PCB Tech - Principles of PCB board design layout based on PROTEL DXP software

Protel pcb DXP is the first board-level design system that integrates all design tools. Electronic designers can implement their own design methods from the initial project module planning to the final production data. Protel DXP runs on an optimized design browser platform, and has all the advanced design features of today, able to handle various complex PCB board design processes. Through the integration of design input simulation, PCB drawing and editing, topology automatic routing, signal integrity analysis and design output, Protel DXP provides a comprehensive design solution.

The principles of PCB board design include the following aspects:

1. Selection of PCB board

2. PCB board size

3. PCB board component layout

4. PCB board wiring

5. PCB board grounding

6. PCB board anti-interference

7. PCB board pad

8. Large area filling of PCB board

9. PCB board jumper

10. High frequency wiring on PCB board

Selection of protel pcb board

PCB boards are generally made of copper-clad laminates, and the selection of the layers should be considered in terms of electrical performance, reliability, processing requirements and economic indicators. Commonly used copper-clad laminates are copper-clad phenolic paper laminates, copper-clad epoxy paper laminates, copper-clad epoxy glass cloth laminates, copper-clad epoxy phenolic glass cloth laminates, and copper-clad polytetrafluoroethylene glass. Epoxy glass cloth for cloth laminates and multilayer printed circuit boards, etc. Laminates of different materials have different characteristics. Epoxy resin and copper foil have excellent adhesion, so the adhesion strength and working temperature of copper foil are relatively high, and it can not bubble in the molten tin at 260°C. Glass cloth laminates impregnated with epoxy resin are less affected by moisture. The UHF circuit board is preferably a copper-clad polytetrafluoroethylene glass cloth laminate. In electronic equipment that requires flame retardancy, flame retardant PCB boards are also required. These PCB boards are laminates impregnated with flame retardant resin.

PCB board size

The thickness of the PCB board should be determined according to the function of the PCB board, the weight of the installed components, the specifications of the PCB board socket, the external size of the PCB board and the mechanical load it bears. Mainly should ensure sufficient rigidity and strength.

The thickness of common PCB boards are: 0.5mm, 1.0mm, 1.5mm, 2.0mm.

Considering cost, copper film line length, and anti-noise capability, the smaller the PCB board size, the better. However, if the PCB board size is too small, the heat dissipation will be poor, and adjacent wires will easily cause interference. The production cost of the PCB board is related to the area of the PCB board. The larger the area, the higher the cost. When designing a PCB board with a chassis, the size of the PCB board is also limited by the size of the chassis shell. The chassis size must be determined before the PCB board size is determined, otherwise the PCB board size cannot be determined. In general, the designated wiring range in the prohibited wiring layer is the size of the PCB board.

The best shape of the PCB board is rectangular, and the aspect ratio is 3:2 or 4:3. When the size of the PCB board is larger than 200*150mm, the mechanical strength of the PCB board should be considered. In short, the pros and cons should be considered comprehensively to determine the PCB board size.

PCB board component layout

Although Protel DXP can automatically lay out, in fact, the layout of PCB components is almost all done manually during design. The layout of PCB board components generally follows the following rules:

1. Special component layout

The layout of special components is considered from the following aspects:

1) High frequency components

The shorter the connection between high-frequency components, the better, try to reduce the distribution parameters of the connection and the electromagnetic interference between each other, and the components that are susceptible to interference should not be too close. The distance between the input and output components should be as large as possible.

2) Components with high potential difference

The distance between the component with a high potential difference and the connection should be increased to avoid damage to the component in the event of an accidental short circuit. In order to avoid the occurrence of creepage phenomenon, it is generally required that the distance between the copper film lines between the 2000V potential difference should be greater than 2mm. For higher potential differences, the distance should be increased. Devices with high voltage should be placed as hard as possible in a place that is not easy to reach during debugging.

3) Components with too much weight

Such components should be fixed with brackets, and components that are large, heavy, and generate a lot of heat should not be installed on the PCB.

4) Heating and heat-sensitive components

Note that heating elements should be far away from heat-sensitive elements.

5) Adjustable components

For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, micro switches, etc., the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the PCB where it is easy to adjust., Its position should correspond to the position of the adjusting knob on the chassis panel.

6) Circuit board mounting holes and bracket holes

The mounting holes of the PCB board and the mounting holes of the bracket should be reserved, because wiring cannot be made near these holes and holes.

