How to design a multi-layer board Protel DXP

In the circuit board PCB drawing design, it is necessary to arrange and position the fixed component positions, and to properly connect the pins of each electronic device. It is very important for the designer to perform each step, because its quality has a great influence on the anti-interference ability of the product. Large and decisive for the performance of the product. Nowadays, electronic technology is developing rapidly, and the technical indicators required in various electronic products are getting higher and higher, such as embedded development, microcontroller development and design, and motor drivers. In the Zigong area, the simulated dinosaur control box is used. In the simulation animal control box, the dimensions of the components and products used are getting smaller and smaller, and the working frequency is getting higher and higher, which makes the density of components on the PCB greatly improved. It increases the difficulty of PCB design and processing.
In the electronics industry, electronic printed circuit boards/PCBs support electronic components and devices in various electronic products. At the same time, in a limited space area, complex electrical connection lines are required to be placed between electronic components and devices. The electrical wiring is connected.

1 layout
The so-called layout is to arrange all the components in the circuit diagram reasonably on the PCB with limited area. From the perspective of the signal
Generally speaking, there are three types of digital signal boards, analog signal boards, and mixed signal boards. Designing mixed signal power
When you use the board, you must carefully consider the components placed manually in the appropriate position of the board so that the numbers and
The analog devices are separated.
In the layout process of the PCB layout, the most critical issues are: switches, buttons, knobs and other operating parts and structural parts (simplified
It is called "special component", etc., and must be arranged in advance to the designated (appropriate) position. After being placed, you can set the element
Attribute, select the LOCK item, so that you can avoid moving it later by mistake; for other components
Arrangement must be balanced with the optimization of the routing and electrical performance of the wiring, as well as the future production process and cost.
Aspect factor. The so-called “consideration” is often a challenge to the level and experience of design staff.
1.1 The layout principle of special components
1 should minimize the wiring between components, try to reduce their distribution parameters and electromagnetic dryness between each other
Disturb. Components that are susceptible to electromagnetic interference should not be placed too close together, and input and output components should be as far apart as possible from each other.
2 Some components or wires may have a high potential difference, so the distance between them should be increased to avoid
Electric causes accidental short circuit; at the same time, from the perspective of safety, components with high voltage should be arranged as much as possible during commissioning.
The place touched.
3 For tantalum devices with a mass exceeding 15g, the brackets should be used for fixing before soldering. Those big and heavy,
Components with large heat generation should not be mounted on the printed board, but should be installed on the bottom plate of the whole machine;
The thermal components should be kept away from the heating elements.
4 For the layout of adjustable components such as potentiometer, inductive wire, variable capacitor, micro switch, etc., the whole machine should be considered.
Structural requirements. If it is adjusted inside the machine, it should be placed on the printed board for easy adjustment; if it is adjusted outside the machine, its position should be
Match the position of the adjustment knob on the chassis panel.
1.2 Layout of common components
1 Arrange the position of each circuit unit according to the flow of the circuit, so that the layout facilitates the circulation of signals and tries to make signals
Ability to maintain a consistent direction.
2 Center around the core components of each functional circuit and arrange it around it. Components should be uniform, tidy and tight
Arranged on the PCB. Minimize and shorten leads and connections between devices.
3 Circuits operating at high frequencies, taking into account the distribution parameters between components. In general, the circuit should be as
The parallel arrangement of components not only achieves an aesthetic effect, but also facilitates welding and mass production.
4 The components located at the edge of the board are generally not less than 2mm from the edge of the board; the optimal shape of the board is rectangular.
The aspect ratio can be 3:2 or 4:3. When the board surface size is larger than 200mm × 150mm, the board should be considered
Mechanical strength. If you can't determine the required size of the PCB board at the beginning of the actual design process, you can design it slightly.
Slightly larger. After the PCB design work is finished, you can select Design→Board Shape→Redefine in Protel DXP.
Board Shape operation, appropriate cutting of the original PCB.
In addition, according to my actual work experience, if you want to expand or reduce certain functions of existing boards, you need to
To redesign a new PCB, you can manually arrange the components by referring to the layout on the motherboard during the actual layout.
