Larry,My opinion below:
I've been working on this problem myself, and would like to hear what the community thinks also.I did a quick search using google with these keywords ".5mm pitch bga guideline"turned up some good info.I believe that most recommend a .25mm (10 mil) pad, do not recommend soldermask defined pads.The soldermask defined pads I have heard cause fractures at the contact (soldermask doesn't allow the solderball to slump over the pad).If at all possible, fan out the first two rows using a 3 mil trace / 3.5 mil space.    Then fan in (if possible), or offset the via from the bga pad.Use a 10 mil via pad with a 5 mil laser hole (no soldermask opening).     I would use the same size solderpaste opening for the bga pad, and let the assembly fabricator adjust as necessary.  
http://www.nxp.com/documents/application_note/AN10778.pdfhttp://www.ti.com/lit/an/sprabb3/sprabb3.pdfhttp://www.latticesemi.com/~/media/Documents/ApplicationNotes/PT/PCBLayoutRecommendationsforBGAPackages.pdf?document_id=671

Karl


> Date: Fri, 21 Mar 2014 08:51:17 -0500
> From: [log in to unmask]
> Subject: [DC] Microvias in pad
> To: [log in to unmask]
> 
> Ok, another question for this very helpful community :o)
> 
> I have a micro BGA, 0.5mm pitch, 121 pads.
> 
> Each pad is 8mil on the PCB footprint as per the recommendation from the manufacture.
> 
> Now I'm going to place a microvia of 12mil with a laser hole of 4mil in some of the inner pads.  
> 
> What would you recommend for the solder resist opening for this via in pad and the solder paste opening for it?
> 
> In advance, many thanks for your help.
> 
> Larry
> 
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