Techniques in high speed (>100MHz) high density PCB design

- Aug 18, 2018-

In the case of fixed circuit board size, if more functions need to be accommodated in the design, the routing density of PCB will often need to be improved, but this may lead to enhanced mutual interference of the routing, and at the same time, excessive routing also makes the impedance unable to be reduced.When designing high density PCB, CAM OEM needs to pay attention to crosstalkinterference, because it has great influence on timing and signalintegrity.Here are a few caveats:

1. Control the continuity and matching of routing characteristic impedance.

2. The spacing between the lines.It is commonly seen that the spacing is twice the width of the line.Simulation can be used to understand the effect of line-spacing on timing and signal integrity and find the minimum tolerable spacing.Different chip signals may have different results.Select the appropriate connection mode.Avoid that the two adjacent layers have the same direction of travel, and even some travel lines overlap up and down, because this kind of crosstalk is bigger than that of adjacent layers.

3. Use blind/buriedvia to increase the line area.But PCB production costs will increase.It's really hard to achieve full parallelism and equal-length in practice, but try to do it anyway.

4. Difference and common mode terminals can be reserved to mitigate the impact on timing sequence and signal integrity.