What Julie describes is defined in IPC-4761, "Design Guide for Protection of Printed Board Via Structures" as a "Type VII Via" or "Filled and Capped Via".  Personally I'm an advocate of using the definitions (tented, covered, plugged, filled, capped) outlined in the IPC-4761 document to reduce the confusion when specifying via protection requirements.

John Parsons

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Julie Silk
Sent: April-26-13 9:56 AM
To: [log in to unmask]
Subject: Re: [TN] Via in pad using filled or plugged vias

We use vias in pad, plated over (vippo) in many designs.  They provide connectivity and heat conduction without adversely affecting the solder joint.  Especially important in the ground pad of QFNs.
"Plugged" or "capped" implies that a via is covered on only one side.  We do not use these because of concerns with nasty chemicals getting trapped inside during the fab or assembly processes.  
Vippos are usually made using the 2-step plating described in one of the other replies.  Drill vippos, plate, fill, planarize, drill non-vippos, plate.  Because of the added copper thickness, the line/space width on the exterior has limits.  It is also critical that the wrap plating meet the specs (mentioned in other reply).  Without this little bit of wrap plating, the stresses on the via can cause the cap to pop off and you'll have an open.  The planarization process seems primitive, and yet is critical.  This is the source of most problems.
Other tips:  do not use conductive filler, besides not helping that much, it messes up the copper plating.  Have these evenly distributed across the board; localized concentrations are difficult to planarize properly.  We were requested to have any specific drill size either all-vippos or all not.  

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________