How to draw a double-layer PCB
Double-layer PCB circuit board, means to draw wires on the top and bottom layers of a PCB board. The double-sided board solves the difficulty of interleaved wiring in the single-sided board, that is, the front and back sides have wiring, the components can be soldered on the front side, or can be soldered on the back side, and the double-sided circuit board has components and wiring on both sides There is no doubt that the difficulty of designing double-layer PCB boards is higher than that of more boards.
Let's take a look at the wiring principle of double-layer PCB circuit board.
So how do you draw a double-layer PCB board?
The double-layer PCB board needs to use the wires on both sides, and there must be a proper circuit connection between the two sides. This "bridge" between circuits is called a via. The pilot hole is a small hole filled or coated with metal on the PCB, which can be connected to the wires on both sides. When using PROTEL to draw a double-sided pcb board, draw the wire connection components on the TopLayer (top layer), which is the top board; select BottomLayer (bottom layer), draw the wire connection components on the bottom layer, which is the board on the bottom layer. The above is the basis for drawing a double-layer PCB. Before drawing the double-layer PCB board, we must first determine the layout of the components, and when wiring, first distribute the key crystal, crystal oscillator circuit, clock circuit, CPU and other signal lines, and must observe the principle of the smallest circulating area.
After the reasonable layout of the components of the double-layer board is determined, the power cable of the ground net copy board is designed first, and then the important lines-sensitive lines, high-frequency lines, and then the general lines-low-frequency lines. It is best to have a separate power supply for the key lead, a ground return, and the lead is very short, so sometimes a ground wire is placed close to the signal line on the edge of the key lead to make it form the smallest working loop.
When drawing a double-layer PCB, follow the layout principle of "big first, then small, hard first, and easy", that is, important unit circuits and core components should be laid out first, and the layout should refer to the principle block diagram. According to the main board The signal flow direction arranges the main components regularly. And the total connection is as short as possible, the shortest key signal line; high voltage, high current signal and small current, low voltage weak signal is completely separated; analog signal is separated from digital signal; high frequency signal is separated from low frequency signal; high frequency components The interval should be sufficient.
It should also be noted that these two double-layer boards have a ground plane on the lower layer of the circuit board. This design is to allow engineers to quickly see the wiring when doing troubleshooting. This method often appears on the demonstration and evaluation boards of device manufacturers. But a more typical approach is to lay a ground plane on the upper layer of the circuit board to reduce electromagnetic interference (EMI).
Steps to draw double-layer PCB board
1. Prepare the circuit schematic
2. Create a new PCB file and load it into the component package library
3. Planning the circuit board
4. Load the netlist and components
5. Automatic layout of components
6. Layout adjustment
7. Network density analysis
8. Wiring rule setting
9. Automatic wiring
10. Manually adjust the wiring
After the components of the double-layer board are drawn in the EDA software, the wiring sequence is first designed as power line, sensitive line, high frequency line, and low frequency line. It is best to have a separate power supply for the key lead, a ground return, and the lead is very short, so sometimes a ground wire is placed close to the signal line on the edge of the key lead to make it form the smallest working loop.
The signal line of the double-layer board is the key line of the key crystal, crystal oscillator circuit, clock circuit, CPU, etc., and the principle of the smallest circulating area must be observed. When the IC circuit of the PCB is working, the circulation area is mentioned many times, and its origin is actually the concept of differential mode radiation.
For example, the definition of differential mode radiation: the working current of the circuit flows in the signal loop. This signal loop will generate electromagnetic radiation. Because this current is differential mode, the radiation generated by the signal loop is called differential mode radiation. The field strength has a calculation formula: It can be drawn that the radiation field strength at the circuit space γ of the E1 --- differential mode copy board is visible from the differential mode radiation formula. Its radiation field strength and operating frequency f2, circulating area A. The working current I is directly proportional. For example, when the working frequency f is determined, the size of the circulating area is a key factor that can be directly controlled in our design. At the same time, as long as the circulating working speed and current meet the reliability, the larger the better, the signal jumps The narrower the edge along the lower jump, the greater its harmonic components, the wider, the higher the electromagnetic radiation, the greater the power, the greater the current, which is undesirable.
The following gives several kinds of logic circuits that can meet the reference value of the circulating area allowed by the radiation class B standard. It can be seen that the faster the circuit switching speed, the smaller the allowable area. The key connecting line can be surrounded by ground wire if possible. In addition, after the wiring of the PCB copy board is completed, all the gaps can be covered with ground wires, but it must be noted that these covered ground wires must be short-circuited with the low-impedance conjoint of the ground layer, which can achieve good results.
