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Decrypting PROTEL DXP software for PCB design techniques

Protel DXP is the first board-level design system that integrates all design tools, and electronic designers can plan their own design from the initial project module to the final production data. Protel DXP runs on an optimized design browser platform with all of today’s advanced design features to handle complex PCB design processes. Protel DXP as a new circuit design software, based on the previous version of the addition of many new features. The new customizable design environment features dual monitor support, fixed, floating and pop-up panels, powerful filtering and object positioning capabilities, and enhanced user interface. Protel DXP provides a comprehensive design solution through the design of input simulation, PCB rendering editing, topology automatic routing, signal integrity analysis and design output.
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PCB board design of the general principles include: circuit board selection, circuit board size, component layout, wiring, pad, fill, jumper and so on.
The circuit board is generally made of copper laminates, board selection from the electrical performance, reliability, processing technology requirements and economic indicators and other considerations. Common copper laminates are copper phenolic paper laminates, copper-coated epoxy paper laminates, copper-coated epoxy glass cloth laminates, copper-clad epoxy phenolic glass cloth laminates, copper-coated polytetrafluoroethylene glass Cloth laminates and multilayer printed circuit boards with epoxy glass cloth and so on. Different materials of the laminate has different characteristics. Epoxy resin and copper foil has an excellent adhesion, so the copper foil adhesion strength and operating temperature is high, can be 260 ℃ in the melting tin does not blister. Epoxy resin-impregnated glass cloth laminates are less affected by moisture. Ultra-high frequency circuit board is best deposited copper polystyrene glass cloth laminate.
In the requirements of flame-retardant electronic equipment, also need flame retardant circuit boards, these boards are immersed in the flame retardant resin laminate. The thickness of the circuit board should be determined according to the function of the circuit board, the weight of the components installed, the specifications of the circuit board socket, the dimensions of the circuit board and the mechanical load.
Mainly should ensure sufficient rigidity and strength.
Common circuit board thickness of 0.5mm, 1.0mm, 1.5mm, 2.0mm
From the cost, copper wire length, anti-noise ability to consider, the smaller the better the size of the circuit board, but the board size is too small, the poor heat dissipation, and adjacent wires easily lead to interference. The production cost of the circuit board is related to the area of the circuit board, the larger the area, the higher the cost. When designing a circuit board with a housing, the size of the board is also limited by the size of the chassis. Be sure to determine the size of the chassis before determining the board size. Otherwise, the board size can not be determined. In general, the wiring range specified in the forbidden wiring layer is the size of the board size. The best shape of the circuit board is rectangular, the aspect ratio is 3: 2 or 4: 3, when the circuit board size is greater than 200mm × 150mm, should consider the mechanical strength of the circuit board. In short, should consider the pros and cons to determine the size of the circuit board.
Although the Protel DXP can automatically layout, but in fact the layout of the circuit board is almost done by hand.
To carry out the layout, generally follow the following rules:
1. The layout of special components
The layout of special components from the following aspects:
1) high-frequency components: the connection between the high-frequency components as short as possible, try to reduce the distribution of the parameters and the electromagnetic interference between each other, vulnerable components can not be too close. The distance between the components belonging to the input and the components belonging to the output should be as large as possible.
2) Components with high potential difference: The distance between the elements with high potential difference and the connection should be increased to avoid damage to the components when an unexpected short circuit occurs. In order to avoid the occurrence of creepage phenomenon, generally require 2000V potential difference between the copper film line distance should be greater than 2mm, if the higher potential difference, the distance should also increase. With high voltage devices, should be arranged as far as possible when debugging difficult to reach the place.
3) too much weight components: such components should have a fixed bracket, and for large and heavy, more heat and more components, should not be installed on the circuit board.
4) heat and thermal components: Note that the heating element should be away from the thermal components.
5) can adjust the components: for the potentiometer, adjustable inductance coil, variable capacitor, micro switch and other adjustable components of the layout should consider the structural requirements of the machine, if the machine is adjusted, it should be easy to adjust on the circuit board Of the place, if the machine outside the adjustment, its position with the adjustment knob in the chassis panel on the corresponding position.
6) Board mounting holes and bracket holes: The mounting holes for the circuit board and the mounting holes of the brackets should be reserved because they can not be routed near the holes and holes.
