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Neck length/width is the same as truncation. What this means is trace that has x width for some distance but has to be smaller to route inside something like a bga. While routing within bga, trace has to neck down or truncate to a smaller width to allow routing without violating rules or causing a short. The neck length would be the distance a trace is necked down or truncated.
I am trying to measure a trace using the Cadence Allegro tool. When I click on the via - I get the Neck length/Neck Width/Line width info and when I click on the other end of the trace I get the total etch distance. Should the "neck" parameters be taken into consideration for calculating the trace length - if so, how?
And what is the difference between the Neck width and Line width?
Yes, neck parameters should be taken into account. Especially if you are trying to control timing. (you will want to know the entire trace length neck included)
Sounds like in this case the normal trace width is impedance controlled, the neck length is usually some minimum distance required to complete the circuit. (the necked down portion of net will no longer have the same impedance value as Line Width) This can be setup in your rules.