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Date: Thu, 12 Sep 1996 22:37:47 +1100
From: Andy Kowalewski <[log in to unmask]>
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Subject: Controlled depth drilling
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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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======== Fwd by: Tony King / N ========
I am also very interested in knowing what manufacturers are supplying blind
vias using a controlled depth drilling process. What volumes are being
produced and what type of process is used for the hole metalization. What do
the cross sections of the blind via holes look like ? Are the drill tools
standard ?  Aspect ratios ? 

I would appreciate talking with any suppliers of printed circuit boards who
might have this information first-hand.

Tony King
Elexsys International Inc.
Nashua N.H.
603-886-0066
[log in to unmask]

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