a printed circuit board, it is necessary to avoid the situation of Td> Trd. The larger the printed circuit board, the faster the system cannot be too fast.

A rule for printed circuit board design is summarized with the following conclusions:

The signal is transmitted on the printed board, and its delay time should not be greater than the nominal delay time of the device used.

(3) Reduce the mutual interference between signal lines:

A step signal with a rise time Tr at point A is transmitted to terminal B through the lead AB. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the signal at point A, 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 Td time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width that is twice the delay time of the signal on the AB line will be induced. This is the mutual interference between signals. The strength of the interference signal is related to the di / at of the C point signal, and it is related to the distance between the lines. When the two signal lines are not very long, what is actually seen on AB is the superposition of two pulses.

The micro-controller manufactured by CMOS process has high input impedance, high noise, and high noise tolerance. The digital circuit is superimposed with 100 ~ 200mv noise and does not affect its operation. If the AB line in the figure is an analog signal, this interference becomes intolerable. If the printed circuit board is a four-layer board, one of which has a large area of ground, or a double-sided board, and when the reverse side of the signal line is a large area of ground, the interference between such signals will become smaller. The reason is that a large area of ground reduces the characteristic impedance of the signal line, and the reflection of the signal at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the dielectric between the signal line and the ground, and proportional to the natural logarithm of the dielectric thickness. If the AB line is an analog signal, to avoid the digital circuit signal line CD from interfering with AB, a large area of ground must be under the AB line. The distance from the AB line to the CD line must be greater than 2 to 3 times the distance between the AB line and the ground. Local shielding ground can be used, and the ground wire should be laid on the left and right sides of the lead with a lead.

(4) reduce the noise from the power supply

When the power supply provides energy to the system, it also adds its noise to the power supply. The reset line, interrupt line, and other control lines of the microcontroller in the circuit are most susceptible to interference from external noise. Strong interference on the electrical network enters the circuit through the power supply, and even in battery-powered systems, the battery itself has high-frequency noise. The analog signal in the analog circuit cannot withstand the interference from the power supply.

(5) Pay attention to the high-frequency characteristics of printed wiring boards and components

Under high-frequency conditions, the leads, vias, resistances, capacitors, and distributed inductance and capacitance of connectors on printed circuit boards cannot be ignored. The distributed inductance of a capacitor cannot be ignored, and the distributed capacitance of an inductor cannot be ignored. The resistance generates reflection of high-frequency signals, and the distributed capacitance of the lead will play a role. When the length is longer than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect will occur, and the noise will be emitted outward through the lead.

 

The vias of the printed circuit board cause a capacitance of approximately 0.6pf.

The packaging material of an integrated circuit itself introduces 2 ~ 6pf capacitors.

A connector on a circuit board has a distributed inductance of 520nH. A dual-in-line, 24-pin integrated circuit socket with 4 ~ 18nH distributed inductance introduced.

These small distribution parameters are negligible for microcontroller systems at this lower frequency; special attention must be paid to high-speed systems.

 

(6) Element layout should be reasonably partitioned

The position of the components on the printed circuit board must be fully considered against electromagnetic interference. One of the principles is that the leads between components should be as short as possible. In the layout, the three parts of the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high-current switches, etc.) must be reasonably separated to minimize the signal coupling between them.

 

Handle the ground wire well

On the printed circuit board, the power and ground wires are the most important. The most important means to overcome electromagnetic interference is to ground.

 

For the double-panel, the layout of the ground wire is particularly particular. By adopting the single-point grounding method, the power supply and ground are connected to the printed circuit board from both ends of the power supply, and the power supply has one contact and one ground contact. On the printed circuit board, there must be multiple return ground wires, which will be gathered on the contact point of the power supply, which is the so-called single-point ground. The so-called analog ground, digital ground, and high-power device ground are divided, which means that the wiring is separated, and finally all come together to this ground point. When connecting to signals other than printed circuit boards, shielded cables are usually used. For high frequency and digital signals, the shielded cable is grounded at both ends. Shielded cables for low-frequency analog signals should be grounded at one end.

Circuits that are very sensitive to noise and interference or circuits with particularly high frequency noise should be shielded with metal covers.