2. Layout according to circuit function

If there is no special requirement, layout the components according to the component arrangement of the schematic diagram as much as possible, the signal enters from the left, outputs from the right, inputs from the top, and outputs from the bottom. According to the circuit flow, arrange the position of each functional circuit unit to make the signal flow more smoothly and keep the direction consistent. With each functional circuit as the core, layout around this core circuit, the arrangement of components should be uniform, neat, and compact. The principle is to reduce and shorten the leads and connections between each component. The digital circuit part should be laid out separately from the analog circuit part.

3. The distance between the component and the edge of the PCB board

All components should be placed within 3mm from the edge of the PCB board, or at least the distance from the edge of the PCB board is equal to the thickness of the board. This is because the assembly line plug-in and wave soldering in mass production should be provided to the guide groove. At the same time, it is also to prevent the edge of the PCB from being damaged due to the shape processing, causing the copper film line to break and leading to waste. If there are too many components on the PCB and it is necessary to exceed 3mm, you can add a 3mm auxiliary edge on the edge of the PCB board, open a V-shaped groove on the auxiliary edge, and break it by hand during production.

4. The order of component placement

First, place the fixed position components that closely match the structure, such as power sockets, indicator lights, switches, and connection plugs. Then place special components, such as heating components, transformers, integrated circuits, etc. Finally, place small components, such as resistors, capacitors, diodes, etc.

PCB board wiring

The rules of PCB board wiring are as follows:

1) Cable length

Copper film lines should be as short as possible, especially in high-frequency circuits. The corners of the copper film line should be rounded or beveled, and the right or sharp corners will affect the electrical performance in the case of high-frequency circuits and high wiring density. When double-sided wiring, the wires on both sides should be perpendicular to each other, diagonally or bent, and avoid parallel to each other to reduce parasitic capacitance.

2) Line width

The width of the copper film line should be based on the criterion that can meet the electrical characteristics and is easy to produce. Its minimum value depends on the current flowing through it, but generally it should not be less than 0.2mm. As long as the board area is large enough, the copper film line width and spacing should preferably be 0.3mm. In general, a line width of 1~1.5mm allows a current of 2A to flow. For example, it is better to choose a line width larger than 1mm for the ground wire and the power wire. When two wires are routed between the IC seat pads, the pad diameter is 50 mils, and the line width and line spacing are both 10 mils. When a wire is routed between the pads, the pad diameter is 64 mils, the line width and the line The spacing is 12mil. Pay attention to the conversion between metric and imperial systems, 100mil=2.54mm.

3) Line spacing

The spacing between adjacent copper film lines should meet electrical safety requirements, and in order to facilitate production, the spacing should be as wide as possible. The minimum distance can at least withstand the peak value of the applied voltage. In the case of low wiring density, the spacing should be as large as possible.

4) Shielding and grounding

The common ground of the copper film line should be placed on the edge of the circuit board as much as possible. Keep as much copper foil as the ground wire on the PCB board, so that the shielding ability can be enhanced. In addition, the shape of the ground wire is best to be looped or meshed. The multi-layer PCB board uses the inner layer as a dedicated layer for power and ground, so it can play a better shielding effect.

PCB board grounding

1. Common impedance interference of ground wire

The ground wire on the circuit diagram represents the zero potential in the circuit and is used as a common reference point for other points in the circuit. In the actual circuit, due to the existence of the ground wire (copper film wire) impedance, it will inevitably bring common impedance interference. When wiring, the points with the ground symbol can not be connected together randomly, which may cause harmful coupling and affect the normal operation of the circuit.

2. How to connect the ground wire

Usually in an electronic system, the ground wire is divided into system ground, chassis ground (shielding ground), digital ground (logical ground) and analog ground. When connecting the ground wire, you should pay attention to the following points:

1) Correctly choose single-point grounding and multi-point grounding

In low-frequency circuits, the signal frequency is less than 1MHz, the inductance between wiring and components can be ignored, and the voltage drop generated on the ground circuit resistance has a greater impact on the circuit, so single-point grounding should be used. When the frequency of the signal is greater than 10MHz, the influence of the ground wire inductance is greater, so the nearby multi-point grounding method should be adopted. When the signal frequency is between 1~10MHz, if the single-point grounding method is adopted, the ground wire length should not exceed 1/20 of the wavelength, otherwise, multi-point grounding should be adopted.

2) Separate digital ground and analog ground

There are both digital circuits and analog circuits on the PCB. They should be separated as much as possible, and the ground wires should not be mixed. They should be connected to the ground terminals of the power supply (preferably the power terminals are also connected separately). Try to increase the area of the linear circuit. Generally, digital circuits have strong anti-interference ability. The noise tolerance of TTL circuits is 0.4~0.6V. The noise tolerance of CMOS digital circuits is 0.3~0.45 times of the power supply voltage. As long as there is microvolt noise in the analog circuit, Enough to make it work abnormally. So the two types of circuits should be laid out and routed separately.

