The four-layer circuit board can be divided into a top layer, a bottom layer and two intermediate layers. Signal lines on the top and bottom floors, The middle LAYER first uses the command DESIGN/LAYER STACK MANAGER to ADD INTERNAL PLANE1 and INTERNAL PLANE2 as the most used power layer such as VCC and the ground such as GND (that is, connect the corresponding network label) with the command ADD PLANE. Be careful not to use ADD LAYER, this will increase the MIDPLAYER, the latter is mainly used as a multi-layer signal line placement), so PLNNE1 and PLANE2 are two layers of copper connecting power VCC and ground GND.
Four-layer Board refers to the line Printed board PCB Printed Circuit Board is made of 4 layers of glass fiber, usually SDRAM will use 4 layers of board, although it will increase the cost of PCB but can avoid noise interference.
General principles of Multi-layer PCB board layout PCB designers need to follow the following general principles during the circuit board layout process:
1. The setting principle of the spacing of the printed wiring of components. The spacing constraint between different networks is the setting principle of the spacing of printed wiring of components by electrical insulation, manufacturing process and components. Size and other factors determine. For example, the pin spacing of a chip component is 8mil, the ClearanceConstraint of the chip cannot be set to 10mil, and the PCB designer needs to set a 6mil PCB design rule for the chip separately. At the same time, the setting of spacing should also take into account the production capacity of the manufacturer.
In addition, an important factor affecting the components is electrical insulation, if the potential difference between two components or networks is large, you need to consider the problem of electrical insulation. The gap safety voltage in the general environment is 200V/mm, that is, 5.08V/mil. Therefore, when there are both high-voltage and low-voltage circuits on the same circuit board, it is necessary to pay special attention to sufficient safety spacing. When there are high-voltage circuits and low-voltage circuits, special attention needs to be paid to adequate safety spacing.
2. line corner routing form selection. In order to make the circuit board easy to manufacture and beautiful, it is necessary to set the corner mode of the line and choose the corner routing form of the line when the PCB is designed. You can choose 45, 90 and arc. Generally do not use sharp corners, it is best to use arc transition or 45 transition, avoid using 90 or more sharp corner transition.
The connection between the wire and the pad should also be as smooth as possible to avoid small pointed feet, which can be solved by using the method of filling tears. When the center distance between pads is less than the outer diameter D of a pad, the width of the wire can be the same as the diameter of the pad; If the center distance between pads is greater than D, the width of the wire should not be greater than the diameter of the pads. When the wire passes between two pads and is not connected with them, it should maintain a maximum and equal spacing with them, and when the wire and the wire of the wire pass between two pads and is not connected with them, it should maintain a maximum and equal spacing with them, and the spacing between them should also be evenly equal and maintain a maximum. The spacing between them should also be evenly equal and maintained at maximum.
3. the method of determining the width of the printed line. The width of the wire is determined by the current level of the conductor and anti-interference and other factors. The larger the current, the wider the wire should be. The power cable should be wider than the signal cable. In order to ensure the stability of the ground potential (the greater the transformer by the ground current, the wider the line should be. Generally, the power line should be wider than the signal line, the power line should be wider than the signal line, the ground line should also be wider. Experiments show that: when the thickness of the copper film of the printed wire is 0.05mm, the current-carrying ground wire of the printed wire should also be wide and can be calculated according to 20A/mm2, that is, a wire of 0.05mm thickness and 1mm width can flow 1A current. So for the general for the general width can meet the requirements; High voltage High voltage, signal line for 10 ~ 30mil width can meet the requirements of high voltage, large current signal line width is equal to 40mil, line to line spacing is greater than 30mil. In order to ensure the peeling strength and reliability of the wire, the widest possible wire should be used to reduce the line impedance and improve the anti-interference performance within the range allowed by the plate area and density.
For the width of the power cord and ground wire, in order to ensure the stability of the waveform, in the case of the circuit board wiring space, as far as possible bold, generally at least 50mil is required.
4. printed wire anti-interference and electromagnetic shielding. The interference on the wire mainly includes the interference introduced between the wires, the interference introduced by the power line) the interference and electromagnetic shielding of the printed wire. The interference on the wire mainly includes the interference introduced between the wires, the crosstalk between the signal lines, etc., and the crosstalk between the signal lines, etc., reasonable arrangement and layout of the wiring and grounding mode can effectively reduce the interference source, so that the PCB design circuit board has better electromagnetic compatibility performance.
For high frequency or some other important signal lines, such as clock signal lines, on the one hand, the line should be as wide as possible, for high frequency or some other important signal lines, such as clock signal lines, on the one hand, the line should be as wide as possible, and on the other hand, the signal line can be wrapped with a closed ground line. Wrap up is equivalent to adding a pack of ground to isolate it from the surrounding signal line is to use a closed ground wire to "wrap up the signal line, grounding shield layer). Layer grounding shield).
For analog and digital to separate wiring, can not be mixed. For analog and digital to separate wiring, can not be mixed. If you need to finally unify the analog and digital ground into a potential, you should usually use a one-point grounding method, that is, only select a point to connect the analog and digital ground to prevent the formation of a ground wire loop, resulting in ground potential deviation.
After the wiring is completed, a large area of ground copper film, also known as copper coating, should be applied to the top and bottom places without laying wires to effectively reduce the wiring, a large area of ground copper film, also known as copper coating, should be applied to the top and bottom places without laying wires to effectively reduce the ground impedance, thus weakening the high frequency signal in the ground wire. At the same time, a large area of ground can inhibit electromagnetic interference. Small ground impedance, thereby weakening the high frequency signal in the ground, while a large area of ground can inhibit electromagnetic interference. A large area of ground can inhibit electromagnetic interference of parasitic capacitors, especially harmful to high-speed circuits; At the same time, one of the excessive holes in the circuit board can introduce about 10pF of parasitic capacitance, which is especially harmful for high-speed circuits and can reduce the mechanical strength of the board. Therefore, when wiring, the number of holes should be reduced as much as possible. In addition, when using penetrant holes in wiring, the number of holes should be reduced as much as possible when wiring (through holes), usually using pads instead. This is because when the circuit board is made, it is possible to cause some penetrating holes (through holes) to be pierced due to processing reasons, and the pad can certainly be pierced during processing, which is equivalent to bringing convenience to the production.
The above is the general principles of PCB layout and wiring, but in actual operation, the layout and wiring of components is still a very flexible work, the layout and wiring of components are not unique, and the results of layout and wiring largely depend on the experience and ideas of PCB designers. It can be said that there is no standard that can judge the right and wrong of the layout and wiring scheme, and can only compare the relative good and bad. Therefore, the above layout and wiring principles are only used as a PCB design reference, and practice is the only standard to judge the advantages and disadvantages.
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