Differential pair nets are routing patterns with a certain parallel clearance apart
on which reversed signals flow at the same time to send information. Differential
pairs must have the same environment to keep a stable signal and coupling status in
terms of trace length, trace width and routing structure including vias. Otherwise,
common mode noise might result which might then cause signal timing and EMI
problems. This item checks for uniformity of the differential pairs design
environments. Differential trace length, width and via count rules are checked in
this section. Because you are keeping the trace lengths exactly the same, comparison
tolerance (%) can be entered for the rule check. Maintaining the same trace width is
easier to achieve and usually its difference must be kept within +/-
5%.
Item: Input item name.
Net: Select Differential Pair Net Group.
Start Component: Upon selecting this option, a test will be performed
between the start component and the end component.
End Component: Upon selecting this option, a test will be performed between
the start component and the end component.
Filter: Enter a filter to choose differential pair net from the selected net
group. Enter Base Net and Pair Net and click Add Filter. The Differential
Pair Filter is added.
Tolerance: Define tolerance measure base.
Value: Tolerance value to measure an allowable length skew between nets in a
differential pair.
Via Existence: Via count check criteria. (Not Check/Min/Max)
Via Qty: Specify the minimum or maximum numbers of vias.
Report display highlights all differential pair nets. It is reported
as failed if a length difference between the two nets in a pair is
bigger than the tolerance.
It is reported as failed if the number of vias does not match with
the specified via count.
Buried Via Merge: Enter a reference value to judge multiple Blind Via as
one.
Width: Upon selecting check, it fails if there is a region in which two nets
have different widths.
Pin Escape: Enter a radius of circular region around pins to be excluded for
the rule check.
Max Total Length: Check maximum trace length.
Include Via Length: DFE includes via length for calculating trace
length.
Include Package Length: DFE includes package length for calculating trace
length.
As described above, Differential Pair Net must keep their distance and trace width
uniform. The trace thickness difference should be kept within ±5%. Figure 1.
It is better to design a whole differential pair net on the same layer and minimize
via usage. Figure 2.
Differential pair nets usually keep minimum 80% of total net length parallel. Figure 3.
