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How to communicate impedance control information to the PCB house from Allegro

likeabletom

All,

Our new designs have a lot of differential pairs running around our boards and I want to communicate this clearly to the board house so they can adjust tracks to their process to maintain impedance control.  We use the ODB++ functionality to communicate the physical board.  I have looked though every file inside the ODB++ file and i see nothing that can be interpreted as netname + impedance value + on this layer information.  I have looked through the entire interface of Allegro and see nothing that can do this.

So, my question is, how can i extract this information from our finished Allegro design and commuincate it in an effective and useful manner?

Robert.

  • redwire
    Nets can have "impedance_rule". A trick used by many fabricators is to set a line width to something easy to pick up on, for example, set a line to 4.876 mils. Then tell them that "all 4.876 mil lines on layer xxx shall have an impedance of 50+/-5 ohms. You can check if "impedance_rule" gets forwarded to ODB++. I personally do not use it.

    However, with Allegro, why not go ahead and set up the stackup with the correct material values and set the impedance in constraint manager to 50 ohms (or 100 ohm diff) and route away? The Allegro solver is very accurate and uses the same closed-form equations as the fab houses. What differs is actual resin content and thickness of the materials. Speak with the fabricator up-front and get this data and enter it into your material database and let Allegro do its job.
  • tltoth

    When you have custom layer stackup, fabs are not really keen on giving you these infos till you place an order.
    However usually there is no problem with poolable designs.

  • techk

    The info really needs to be on the fabrication drawing or at least in a text file included with the Gerber set. It should be easy for the fabricator to see, and allows the customer to reject a board if it does not meet the specs.

  • redwire

    tltoth said:

    When you have custom layer stackup, fabs are not really keen on giving you these infos till you place an order.

    Very true.   I would highly recommend working with a fabricator up-front to develop this trust relationship and give them the work promised.   Otherwise it's best to build your own material / stackup databases and use the built-in solver.  The results are highly accurate when setup correctly.

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