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Problem connecting Power Pads to GND polygon

autodeskguest

I am designing a board that includes several kinds of ICs

with "Power Pads" under the IC package that need

to be grounded.  For some of the ICs, the GND polygon

connects to the Power Pad as it should.  For other

ICs, the GND polygon is not connecting to the Power Pad.

Examining the devices in the library, they appear to be

constructed and labeled the same way.  I can't see

a difference in labeling, pin declaration, or grounding

between the ones that connect to ground and the ones

that don't.

 

Can you suggest what might be different between the

ICs or the libraries to cause this?

 

Thank you,

--- Graham

 

==

 

Parents

  • Gary:

     

    The issue is not weaving grounds between the SMDs, but

    rather getting the SMDs that are named as GND to automatically

    connect to the PowerPad that is named GND and is only 0.5 mm away,

    with nothing in between or nearby, other than the other SMDs for

    adjacent package leads. And yes, I will have vias to inner layer GND

    in the PowerPad.  All Isolate settings at 8 mil, or 0.2 mm.

     

    This is a very fine pitch part.  0.5 mm pitch on the leads.

    So SMDs 0.28 mm wide, and on 0.5 mm centers.  The PowerPad comes

    within 0.5 mm of the SMDs for the leads.  I am having the same problem

    with a QFN-16 package with a relatively large PowerPad.  Smaller

    PowerPads do not have the problem.

     

    If the PowerPad is smaller in size, and is perhaps shrunk so that

    it is 1.5 mm away, the Ground Polygon will connect the SMDs named

    GND to the PowerPad named GND.  If it is much closer, set to the

    0.5 mm specified in the Texas Instruments footprint, the Polygon

    will not connect without turning off "Thermals" on the PowerPad.

     

    That is OK as a workaround, but I find the behavior confusing.

     

    Software is 5.6.0 Professional on Windows XP.

     

    --- Graham

     

    ==

     

    Gary Gofstein wrote:

    well, i haven't seen your layout, but I will guess that any trace that

    will fit between your SMD's will have too much inductance at RF to be

    any use, may even form resonators with the ground plane if you're really

    at high freq/speed. i would say the main advantage is that they might

    help isolate one SMD from another crosstalkwise. To get a low impedance

    connection, i would just load the pad with vias to ground, but that's

    just my feeling, haven't tested it ever. Best to copy whatever reference

    layout is shown with the part, a lot of times they are on a separate app

    note or white paper. good luck!

     

Reply

  • Gary:

     

    The issue is not weaving grounds between the SMDs, but

    rather getting the SMDs that are named as GND to automatically

    connect to the PowerPad that is named GND and is only 0.5 mm away,

    with nothing in between or nearby, other than the other SMDs for

    adjacent package leads. And yes, I will have vias to inner layer GND

    in the PowerPad.  All Isolate settings at 8 mil, or 0.2 mm.

     

    This is a very fine pitch part.  0.5 mm pitch on the leads.

    So SMDs 0.28 mm wide, and on 0.5 mm centers.  The PowerPad comes

    within 0.5 mm of the SMDs for the leads.  I am having the same problem

    with a QFN-16 package with a relatively large PowerPad.  Smaller

    PowerPads do not have the problem.

     

    If the PowerPad is smaller in size, and is perhaps shrunk so that

    it is 1.5 mm away, the Ground Polygon will connect the SMDs named

    GND to the PowerPad named GND.  If it is much closer, set to the

    0.5 mm specified in the Texas Instruments footprint, the Polygon

    will not connect without turning off "Thermals" on the PowerPad.

     

    That is OK as a workaround, but I find the behavior confusing.

     

    Software is 5.6.0 Professional on Windows XP.

     

    --- Graham

     

    ==

     

    Gary Gofstein wrote:

    well, i haven't seen your layout, but I will guess that any trace that

    will fit between your SMD's will have too much inductance at RF to be

    any use, may even form resonators with the ground plane if you're really

    at high freq/speed. i would say the main advantage is that they might

    help isolate one SMD from another crosstalkwise. To get a low impedance

    connection, i would just load the pad with vias to ground, but that's

    just my feeling, haven't tested it ever. Best to copy whatever reference

    layout is shown with the part, a lot of times they are on a separate app

    note or white paper. good luck!

     

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