Stub
This stub rule checks unnecessary stubs for specific use cases.
Stubs may not affect the signal path among pins, but they could act as antenna and cause EMI problems. If a stub is longer than a certain length, it becomes a good antenna, so you should avoid long stubs in designs. Some types of stubs have no purpose at all from a design point of view. However, a certain type of stub has an intended purpose. One of such examples is a test point. If a pattern is branched to make a test point, this structure can act as an antenna. Another is an optional component. You can route to a resistor point, but if a manufacturing engineer does not use resistor, the rest of pattern can act as an antenna. This stub rule checks unnecessary stubs for those three cases mentioned above.
- Item: Input item name.
- Net: Define net group to be checked for the existence of stub.
- Stub Length: Length criteria to be recognized as a stub/branch.
- Stub Component: Define component group to be treated as stub/branch component such as test points and resistors.
- PU/PD Net: Define Power/Ground signals connected to pull-up/pull-down resistors. The DFE considers the resistor that is connected to power/ground net as a pull-up/pull-down resistor.
- Start Component: Define start component of net topology. The DFE considers the path which is connected to the start component as a main route.
- Passive Component: Define passive components. The DFE ignores trace which is connected to the passive component directly.
- Composite Power Net: DFE uses composited power net instead of single power
net. Double click the item field. The Composite Power
Net dialog displays.
- Passive Comp: The DFE makes composite net which are connected through this passive component.
- Exception Net: Nets that should not be merged into the composite net.
- Some types of stubs have no purpose at all in design point of view.
- However, a certain type of stub has an intended purpose. One of such examples is a test point. If a pattern is branched to make a test point, this structure act as an antenna.
- Another is an optional component. Board designer may route to a resistor
point, but if a manufacturing engineer does not use the resistor, the rest
of the pattern can be act as an antenna.
Figure 1. 
Trace stubs should be minimized if absolutely required or eliminated if not.
A stub encounters an impedance mismatch at the high-impedance input of the receiver, causing reflections.
The loss of energy resulting from a reflection causes attenuation of the signal as it proceeds to its destination. The reflection itself can result in ringing and jitter as it collides with other signals being generated by the source.
