When the fab shop wants to make an impedance adjustment to differential pairs, they are most likely to change the width of
the lines which also changes the gap. They are working with what you've given them in terms of the pitch of the two traces.
Their line width updates are usually pretty small but the air gap changes along with the line width. Their options are limited
but they always seem to figure it out in the DFM cycle.
Figure 3. The second tab down on the Spacing worksheet is labeled "Net/All Layers". This is where the rules created in the first spacing tab are
applied to groups, pairs and individual nets. The spacing values presented in the Nets tab impact all layers rather than the layer-by-layer
approach in the Spacing Constraint Set. If the numbers are replaced by the three asterisk trio, it means that different values are in effect for
the various layers while a number there means that all spacing values are the same for that item.
Can You Give Me Some Space?
Meanwhile, loosely coupled pairs present a challenge of their own. The ideal geometry can be difficult to implement on fine
pitch BGA devices. There may be a transition area where you work your way out of a congested pin field. Being the opposite of
tightly coupled, the trace that takes the outside of a bend goes considerably further than the one with the inside route.
The disparity works against what it means to be differential and suggests that we do a phase tune for every bend that isn't
immediately countered by a bend in the other direction. We could ignore a short S-turn but would compensate for an isolated
90 degree bend when using loosely coupled differential pairs. The goal is to have two signals making their way down the
conductors at the exact same instant. Any transient conditions along the way should affect both signals so that it all cancels
out. It sounds simple but things are happening really fast.
… the higher of the two (spacing) values apply when traces with different spacing rules meet.
4 www.cadence.com
Controlling Air Gaps Using OrCAD X and Allegro X Tools
