andyk,
    These are very common questions that come up with blind vias.  I'd
    like to try to answer them from our perspective:
    a) Certainly, controlled depth blinds are less costly to process
    and therefore will be a lower price to you.
    b) Repeatability of depth should not be a real mystery to your
    supplier.  Most drilling equipment configured for depth control
    utilizies a Z-axis encoder that is triggered by surface contact of
    the pressure foot.  So long as the laminate thickness and surface
    topography is kepth under control, there should be repeatability
    in depth within a reasonable tolerance window (.003" is not
    uncommon).
    c) I don't have any data to support long term reliability for
    these holes, but aboviously, very many suppliers are making blind
    vias in this fashion, and have been for a number of years.  The
    key issue surrounds the ability to properly prepare and metalize
    the holes, which leads to...
    d) The upper limit on aspect ratio for CDBV's, generally in the
    industry, seems to be around 1:1.  The obvious problem here is
    solution movement during metalization.  Hole size also plays a big
    role in this.  Just because a shop can do 1:1 with a .015" drill
    doesn't mean they are capable of 1:1 with a .010" drill.  Each
    shop is a little different.
    Hope this helps.

    Andy Slade
    HADCO Tech Center One
    7 Manor Pkwy
    Salem, NH 03079
    (603)-896-2210

    ------------------------------------------------------------------
     Hi, I'n in the end run of a revision to a board design which is
    double sided eight layers with blind vias on layers 1/2, 1/4, 5/8
    and 7/8, as well as the normal through-hole 1/8 vias. Sizes are
    0.3mm (12mil) for in-pad blind vias and 0.6mm (24mil) wherever I
    can get away with it.

We've already had batches of Rev A boards made up with sequential lamination and
with controlled depth drilling
and we're in the process of evaluating suppliers and their boards. My problem is
that we are having trouble nailing
down any sort of specification for our next run. These will also be prototypes
but we're getting close to volume
production and need something for QA to hang its hat on for incoming inspection.

Factors we are considering are:
a) Boards manufactured with controlled depth drilling promise to be considerably
less expensive (30% on one estimate)
b) The repeatability of the depth of drilling is an unknown, from stack to stack
and from batch to batch.
c) The long term reliability of a blind via defined this way is unknown to us
d) The aspect ratio of a blind hole appears to be critical, but what is optimum
and what is acceptable?

and so on and on.

Can anyone help?

Thanks in advance

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