Via types

IPC-4761 (Design Guide for Protection of Printed Board Via Structures) defines several types of methods for protecting PCB vias. These are:

• Type I: Tented
  Dry film solder mask is stretched over the hole. No other materials are added.

• Type II: Tented and covered
  An additional dry film or liquid solder mask layer covers the tenting of a Type I tented via.

• Type III: Plugged
  A non-conductive material penetrates partially into the hole, but does not go all the way through.

• Type IV: Plugged and covered
  An additional dry film or liquid solder mask layer covers the plugging of a Type III plugged via.

• Type V: Filled
  A conductive or non-conductive material penetrates completely into the hole, filling the hole all the way through.

• Type VI: Filled and covered
  An additional dry film or liquid solder mask layer covers the filled material of a Type V filled via.

• Type VII: Filled and capped
  A metalized coating covers the filled material of a Type V filled via.

In general, production costs increase as you go down the list, with type III being more expensive than type I, etc.

Additional info:

• Via Tenting, Plugging, and Filling by PCB Universe
• Filled, Plugged, Plated Via-In-Pad by John Steinar Johnsen, Elmatica