2. According to the circuit function layout
If there is no special requirements, as far as possible in accordance with the schematic components of the arrangements for the layout of the components, the signal from the left into the output from the right, from the top input from the bottom of the output. In accordance with the circuit process, the arrangement of the various functional circuit unit position, so that the signal flow more smoothly and maintain the same direction. With each functional circuit as the core, around the core circuit layout, component arrangement should be uniform, neat, compact, the principle is to reduce and shorten the lead between the various components and connections. The digital circuit part should be separated from the analog circuit section.
3. The distance of the component from the edge of the board
All components should be placed within 3mm from the edge of the board, or at least the distance from the edge of the board is equal to the thickness of the board, which is due to the use of high-volume production line plug and wave soldering, But also to prevent the deformation caused by the shape of the circuit board edge, causing copper film broken lead to waste. If the components on the circuit board too much, had to be more than 3mm, you can add the edge of the circuit board 3mm auxiliary side, in the auxiliary side to open V-shaped groove, in the production by hand breaking apart.
4. The order in which the components are placed
First place the fixed position with the structure in close contact with the components, such as power outlets, lights, switches and plug-ins and so on. And then placed special components, such as heating elements, transformers, integrated circuits. Finally, place small components, such as resistors, capacitors, diodes, and so on.
The rules of wiring are as follows:
1) line length: copper film should be as short as possible, in the high-frequency circuit should be the case. The fillet should not be rounded or beveled, and the right angle or sharp corners may affect electrical performance in the case of high frequency circuits and high wiring densities. When the double-panel wiring, the two sides of the wire should be perpendicular to each other, oblique or curved alignment, to avoid parallel to each other to reduce the parasitic capacitance.
2) line width: the width of the copper film should be able to meet the electrical characteristics of the requirements and easy to produce as a criterion, its minimum value depends on the current flowing through it, but generally should not be less than 0.2mm. As long as the board area is large enough, copper wire width and spacing the best choice 0.3mm. Under normal circumstances, 1 ~ 1.5mm line width, allowing the flow of 2A current. For example, the ground and power lines are best to use a line width greater than 1mm. When the two wires are between the integrated circuit board pads, the pad diameter is 50 mils, the line width and line spacing are 10 mils. When the pad is taken between a line, the pad diameter is 64 mils, line width and line The spacing is 12mil. Note the conversion between metric and imperial, 100mil = 2.54mm.
3) Line spacing: the spacing between adjacent copper film lines should meet the electrical safety requirements, and in order to facilitate the production, the spacing should be as wide as possible. The minimum spacing can withstand at least the peak of the applied voltage. In the case of low wiring density, the spacing should be as large as possible.
4) Shield and ground: copper wire of the public ground should be placed on the edge of the circuit board as much as possible. In the circuit board should be as much as possible to retain the copper foil to do the ground, so that can enhance the shielding capacity. In addition, the shape of the ground wire is preferably made into a loop or a grid. Multi-layer circuit board as a result of the use of inner power and ground special layer, which can play a better shielding effect.
The size of the pad size of the pad must be determined by the diameter of the metal lead and the thickness of the solder layer, the thickness of the tin layer, the pore size tolerance, and the thickness of the hole metallized plating. The diameter of the hole of the disk. For example, the resistance of the metal pin diameter of 0.5mm, the pad hole diameter of 0.7mm, and the pad diameter should be pad diameter plus 1.2mm, the minimum should be pad diameter plus 1.0mm. When the pad diameter is 1.5 mm, a square pad may be used in order to increase the peel strength of the pad. For holes with a hole diameter less than 0.4 mm, the outer diameter of the pad / pad hole diameter is 0.5 to 3. For holes with a hole diameter greater than 2 mm, the pad diameter / pad hole diameter is 1.5 to 2.
Common pad size
Pad 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
Consider the following when designing the pad:
1) The edge of the pad hole to the edge of the circuit board is greater than 1mm, which can avoid the processing lead to pad defects.
2) When the copper film is connected with the pad, the connection between the pad and the copper film is designed as a teardrop-like, so that the pad can not be easily peeled off. The connection between the copper film and the pad is not easy to disconnect.
3) adjacent pads to avoid an acute angle.
Large area filled
The large area on the circuit board filled with two purposes, one is the heat, the other is to use shield to reduce interference, in order to avoid the heat generated when the welding of the circuit board to produce no gas emissions leaving the copper film off, The area is filled with a window, which fills the grid. The use of copper can also achieve the purpose of anti-interference, and copper can automatically bypass the pad and can be connected to the ground.