In the proper position; in the wiring process, adjust according to actual needs to further improve the routing rate.

2 wiring
The wiring is after the layout, through the wiring diagram of the juice copper foil, according to the schematic diagram to connect all the traces. Obviously, the layout
The reasonable degree will directly affect the success rate of the wiring, so the layout is often required throughout the wiring process.
Appropriate adjustments. Wiring design can use double-layer routing and single-layer routing; for extremely complex designs, consider more
Layer wiring scheme.
In PCB design, routing is an important step in completing product design. It can be said that all the preparations in front are
Do it for it. PCB layouts are single-sided, double-sided, and multi-layer. There are two ways to route: automatic routing and
Interactive wiring.
In PCB design, designers often want to be able to use automatic routing. Under normal circumstances, for pure numbers
Word signal boards (especially when the signal level is relatively low and the board density is relatively small) are not problematic with automatic routing. but
Yes, in designing analog signals. When mixing signals or high-speed boards, if you use automatic routing completely, you may have problems.
It may even cause serious circuit performance problems.
At present, although there are already some automatic routing tools that are very powerful, they can usually achieve a 100% pass rate.
However, the overall appearance is not very beautiful, sometimes the wiring is disorderly, and the connection between the two pins is not the shortest (most
Excellent) path.
For designs with relatively complex circuits, try not to use automatic routing. It is recommended to use automatic cloth
Before the line, the lines that are more demanding are pre-wired in an interactive manner. Simultaneous input and output
The edges of the edges should avoid adjacent parallels to avoid reflection interference; the wiring of two adjacent layers should be perpendicular to each other, and the parallelism is easy to generate.
Raw coupling, this constraint can be added in the routing rules.
The routing rate of automatic routing depends on a good layout. The wiring rules should be preset, including the number of bends of the traces,
The number of via holes and the number of steps, and the like. Generally, explore the city line first, and quickly connect the short lines first; then proceed
Labyrinth wiring, first optimize the routing path of the line to be clothed, it can disconnect the laid line as needed, and
New wiring to improve overall performance.
In manual wiring, in order to ensure the correct implementation of the circuit, it is necessary to follow some general design rules: try to use the ground level
The surface acts as a current loop; separates the analog ground plane from the digital ground plane; if the ground plane is blocked by the signal line, then
The interference to the ground current loop should be such that the signal trace is perpendicular to the ground plane; the analog circuit is placed as close as possible to the edge of the circuit board.
Place the word circuit as close as possible to the power connection, so as to reduce the di/dt effect caused by the digital switch.
3 PCB circuit and circuit anti-interference measures
The anti-jamming design has a close relationship with the specific circuit and is also a very complicated technical problem. Combined here on the PCB
Do some simple introductions to the experience in the design process.
1 power cord design. Depending on the PCB current, try to increase the width of the power line (in the wiring design rules,
You can make new constraint rules for the line width of the power line and ground line separately, reduce the loop resistance, especially pay attention to the power line,
The grounding direction of the ground wire is opposite to the direction of data and signal transmission, which helps to enhance the anti-noise ability.
2 ground wire design. The ground wire is both a special power line and a signal line. In addition to following the design principles of the power cord,
Should also be done: digital ground and analog ground; if the circuit board has both logic and linear circuits, they should be made as far as possible
Separate; the ground of the low-frequency circuit should be grounded in parallel at a single point. When the actual wiring is difficult, it can be partially connected and then connected in parallel.
The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and thick, and the high-frequency components should be applied as large as possible in a grid shape.
Copper; try to widen the power and ground line width, preferably the ground line is wider than the power line, their width relationship is, the ground line >
Power line > Signal line.
3 The grounding of the digital circuit system constitutes a closed-loop road, which constitutes a ground network, which can improve the anti-noise capability.
4 Digital current should not flow through the analog device, high-speed current should not flow through the low-speed device.
5 Add a decoupling capacitor between the power ground to improve the anti-interference ability of the power circuit.
4 specific examples
The following example is a PCB design based on ARM, autonomous mobile embedded system core board.