3) Thicken the ground wire as much as possible

If the ground wire is very thin, the ground potential will change with the current change, causing the signal of the electronic system to be interfered, especially the analog circuit part, so the ground wire should be as wide as possible, generally greater than 3mm.

4) Form the ground wire into a closed loop

When there are only digital circuits on the PCB, the ground wire should form a loop, which can significantly improve the anti-interference ability. This is because when there are many integrated circuits on the PCB, if the ground wire is very thin, it will cause a larger ground. Potential difference, and the ring ground wire can reduce the ground resistance, thereby reducing the ground potential difference.

5) Grounding of circuits of the same level

The grounding point of the circuit of the same level should be as close as possible, and the power filter capacitor of the circuit of this level should also be connected to the grounding point of this level.

6) Connection of the general ground wire

The main ground wire must be connected from weak current to strong current in strict accordance with the order of high frequency, intermediate frequency, and low frequency. It is best to use a large-area surrounding ground wire for the high-frequency part to ensure a good shielding effect.

PCB board anti-interference

For electronic systems with microprocessors, anti-interference and electromagnetic compatibility are issues that must be considered in the design process, especially for systems with high clock frequencies and fast bus cycles; systems with high-power and high-current drive circuits; and weak simulations. Signal and high-precision A/D conversion circuit system. In order to increase the system's anti-electromagnetic interference capability, the following measures should be considered:

1) Choose a microprocessor with a low clock frequency

As long as the controller performance can meet the requirements, the lower the clock frequency, the better. A low clock can effectively reduce noise and improve the anti-interference ability of the system. Since the square wave contains various frequency components, its high-frequency components can easily become noise sources. In general, high-frequency noise with 3 times the clock frequency is the most dangerous.

2) Reduce distortion in signal transmission

When high-speed signals (high signal frequency = signals with fast rising and falling edges) are transmitted on the copper film line, the signal will be distorted due to the influence of the inductance and capacitance of the copper film line. When the distortion is too large, the signal will be distorted. The system is not working reliably. It is generally required that the shorter the copper film line for signal transmission on the PCB, the better, and the fewer the number of vias, the better. Typical value: The length does not exceed 25cm, and the number of vias does not exceed 2.

3) Reduce cross interference between signals

When a signal line has a pulse signal, it will interfere with another weak signal line with high input impedance. At this time, it is necessary to isolate the weak signal line by adding a grounded contour line to surround the weak signal, or Increase the distance between the lines, and the interference between different levels can be solved by increasing the power and ground levels.

4) Reduce the noise from the power supply

While the power supply provides energy to the system, it also adds its noise to the power supply system. The reset, interrupt and other control signals in the system are most susceptible to interference from external noise. Therefore, capacitors should be appropriately added to filter these Noise from the power supply.

5) Pay attention to the high frequency characteristics of PCB board and components

In the case of high frequency, the distributed inductance and capacitance of the copper film lines, pads, vias, resistors, capacitors, and connectors on the PCB cannot be ignored. Due to the influence of these distributed inductances and capacitances, when the length of the copper film line is 1/20 of the signal or noise wavelength, an antenna effect will occur, causing electromagnetic interference to the internal, and emitting electromagnetic waves to the outside. Under normal circumstances, vias and pads will generate 0.6pF capacitance, an integrated circuit package will generate 2~6pF capacitance, a PCB board connector will generate 520mH inductance, and a DIP-24 socket has 18nH Inductance, these capacitances and inductances have no effect on circuits with low clock frequencies, and attention must be paid to circuits with high clock frequencies.

6) The layout of the components should be divided reasonably

The position of the components on the circuit board should fully consider the problem of anti-electromagnetic interference. One of the principles is that the copper film lines between the various components should be as short as possible. In the layout, the analog circuits, digital circuits, and circuits that generate large noises (relays, high current switches, etc.) should be reasonably separated so that they are mutually connected. The signal coupling is minimal.

7) Handle the ground wire

Dispose of the ground wire in accordance with the single-point grounding or multi-point grounding method mentioned earlier. Separately connect the analog ground, digital ground, and high-power device ground, and then converge to the ground point of the power supply. Use shielded wires for leads outside the PCB. For high-frequency and digital signals, both ends of the shielded cable must be grounded. For low-frequency analog signals, single-ended grounding is generally used. Circuits that are very sensitive to noise and interference or circuits that are particularly high-frequency noise should be shielded with a metal shield.

