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Differential Pair BGA Routing

Avalex Steven

I am having a hard time routing differential pairs out from a BGA.  OrCAD seems to keep snapping the route in awkward paths that don't make sense to me.  See image for example with lines in red being what I'm trying to accomplish.  Any settings I can change to fix this? 

 

  • lcan

    One way to achieve your red path planning lines is to change methodology rather than settings. Start your breakout etch from the outermost via, RMB and switch to 'Single Trace mode', after achieving the desired length for the first cline, RMB and turn off Single trace mode, the second net should follow your desired path. (Assigning a funckey to "Single Trace mode" avoids having to use the Right mouse Button if you have many diff pairs.)

    Bill

  • Avalex Steven

    Thanks for the reply.  The single trace mode solution works pretty good, but it seems like there should be a better solution using the normal differential routing.

  • lcan
    in reply to Avalex Steven

    An alternative is to 1) assign your diff pairs to a net class.  2)In the Physical domain of Constraint Manager create a region then assign values for  Neck 'Min Width' and 'Neck gap".  3) define a shape of type 'Constraint Region' on the Subclasses you will be using for routing while assigning the newly created Region name to the shape. Your diff pairs will now use these constraints to route between vias.This method however will not follow exactly your original red path planning lines, they will still seek to "gather" before exiting the via array.

    Bill

  • redwire

    That is caused by a clearance issue that OrCAD is trying to solve.  Turn the bubble mode to "OFF" and "Allow DRC" to ON and route.  If it breaks out correctly then you will get DRCs.  Look at what's causing that.

    Necking might be required in that area and once you have that set up properly it will route out as shown with no issue.

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