4.1 Core board schematic
In the core board schematic, the core component is the S3C44BOX. This component is a high performance 16/32 bit RISC micro
Processor, 66MHz main frequency, integrated LCD controller, suitable for handheld devices, in 160-QFP package. in
In this system, the power supply circuit needs to use 5V and 3.3V DC regulated power supply, and the S3C44BOX uses 2.5V power supply.
Peripheral devices require a 3.3V supply, and other devices require a 5V supply; crystal oscillator circuits are used to provide CPU and other circuits.
Working clock, S3C44BOX uses commonly used passive crystal oscillator; U6, U7 and U9 constitute the storage system of this system.
4.2 Four-layer circuit board wiring rules
The software tool used in this PCB design is Protel DXP. The core board is a four-layer circuit board with the top and bottom layers
The signal layer, the middle layer is the power layer and the ground layer. The top and bottom PCB diagrams are shown in Figures 2 and 3.
2 The original PCB size of the motherboard is 55mm×70mm, but in the actual design process, if the PCB size is also designed as
This value will encounter practical difficulties in the wiring process, so the board size is designed to be slightly larger.
After the wiring is successful, select Design→Board Shape→Redefine Roard Shape to trim the PCB.
3 When generating the PCB drawing from the schematic diagram, some components with strict requirements (U5, U6,
U7 and U9) are placed in the appropriate position and locked; the crystal is placed near U5. Put other yuan in
Pay attention to the distance between components when the device is too close. If the distance between the two components is too close, it will cause interference and be green.
Show.
4 In the system, the on-chip operating frequency of the S3C44BOX is 66MHz. Therefore, during the design process of the printed circuit board,
The basic principles of design of some high-frequency circuits should be followed, otherwise the system will work unstable or not work properly.
5 For the current high-density PCB design, it has been felt that the through-hole is not well adapted, and a lot of valuable wiring is wasted.
aisle. In order to solve this contradiction, blind hole and buried hole technology has emerged, which not only realizes the function of the via hole, but also saves
Many routing channels make the wiring process more convenient, smooth, and more complete. In most tutorials, it is also advocated in
Blind and buried via technology is used in the design of multilayer boards. This can make wiring work easier, but at the same time
It also increases the cost of PCB design. Therefore, whether to choose this technology depends on the actual circuit complexity and economic ability.
Decide. I did not use this technology in the process of designing four-layer boards. If I think that there are too many through-holes, I can
The upper limit of the punching is limited in the wiring rules before wiring.
6 Before wiring, the horizontal layer is set in the top layer in the wiring rule, and the vertical layer is used in the bottom layer.
This allows the top and bottom wiring to be perpendicular to each other to avoid parasitic coupling;
Do not use right angles or sharp angles as much as possible because they affect electrical performance in high frequency circuits.
The 7PCB design uses interactive routing. First, manually connect the pins between components J1, J2 and U5.
Pre-wiring, in the input/output signals of the microprocessor, a considerable part is of the same type, such as data lines, address lines
And signal lines. For these same types of signal lines should be grouped, parallel distributed, and pay attention to the difference between the lengths of them
Too large, which can reduce interference, enhance system stability, and make wiring simple, printed circuit board
The appearance is more neat and beautiful. Then use automatic routing for the remaining components (it can try different routing strategies)
After the line is successful, some unsatisfactory wiring can be modified and optimized.
8 After the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and the electricity needs to be confirmed.
Recognize whether the rules are in line with the requirements of the printed board production process; then fill the tears on the pads to make the pads not easy to peel,
The trace and the pad are not easily disconnected; finally, a large area of copper is applied to the printed circuit board, which is advantageous for heat dissipation and shielding, and reduces
interference. The volatility occurs when the adhesive between the substrate of the printed circuit board and the copper foil is dipped or heated for a long time.
The gas cannot be removed, and the heat is not easily dissipated, so that the copper foil expands and falls off. Therefore, when it is applied in a large area, it should be
Its open window is designed as a mesh.