8) Decoupling capacitor

Ceramic capacitors or multilayer ceramic capacitors have better high-frequency characteristics for decoupling capacitors. When designing the PCB board, a decoupling capacitor must be added between the power and ground of each integrated circuit. The decoupling capacitor has two functions. On the one hand, it is the energy storage capacitor of the integrated circuit, which provides and absorbs the charging and discharging energy at the moment of opening and closing the integrated circuit. On the other hand, it bypasses the high-frequency noise generated by the device. The typical decoupling capacitor in the digital circuit is 0.1μF, such a capacitor has

The distributed inductance of 5nH can have a better decoupling effect on noise below 10MHz. Generally, a capacitor of 0.01~0.1μF can be selected.

Generally, it is required to add a 10μF charge and discharge capacitor to less than 10 integrated circuits. In addition, a 10~100μF capacitor should be connected across the power supply terminal and the four corners of the circuit board.

PCB board pad

Pad size: The inner hole size of the pad must be considered from the component lead diameter and tolerance size, as well as the thickness of the tin plating layer, the hole diameter tolerance, and the thickness of the hole metalization plating layer. Normally, the diameter of the metal pin plus 0.2mm is taken as The diameter of the inner hole of the pad. For example, if the diameter of the metal pin of the resistor is 0.5mm, the diameter of the pad hole is 0.7mm, and the outer diameter of the pad should be the pad diameter plus 1.2mm, and the minimum should be the pad diameter plus 1.0mm. When the pad diameter is 1.5mm, in order to increase the peel strength of the pad, a square pad can be used. For pads with a hole diameter of less than 0.4mm, the outer diameter of the pad/the diameter of the hole of the pad=0.5~3. For pads with a hole diameter greater than 2mm, the outer diameter of the pad/the diameter of the pad hole=1.5~2.

Commonly used pad size:

Land hole diameter/mm

0.4; 0.5; 0.6; 0.8; 1.0; 1.2; 1.6; 2.0

Pad outer diameter/mm

1.5; 1.5; 2.0; 2.0; 2.5; 3.0; 3.5; 4

The precautions when designing the pad are as follows:

1) The distance between the edge of the pad hole and the edge of the PCB board should be greater than 1mm, so as to avoid damage to the pad during processing.

2) The pad fills up teardrops. When the copper film line connected to the pad is thin, the connection between the pad and the copper film line should be designed in a teardrop shape, so that the pad is not easily peeled off. The connection between the copper film line and the pad is not easy to disconnect.

3) Adjacent pads should avoid sharp angles.

Large area filling of PCB board

There are two purposes for the large area filling on the PCB. One is to dissipate heat, and the other is to use shielding to reduce interference. The area is filled with a window, which makes the filling grid-like. The use of copper coating can also achieve the purpose of anti-interference, and the copper coating can automatically bypass the pad and connect to the ground wire.

PCB board jumper

In the design of a single-sided PCB board, when some copper films cannot be connected, the usual way is to use jumper wires. The length of the jumper wires should be selected as follows: 6mm, 8mm and 10mm.

PCB board high frequency wiring

In order to make the design of the high-frequency PCB board more reasonable and have better anti-interference performance, the following aspects should be considered when designing the PCB board:

1) Reasonably choose the number of layers

Using the middle inner plane as the power and ground layer can play a shielding role, effectively reducing parasitic inductance, shortening the length of signal lines, and reducing cross-interference between signals. In general, a four-layer board has lower noise than a two-layer board 20dB.

2) Wiring method

The wiring must be turned at an angle of 45°, which can reduce the emission of high-frequency signals and the coupling between them.

3) Cable length

The shorter the trace length, the better, and the shorter the parallel distance between the two lines, the better.

4) Number of vias

The smaller the number of vias, the better.

5) Interlayer wiring direction

The wiring direction between layers should be vertical, that is, the top layer is the horizontal direction, and the bottom layer is the vertical direction, so that interference between signals can be reduced.

6) Copper coating

Increasing the grounding copper can reduce the interference between signals.

7) Land package

Packing processing of important signal lines can significantly improve the anti-interference ability of the signal. Of course, it is also possible to package the interference source so that it cannot interfere with other signals.

8) Signal line

The signal wiring cannot be looped and needs to be wired in a daisy chain manner.

9) Decoupling capacitor

Connect a decoupling capacitor across the power supply terminal of the integrated circuit.

10) High frequency choke

When digital ground, analog ground, etc. are connected to a common ground, a high-frequency choke device should be connected, which is generally a high-frequency ferrite bead with a wire through the center hole.