In general, we use automatic wiring for efficient work. In most cases, automatic wiring will not cause problems with pure digital circuits, especially low-frequency signals and low-density circuits. However, when you try to use the automatic routing tool for analog, mixed signal or high-speed circuit wiring, some problems may occur, and it may cause extremely serious circuit performance problems. There are many things to consider about wiring, the biggest problem is the grounding method.
If the ground path starts from the upper layer, the ground of each device is connected to the ground through the pull wire on the layer. For each device in the lower layer, the through hole on the right side of the circuit board is connected to the upper layer to form a ground loop. The immediate red flag you see when checking the wiring method indicates that there are multiple ground loops. In addition, the ground loop of the lower layer is horizontal. It can reduce the impact of digital switching δi / δt on analog circuits. It should be noted that these two double-layer boards have a ground plane on the lower layer of the circuit board. This design is to allow engineers to quickly see the wiring when doing troubleshooting. This method often appears on the demonstration and evaluation boards of device manufacturers. But a more typical approach is to lay a ground plane on the upper layer of the circuit board to reduce EMI.
(1) Components are best placed on one side. If you need to place components on both sides and place pin components on the bottom layer, it may cause the circuit board to be difficult to place and not conducive to soldering. Common component layout method on the computer graphics PCB board. When placed on one side, it is only necessary to make a screen printing layer on one side of the circuit board, which is convenient for reducing costs.
(2) Reasonably arrange the position and direction of interface components. Generally speaking, as the connector components of the circuit board and the outside world (power supply, signal line), it is usually placed on the edge of the circuit board, such as serial port and parallel port. Putting it in the center of the circuit board is not conducive to wiring, and it may not be connected due to the obstruction of other components. Also pay attention to the direction of the interface, so that the connection cable can be led out smoothly, away from the circuit board. After the interface is placed, the type of the interface should be clearly marked with the String of interface components; for the power interface, the voltage level should be marked to prevent the circuit board from being burned due to wiring errors.
(3) It is better to have a wide electrical isolation zone between the high-voltage components and the low-voltage components. Do not put components with very different voltage levels together, which is not only beneficial for electrical insulation, but also great for signal isolation and anti-interference.
(4) Components with close electrical connections are best placed together. This is the modular layout idea.
(5) For components that are prone to noise, such as high-frequency devices such as clock generators and crystal oscillators, they should be placed as close to the clock input of the CPU as possible during layout. High-current circuits and switching circuits are also susceptible to noise. These components or modules should also be kept away from high-speed signal circuits such as logic control circuits and storage circuits. If possible, use control boards in combination with power boards and use interfaces to connect to improve The overall anti-interference ability and working reliability of the circuit board.
(6) Place decoupling capacitors and filter capacitors around the power supply and the chip as much as possible. This is an important measure to improve the power quality of the circuit board and the anti-interference ability. In practical applications, the traces, pin connections and wiring of the printed circuit board may bring a large parasitic inductance, causing high-frequency ripples and glitches in the power supply waveform and signal waveform. Place a 0.1μF or larger capacitor between them to further improve power quality. For the power conversion chip or the power input terminal, it is best to arrange a 10μF decoupling capacitor to effectively filter out these high-frequency ripples and glitches. If a chip capacitor is used on the circuit board, the chip capacitor should be placed close to the power supply pin of the component.
(7) The serial number of the component should be arranged close to the frame of the component, the size is uniform, the direction is neat, and it does not overlap with the component, via and pad. The first pin of the component or connector indicates the direction; the positive and negative signs should be clearly marked on the PCB and are not allowed to be covered; power conversion components (such as DC / DC converters, linear conversion power supplies and switching power supplies) There should be enough heat dissipation space and installation space beside, and enough welding space should be left on the periphery.
(8) The double-layer board ground wire is designed as a grid frame, that is, there are many parallel ground wires on one side of the printed board, and the vertical ground wire on the other side, and then use metalized vias where they cross Connect them (via resistance should be small).
(9) In order to consider that there should be a ground wire near each IC chip, a ground wire is often placed every 1 ~ 115cm. Such a dense ground wire makes the area of the signal loop smaller, which is helpful to reduce radiation. The design method of the ground network should be before the signal line, otherwise it is more difficult to achieve.
(10) Factors that need to be considered: electromagnetic compatibility, impedance matching at the beginning and end, and clock synchronization.
(11) The high-speed line is best to go through the inner layer. The top and bottom layers are easily affected by external temperature, humidity, and air, and are not stable. If you need to test, you can lead through the test via. No more illusions of flying and cutting.