Hello Steven,

From my experience all board vendors try to stack boards for drill for the
simple reason of maximizing throughput and efficiency.

The major factor that influences stacking is the ratio of the smallest drill
hole to the thickness of the board stack.  Where you might drill a .062"
board (with a .010" finished via)"two-high", you might drill a .093" thick
only "one-high". (of course anywhere where you have gone to 50% throughput,
the cost will be increased because efficiency has diminished)

The reasons for this are fairly simple:

1.) the smaller the bit the greater the chance of it breaking.   And the
higher the thickness the greater the strain on the bit.

2.) drill wander.   As bits get smaller in size the are more affected by the
glass bundles in the material.   They tend to deflect around them to a degree
and will wander more with either smaller bits or thicker stack during drill.

This only covers actual drilled holes.  I am not too familiar with micro-vias
or laser drilling, but do know enough to know they have their own
complications caused by increasing the thickness of the stack for drill.

Let me know if you have any questions.

Hope this helps.

Thanks,

Dave Rist
Sales Manager-PCB's
Evergreen Circuits
888-593-8217