Tom
10-23-2002, 10:05 AM
IPC Advanced Study Guide Page Reference: Page 239, Section 4.9
To maintain coupling of 10% or under, with conductors referenced to a single plane (Microstrip- outer layer conductors), keep spacing between conductors at least twice the distance to the nearest power or ground plane. If conductor to plane distance is 0.18 mm [0.007”], the conductor to adjacent conductor spacing must be at least 0.36 mm [0.014”]
The two easiest ways to reduce a backward crosstalk problem are to minimize parallelism between the source and receptor (victim) or increase spacing between the conductors. Though it may be difficult to route an entire board with maximum spacing or minimum parallelism, it’s generally not difficult to re-route a handful of signals to accomplish the desired goals.
Percentage of crosstalk coupling is proportional with length of parallelism, up to critical length. If coupling must be reduced by 50%, simply force parallelism to be ½ critical length or less. Conductor spacing can also be increase by 50%. Doubling conductor spacing reduces crosstalk by a factor of four, hence increasing spacing by 50% (i.e.- from 0.2 mm to 0.3 mm) will reduce coupling by 50%.
Guard conductors are not the answer to control crosstalk, unless you have a via to ground every 1/20th wavelength of the maximum harmonic frequency of the rising or falling edge of the active signal. Even with multiple vias, the majority of benefit occurs because of the wide separation between the source and receptor conductors, not because of the existence of the guard conductor. In most digital circuits, spacing the source and receptor as far apart as they would be spaced with a guard conductor will yield as much benefit as having the guard conductor.
Microstrip
To maintain coupling of 10% or under, with conductors referenced to a single plane (Microstrip- outer layer conductors), keep spacing between conductors at least twice the distance to the nearest power or ground plane. If conductor to plane distance is 0.18 mm [0.007”], the conductor to adjacent conductor spacing must be at least 0.36 mm [0.014”]
The two easiest ways to reduce a backward crosstalk problem are to minimize parallelism between the source and receptor (victim) or increase spacing between the conductors. Though it may be difficult to route an entire board with maximum spacing or minimum parallelism, it’s generally not difficult to re-route a handful of signals to accomplish the desired goals.
Percentage of crosstalk coupling is proportional with length of parallelism, up to critical length. If coupling must be reduced by 50%, simply force parallelism to be ½ critical length or less. Conductor spacing can also be increase by 50%. Doubling conductor spacing reduces crosstalk by a factor of four, hence increasing spacing by 50% (i.e.- from 0.2 mm to 0.3 mm) will reduce coupling by 50%.
Guard conductors are not the answer to control crosstalk, unless you have a via to ground every 1/20th wavelength of the maximum harmonic frequency of the rising or falling edge of the active signal. Even with multiple vias, the majority of benefit occurs because of the wide separation between the source and receptor conductors, not because of the existence of the guard conductor. In most digital circuits, spacing the source and receptor as far apart as they would be spaced with a guard conductor will yield as much benefit as having the guard conductor.
Microstrip