Jumper wire
In the design of single-sided circuit boards, when some copper film can not be connected, the usual practice is to use the jumper, the length of the jumper should choose the following: 6mm, 8mm and 10mm.
1. Ground impedance of the common impedance of the circuit on the ground diagram of the circuit that the zero potential and used as a common reference point in the other points in the circuit, in the actual circuit due to ground (copper film line) impedance exists, Bring together a total impedance interference, so when wiring, can not be connected with the ground symbol of the points together, which may cause harmful coupling and affect the normal work of the circuit.
2. How to connect the ground is usually in an electronic system, the ground is divided into systematically, chassis (shield), digital (logical) and analog ground, etc., in the connection should pay attention to the following ground point:
1) Properly select single point ground and multipoint ground. In the low-frequency circuit, the signal frequency is less than 1MHz, wiring and components between the inductance can be ignored, and ground circuit resistance generated by the voltage drop on the circuit a greater impact, it should be a single point grounding method. When the frequency of the signal is greater than 10MHz, the impact of ground inductance, it is appropriate to use the nearest ground multi-point grounding method. When the signal frequency between 1 ~ 10MHz, if the use of single-point grounding method, the ground length should not exceed the wavelength of 1/20, or should be multi-point grounding.
2) digital ground and analog ground separately. The circuit board has both digital circuits and analog circuits, so that they should be separated as much as possible, and the ground wire can not be muted and connected to the ground side of the power supply (preferably the power supply terminals are also connected separately). To maximize the area of the linear circuit. General digital circuit anti-interference ability, TTL circuit noise margin of 0.4 ~ 0.6V, CMOS digital circuit noise margin for the power supply voltage of 0.3 ~ 0.45 times, while the analog circuit as long as the micro-level noise, Enough to make it work is not normal. So the two types of circuits should be separated from the layout and wiring.
3) as thick as possible. If the ground is very fine, the ground potential will change with the current changes, resulting in the interference of the electronic system signal, especially the analog circuit part, so the ground should be as wide as possible, generally more than 3mm is appropriate.
4) The ground wire forms a closed loop. When there is only a digital circuit on the circuit board, it should make the ground loop, which can significantly improve the anti-jamming capability, because when the circuit board has a lot of integrated circuits, if the ground is very fine, will cause a larger grounding The potential difference, and the ring ground can reduce the grounding resistance, thereby reducing the ground potential difference.
5) the same level of circuit grounding should be as close as possible, and the level of the power supply filter capacitor should also be connected to the level of the ground.
6) the connection of the ground line. The total ground line must be strictly in accordance with the high frequency, intermediate frequency, low frequency order level from weak to strong electrical connection. High-frequency part of the best use of large area surrounded by ground to ensure a good shielding effect.
Electronic systems with microprocessors, anti-jamming and electromagnetic compatibility are issues that must be considered in the design process, especially for systems with high clock rates and fast bus cycles; systems with high power, high current drive circuits; Signal and high-precision A / D conversion circuit system. In order to increase the system anti-electromagnetic interference ability should consider the following measures:
1) Select the microprocessor with low clock frequency. As long as the controller performance to meet the requirements, the lower the clock frequency the better, low clock can effectively reduce noise and improve the system’s anti-jamming capability. As the square wave contains a variety of frequency components, the high-frequency components can easily become a noise source, under normal circumstances, the clock frequency of 3 times the high-frequency noise is the most dangerous.
2) to reduce the distortion in signal transmission. When the high-speed signal (high signal frequency = rising edge and falling edge of the fast signal) in the copper film transmission, due to the copper film line inductance and capacitance, the signal will be distorted, when the distortion is too large, it will make System work is unreliable. General requirements, the signal on the circuit board to spread the copper film as short as possible, the less the number of viable the better. Typical values: no more than 25cm in length, no more than 2 holes.
3) to reduce the cross-interference between signals. When a signal line has a pulse signal, it will interfere with another weak signal line with a high input impedance. In this case, it is necessary to isolate the weak signal line by adding a grounded contour to surround the weak signal, or Increase the distance between lines, for different levels of interference can be used to increase the power and ground level solution.
4) Reduce the noise from the power supply. Power supply to the system while providing energy, but also its noise added to the power supply system, the system reset, interrupt and some other control signals are most vulnerable to external noise interference, so should be appropriate to increase the capacitance to filter out these Noise from the power supply.
5) pay attention to the circuit board and components of the high-frequency characteristics. In the case of high frequency, the circuit board on the copper film, pads, vias, electricity