A 1:1 aspect ratio for blind vias should not be considered the limit.  

There are some DM processes that can "coat" blind vias to make them
conductive rather than rely on the electroless copper process to get into the
bottom of the hole.  Since the gassing of electroless copper precludes going
much higher than a 1:1 aspect ratio, i.e.the gassing limits deposition in the
bottom of a blind via, the next limitation becomes that of electrodeposition
itself.  Providing the entire hole is conductive, I've seen aspect ratios of
closer to 2:1 for blind vias.  And that's only with conventional
electroplating technology.  

Imagine the possibilities when the industry gets to working more on the
electroplating aspects of blind via board construction.

Roger Mouton
INSULECTRO
Lake Forest, CA

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