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(7) Use good decoupling capacitors.

A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHZ. The high frequency characteristics of ceramic chip capacitors or multilayer ceramic capacitors are better. When designing a printed circuit board, add a decoupling capacitor between the power supply and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, the energy storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy of the integrated circuit when the door is opened and closed; on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor in digital circuits is a 0.1uf decoupling capacitor with a 5nH distributed inductance. Its parallel resonance frequency is about 7MHz, which means that it has a good decoupling effect for noise below 10MHz. Noise has little effect.

 

1uf, 10uf capacitor, parallel resonance frequency is above 20MHz, the effect of removing high frequency noise is better. Where the power source enters the printed board and a 1uf or 10uf high frequency capacitor is often advantageous, even a battery-powered system requires such a capacitor.

 

A charge and discharge capacitor, or storage capacitor, should be added for every 10 or so integrated circuits, and the capacitance can be selected as 10uf. It is best not to use electrolytic capacitors. Electrolytic capacitors are rolled up with two layers of diaphragm. This rolled up structure behaves as an inductor at high frequencies. It is best to use bile capacitors or polycarbonate capacitors.

 

The selection of the decoupling capacitor value is not strict, and it can be calculated according to C = 1 / f; that is, 10uf takes 0.1uf, and for a system composed of a microcontroller, it can be taken between 0.1 ~ 0.01uf.

3. Some experience in reducing noise and electromagnetic interference.

(1) Low-speed chips can be used instead of high-speed ones. High-speed chips are used in key places.

(2) A resistor can be connected in series to reduce the transition rate of the upper and lower edges of the control circuit.

(3) Try to provide some form of damping for relays, etc.

(4) Use the lowest frequency clock that meets the system requirements.

(5) The clock generator should be as close as possible to the device using the clock. Ground the quartz crystal oscillator case.

(6) Use a ground wire to circle the clock zone, and keep the clock wire as short as possible.

(7) I / O drive circuit should be as close as possible to the edge of the printed board, and let it leave the printed board as soon as possible. Filter the signal entering the printed board, and filter the signal from the high-noise area. At the same time, use string termination to reduce the signal reflection.

(8) The MCD useless terminal should be connected high, or grounded, or defined as an output terminal. The terminal connected to the power ground on the integrated circuit must be connected, do not float.

(9) Do not leave the unused gate circuit inputs unconnected. The unused op amps have their positive inputs connected to ground and their negative inputs connected to the output.

(10) Printed boards use 45-fold lines instead of 90-fold lines as much as possible to reduce the external emission and coupling of high-frequency signals.

(11) Printed boards are partitioned according to frequency and current switching characteristics. Noise components and non-noise components should be farther away.

(12) Single-point power supply and single-point grounding are used for single-panel and double-panel. The power and ground wires should be as thick as possible. If the economy is affordable, use multi-layer boards to reduce the power and ground inductance.

(13) The clock, bus, and chip select signals should be kept away from I / O lines and connectors.

(14) Analog voltage input lines and reference voltage terminals should be kept as far away from digital circuit signal lines as possible, especially clocks.

(15) For A / D devices, the digital part and the analog part should be unified rather than handed over.

(16) The clock line perpendicular to the I / O line has less interference than the parallel I / O line, and the clock element pins are far away from the I / O cable.

(17) The component pins are as short as possible, and the decoupling capacitor pins are as short as possible.

(18) The key lines should be as thick as possible, and protective grounds should be added on both sides. The high-speed line should be short and straight.

(19) The noise-sensitive lines should not be parallel to high-current, high-speed switching lines.

Do not run traces under the (20) quartz crystal and under noise-sensitive devices.

(21) Weak signal circuits, do not form a current loop around low-frequency circuits.

(22) Do not form a loop for any signal. If unavoidable, keep the loop area as small as possible.

(23) One decoupling capacitor for each integrated circuit. Add a small high-frequency bypass capacitor to each electrolytic capacitor.

(24) Use large-capacity tantalum capacitors or polymer capacitors instead of electrolytic capacitors for circuit charging and discharging energy storage capacitors. When using a tubular capacitor, the case must be grounded.


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