With the development of electronic technology, large-scale and high-precision PCB boards have been widely used, and the mounting density of components on PCB boards is getting higher and higher. Simple single-sided and double-sided wiring can no longer meet the requirements of high-performance circuits, so multilayer PCB boards are needed for wiring.
High-frequency circuits tend to have high integration and high wiring density, so using multilayer boards is not only necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, a reasonable selection of the circuit board size of a certain number of layers can make full use of the middle layer to set the shield, better realize the grounding nearby, effectively reduce the parasitic inductance and shorten the transmission length of the signal, and at the same time, greatly reduce the cross-interference of the signal, etc. All these methods are beneficial to the reliability of the high-frequency circuit.
The skills of wiring rules for multilayer PCB are as follows:
1, more than 3 o’clock connection, try to let the line through each point in turn, easy to test, the line length as short as possible.
2. The lines between different layers should not be parallel as far as possible, so as not to form actual capacitance.
3. Try not to pay off wires between pins, especially between and around IC pins.
4. Wiring should be straight line or 45-degree broken line as far as possible to avoid electromagnetic radiation.
5. Try to be as neat as possible between lines, try to connect the polysemy lines on the floor, and increase the grounding area.
6. Pay attention to uniform discharge of components, so as to facilitate installation, plug-in and welding operation. The characters are placed on the current character layer, and the position is reasonable. Pay attention to the orientation, avoid being blocked, and facilitate production.
7. The structure should be taken into account in the discharge of components. The positive and negative poles of SMD components should be marked in the package and at the end to avoid space conflict.
8. The functional block components should be put together as much as possible, and the components near LCD such as zebra stripes should not be too close together.
9. After the wiring is completed, carefully check whether each line is really connected (lighting method can be used).
The less lead bending between pins of high-speed electronic devices, the better.
It’s best to use all straight wires for high-frequency circuit wiring, which need to be turned. It can be turned with 45-degree broken lines or circular arcs. This requirement is only used to improve the fixation strength of copper foil in low-frequency circuits, but in high-frequency circuits, meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals.
When the wiring space allows, inserting a ground wire or ground plane between two lines with serious crosstalk can play a role in isolation and reduce crosstalk.
When there is a time-varying electromagnetic field in the space around the signal line itself, if parallel distribution cannot be avoided, a large area of “ground” can be arranged on the opposite side of the parallel signal line to greatly reduce the interference.
On the premise that the wiring space permits, increase the distance between adjacent signal lines, reduce the parallel length of signal lines, and try to make the clock line perpendicular to the key signal lines instead of parallel.
HDMI wiring rules. It is required that HDMI signals be routed differentially, with a line width of 10mil and a line spacing of 6mil, and the spacing between two groups of HDMI differential signal pairs is more than 20mil.
LVDS wiring rules. LVDS signal differential wiring is required, with a line width of 7mil and a line pitch of 6mil, in order to control the differential signal pair impedance of HDMI to 100+-15% ohm.
DDR wiring rules. DDR1 wiring requires that signals should not go through holes as far as possible, with signal lines of equal width and equidistant lines. Wiring must meet the 2W principle to reduce the crosstalk between signals. For DDR2 and above high-speed devices, high-frequency data wiring should be of equal length to ensure the impedance matching of signals.
