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Via tenting and plugging

autodeskguest

I want to put vias in the central pads of several QFN/MLF devices to

improve grounding and dissipate heat. To reduce costs, the vias are

0.3mm which is fairly large compared to a small QFN20 part's central

pad. To prevent solder paste from draining away during reflow, I believe

I need to plug those vias.

 

I'm considering plugging and tenting the vias from the bottom-side only

so that there is no top-side solder mask to interfere with good reflow

bonding to the chips. Another approach might be to only plug the vias

from the top (no tenting), but I'm not sure how accurately this can be

done in "prototype" runs.

 

Two questions:

 

1. Do either of those approaches seem practical?

 

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

 

Thanks for any input.

 

-Brad

 

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  • Brad Schick schrieb:

     

    But I'd still says Eagle's implementation is rather odd. It seems like a

    much simpler (and more useful) behavior would be for the "change, stop,

    off" command to always work for all components on a board.

     

    I told you it can be somewhat confusing. image

     

    I think this has historical reasons. Earlier versions of EAGLE didn't

    support the "stop" property for vias, and their solder stop mask was

    globally defined by the masks/limit value in the DRC settings.

    Later, the stop on/off property was added, but the masks/limits

    parameter is still working the same way for compatibility.

     

    I'm not sure how common tented vias are, but another nice feature would

    be to let users change the stop mask setting to the following 4 values:

    "off, on, top only, bottom only". Then I wouldn't have to draw it

    manually and make sure it gets moved together with the via (which is

    currently one advantage of putting in into the package).

     

    For unplugged vias, it is a very bad idea to add solder stop laque only

    at one side. Normally, liquid laques are used, so it will also stain the

    other side. Tented (or partially tented) vias are a specialty that

    always has to be communicated with the board house. I think providing

    such options would be somewhat "dangerous" for all those around who

    don't even know about plugging and tenting.

     

    Tilmann

     

Reply

  • Brad Schick schrieb:

     

    But I'd still says Eagle's implementation is rather odd. It seems like a

    much simpler (and more useful) behavior would be for the "change, stop,

    off" command to always work for all components on a board.

     

    I told you it can be somewhat confusing. image

     

    I think this has historical reasons. Earlier versions of EAGLE didn't

    support the "stop" property for vias, and their solder stop mask was

    globally defined by the masks/limit value in the DRC settings.

    Later, the stop on/off property was added, but the masks/limits

    parameter is still working the same way for compatibility.

     

    I'm not sure how common tented vias are, but another nice feature would

    be to let users change the stop mask setting to the following 4 values:

    "off, on, top only, bottom only". Then I wouldn't have to draw it

    manually and make sure it gets moved together with the via (which is

    currently one advantage of putting in into the package).

     

    For unplugged vias, it is a very bad idea to add solder stop laque only

    at one side. Normally, liquid laques are used, so it will also stain the

    other side. Tented (or partially tented) vias are a specialty that

    always has to be communicated with the board house. I think providing

    such options would be somewhat "dangerous" for all those around who

    don't even know about plugging and tenting.

     

    Tilmann

     

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