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packages with vias in smd pads

Former Member

I have a part with a QFN package with a thermal pad in the centre.  The mfg

suggests an array of vias in the thermal-pad.  Is there a way to do this

without having the pads counted as pins, and freaking out the drc?

 

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  • stderr wrote:

    I have a part with a QFN package with a thermal pad in the centre.

    The mfg suggests an array of vias in the thermal-pad.  Is there a way

    to do this without having the pads counted as pins, and freaking out

    the drc?

     

     

    There are a few different approaches in use .Which you use depends on your

    thoughts on DRC errors/warnings

    Remember that vias can only be applied to the board and not defined in the

    library.

     

    1) Use a library package that has the heat pad SMD pad drawn. This

    automatically takes care of Stop and Cream requirements.  Later, on the

    board you place vias as required onto the heatpad.  The DRC will have

    overlap errors which you approve.

     

    2) Use a library package that does not have a heat pad. On the board you

    draw the heatpad as a signal polygon and add the vias. Name the polygon and

    the vias the same. You may have to set the rank of the polygon as well if a

    larger polygon a can reach your heat pad. I would outline the heatpad in the

    library on a non copper layer to act as a guide when drawing it on the

    board..You will have to also manually define the tStop and tCream areas for

    the package in the library.  Markers for the vias could be there as well.

    There should be no DRC errors.

     

    HTH

    Warren

     

     

     

     

  • in reply to Former Member

    The amazing "Warren Brayshaw" <warrenbrayshaw@paradise.net.nz> wrote in

    news:irhpc0$a14$1@cheetah.cadsoft.de:

     

    stderr wrote:

    >> I have a part with a QFN package with a thermal pad in the centre.

    >> The mfg suggests an array of vias in the thermal-pad.  Is there a way

    >> to do this without having the pads counted as pins, and freaking out

    >> the drc?

    >>

    1) Use a library package that has the heat pad SMD pad drawn. This

    automatically takes care of Stop and Cream requirements.  Later, on the

    board you place vias as required onto the heatpad.  The DRC will have

    overlap errors which you approve.

     

    2) Use a library package that does not have a heat pad. On the board you

    draw the heatpad as a signal polygon and add the vias. Name the polygon

    and the vias the same. You may have to set the rank of the polygon as

    well if a larger polygon a can reach your heat pad. I would outline the

    heatpad in the library on a non copper layer to act as a guide when

    drawing it on the board..You will have to also manually define the tStop

    and tCream areas for the package in the library.  Markers for the vias

    could be there as well. There should be no DRC errors.

     

     

    Thanks Warren,

     

    I was hoping I had missed a more sophisticated way of doing it, like being

    able to create an smd in the library part which would include the vias. 

    Maybe the next generation of Eagle will support more complex smd pads.

     

    Cheers,

    Lawrence

     

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