Electronics > PCB/EDA/CAD

Multi-via objects and transitions

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Alex Eisenhut:
For high-speed signals that require specific signal and ground via configurations, do you
1) Create a footprint and schematic symbol
2) Just create it in the layout and copy paste in the PCB with no special schematic involvement.

For 1, if there's a simulate/layout cycle, all you do is update the footprint, but 2 is easier to get going quickly.

So, WWYD?

Sagar:
I always go with the second approach, after proper R&D once next revisions are easy to carry out.

T3sl4co1l:
Well, "high speed" is a wide net.  You may be interested to know that schematics can be layout-precise, specifying lengths and widths of transmission lines, junctions, stubs, etc., and this is a reasonable way to do RF design and layout -- usually with a check step of EM field simulation of the tentative/finished PCB design, or perhaps model extraction to verify that the output (an n-port matrix) matches the design (when expressed in the same way) within expected tolerances.

For digital signals, there is only one impedance to choose, length to various features is variable, and geometric limits apply instead: length maximum, length matching (within a pair or group, between pairs/groups, etc.), maximum vias, via geometry and clearance, etc.  Any limitations on stub length are handled by inspection, or specifying back-drill when via stubs are the concern.  Many of these can be specified for autorouting purposes, but in a simple hand layout it's less work to just review things manually.

Tim

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