1. Layout
First, consider the size of the PCB. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if it is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. After determining the PCB size, determine the position of special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
The following principles should be followed when determining the location of special components:
(1) Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference cannot be too close to each other, and input and output components should be kept as far away as possible.
(2) There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be arranged in places that are not easily accessible by hand during debugging.
(3) The position occupied by the printing pulley positioning hole and the fixing bracket should be reserved.
According to the functional unit of the circuit, when laying out all the components of the circuit, the following principles must be complied with:
(1) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal circulation, and the direction of the signal is kept as consistent as possible.
(2) Take the core component of each functional circuit as the center and make layout around it. Components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten the leads and connections between components.
(3) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the circuit should arrange the components in parallel as much as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.
(4) Components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape for a circuit board is a rectangle. The aspect ratio is 3:2 into 4::3. When the size of the circuit board is larger than 200x150mm, the mechanical strength of the circuit board should be considered.
2. Wiring
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should avoid being adjacent to each other in parallel as much as possible. It is best to add a ground wire between the lines to avoid feedback coupling.
(2) The minimum width of the printed photoconductive wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them.
(3) The corners of printed wires are generally arc-shaped, while right angles or included angles will affect electrical performance in high-frequency circuits. In addition, try to avoid using a large area of copper foil, otherwise, when heated for a long time, it is easy to cause copper foil to expand and fall off. When a large area of copper foil must be used, it is best to use a grid. This is beneficial to eliminate the volatile gas generated by the adhesive between the copper foil and the substrate when heated.
3. Pad
The central hole of the pad (in-line device) is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a virtual solder joint. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad may be (d+1.0)mm.
PCB and circuit anti-interference measures:
The anti-jamming design of printed circuit boards is closely related to specific circuits. Here we will only explain some common measures for PCB anti-jamming design.
1. Power cord design
According to the size of the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the ability to resist noise.
2. Lot design
The principles of ground wire design are:
(1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be connected in parallel with a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then connected in parallel. The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and short, and the high-frequency components should be surrounded by grid-shaped and large-area ground foil. .
2) The ground wire should be as thick as possible. If the grounding wire is made of a thin wire, the grounding potential will change with the change of the current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2~3mm.
(3) The ground wire forms a closed loop. For printed boards composed only of digital circuits, the grounding circuit is usually arranged in a group loop to improve the anti-noise ability.
4. Decoupling capacitor configuration
One of the conventional practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board. The general configuration principle of the decoupling capacitor is:
(1) Connect the electrolytic capacitor of 10~100uf across the input terminal of the power supply. If possible, it is better to connect more than 100uF.
(2) In principle, each integrated circuit chip should be equipped with a 0.01uf~0.1uf ceramic capacitor. If there is not enough space in the printed board, a 1~10pF capacitor can be arranged every 4~8 chips.
(3) For devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.
5. Via design
In high-speed PCB design, seemingly simple vias often bring great negative effects to circuit design. In order to reduce the adverse effects of parasitic effects of vias, we can try our best to
(1) Considering both cost and signal quality, choose a reasonable size via hole size. For example, for 6-10 layers of memory module PCB design, it is better to use 10/20mil (drilling/pad) vias. For some high-density and small-sized boards, you can also try to use 8/18Mil vias. hole. Under the current technical conditions, it is difficult to use smaller-sized via holes (when the depth of the hole exceeds 6 times the diameter of the drilled hole, it is impossible to ensure that the hole wall can be evenly plated with copper); Consider using a larger size to reduce impedance.
(2) Try not to change the layer of the signal trace on the PCB, that is, try not to use unnecessary vias.
(3) The pins of the power supply and ground should be drilled through holes nearby, and the shorter the lead between the via hole and the pin, the better
(4) Place some grounded vias near the vias where the signal changes layers to provide the closest loop for the signal. It is even possible to place a large number of redundant ground vias on the PCB.
6. Some experience in reducing noise and electromagnetic interference
(1) If you can use low-speed chips, you don’t need high-speed ones. High-speed chips are used in key places
(2) A resistor can be connected in series to reduce the jump rate of the upper and lower edges of the control circuit.
(3) Try to provide some form of damping for relays, such as RC setting current damping
(4) Use the lowest frequency clock that meets the system requirements.
(5) The clock should be as close as possible to the device that uses the clock. The shell of the quartz crystal oscillator should be grounded, and the clock area should be surrounded by a ground wire. The clock, bus, and chip select signals should be kept away from I/O lines and connectors, and the interference of the clock line perpendicular to the I/O line is smaller than that parallel to the I/O line.
(6) Do not suspend the input terminal of the unused gate circuit, the positive input terminal of the unused operational amplifier is grounded, and the negative input terminal is connected to the output terminal
