3. Layout for Double & Multilayer PCB
A. After the Schematic Diagram was completed, the whole board should be conducted for layout, which is the most time consuming and important part of multilayer PCB design.
B. First of all, it must be mentioned that some antennas and differential pairs need impedance matching. Therefore, before the layout, it should be marked out and carried out firstly.
Impedance matching sometimes needs to dig copper. at the same time, it is necessary to avoid layout wire under digging copper, so as to avoid cross-talk of different signals.
C. Secondly, the power supplement parts. Because it is a multi-layer board, we are used to drawing a plane as the power supply plane, which can ensure the continuity of the power supplement and the ability to carry current.
D. Generally, the power supplement is divided from the main power supplement through rectification and filtering into multiple power supplements for different chips to work.
Therefore, when layout the distribution of the power cord, firstly, you can layout the main power supplement into the board, and then spread out from the center to the outside parts, so as to ensure that each power supplement has sufficient power source.
E. The power plane is divided into some parts according to the demand, and the chip is powered by the power plane. At the same time, we need to complete the DC filter design of all chip pins. Many chip power supply pins need a DC filter, which should be considered in the design of a powder supplement.
G. After the completion of such a layout of a power supplement, The power will be provided for all chips after the power part is connected. The power plane also needs to be DC filtered at a certain distance between the forked intersection and the main road to ensure the purity of the power plane.
H. After the power supplement layout is finished, the differential pair shall be placed away from other signal lines to avoid coupling. At the same time, a certain position should be reserved for the equal length between pairs and within pairs. Among them, the equal length between pairs can be adjusted according to the chip requirements.
The equal length within the team must be strictly controlled, and the equal length gap should be less than 5 MIL. No matter the equal length in the pairs or between pairs, try to be in the places where the length does not match, such as corners, exits, etc., so as to ensure that most of the line routes are in the equal length state.
I. At the end of the above layout, the local circuit is started to go through. It is suggested that in this part, one circuit module should go through all before the other parts. When wire layout, it needs to be all through, especially the grounding pin, according to the principle that one pin corresponds to one grounding hole.
If it needs to be adjusted later, it needs to be adjusted again to avoid that the board is too dense to be grounded or the grounding needs to be long-distance wiring. The through-hole placement needs to be placed between the pads to avoid oblique angle wiring. The through-hole can not be punched on the pad, which will affect the through-hole successful rate.
J. During the layout, attention should be paid to an analog signal and a clock signal, which should be at least 3W Rule away from other signals to avoid signal interference caused by long-distance equal length routing with other signals. At the same time, if these important signals need to go to the inner layer, it is necessary to find the nearest inner layer.
In order to ensure that the distance between lines is large enough, when the distance between lines and the center of lines is not less than 3 times of the line width, if the distance between lines is not less than 3 times of the line width, 70% of the electric fields between lines can be kept without mutual interference, which is called 3W rule. To achieve a 98% electric field without mutual interference, a 10 W rule can be used.
The 3W principle is also related to the physical factors of the circuit board. Considering the physical significance of cross-talk, cross-talk should be prevented effectively. The distance is related to stack height and line width.
3W is enough for the height distance between the line and the reference plane (5-10 mils) of the four-layer plate.
However, for the two-layer board, the distance between the routing and the height of the reference layer (45-55 mils), 3W routing may not be enough for high-speed signals.
The 3W principle is generally established under the condition of 50 ohm characteristic impedance transmission line