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On Sun, Sep 13, 2015 at 11:33:02AM -0400, DJ Delorie wrote:
> 
> > I don't really understand what is the use of a via without copper. Can you
> > elaborate?
> 
> A plugged via would be used in, for example, via-in-pad design BGA
> footprints.  The plug is there to keep the solder balls or paste from
> wicking into the via and away from the connection area.


Yes, but in this case you really want the hole to be covered by
solderable material. It is what my PCB house calls a filled and capped via:

http://www.multi-circuit-boards.eu/en/pcb-design-aid/design-parameters/via-covering.html


Note that: 
http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/XLamp%20Application%20Notes/XLamp_PCB_Thermal.pdf

inidcates how poor soldermask thermal conductivity is.