1. in doing PCB design, in order to meet a group of all the signal line to meet the total length of a tolerance range, usually use a snake to the total length of the shorter signal line around the group with the longest signal line length tolerance range, the snake around the long signal line process, is commonly known as PCB signal isometric processing. The goal of equal length is to meet the timing matching requirements of the same group of signals.
2. Isometric range should strictly comply with the requirements of different interfaces or signals, specific reference module specification content. If there is any doubt, confirm with the customer in time.
3. Before dealing with the length of the longest signal line in the same group should be found, the first step to optimise the length of the longest signal.
4. Isometric processing should be completed after the entire signal line path to check, check whether it avoids interference sources, check whether it is qualified.
5. Isometric processing, the pitch is preferred 4W, such as space constraints, can be adjusted to 3W; isometric height control in 40-150mil is appropriate, can not be too high; corner length is not less than 1.5 times the width of the line, the general control of the size of 6-10mil, such as the line width of 4mil, the size of the corner to control the size of the corner of the 6 mil, the corner can not be too small, as shown in Figure 1.
Figure 1 snake alignment requirements
6. Equal length should try to deal with in the horizontal or vertical direction, to avoid dealing with the diagonal; processing can not be too scattered, should try to focus on processing, to ensure its aesthetic, in principle, is to deal with a signal, the second first repair line next to its side, and then on the second for the longest processing to ensure that the utilisation of space and the overall design of the aesthetics. Within the same design, the height of equal length should be controlled almost the same, do not differ too much, such as Figure 2.
Figure 2 PCB snake bite isometrics
7. after dealing with isometric, should check its reference layer, do not allow its cross-segmentation. PCB clock signals (generally end in clk) and other important signals to avoid cross-segmentation, cross-segmentation will lead to sudden changes in signal impedance.
8. When the reference plane of the alignment across the power supply layer, it is recommended that the two power supply layer were added to the ground capacitance to provide a complete return path, as shown in Figure 3.
Figure 3 alignment qua split processing
9. Differential signals should be equal to the position of equal length within the uncoupled generated near the position of equal length, equal height and width of the internal reference to the following Figure 4 requirements.
