Andrew Buonviri wrote Mon, 9 Dec 1996 08:26:10
Subject: blind vias
  My company is looking to design a multilayer PCB with blind vias. Another
  company will do the actual fab. I've heard there's more than one way to
  do blind vias: either controlled depth drilling or fabbing and drilling
  two halves of a board separately and sticking them together. Does anyone
  have any experience with either of these methods, and is there another
  way I don't know of? Which way is better, what are the risks, etc? Any
  info would be appreciated. Thanks, ---- End of mail text


We manufacture large volumes of blind via product using the sequential
lamination process sequence. Reasons for manufacture this way are many:
firstly depth drilling is aspect ratio dependent, and most products we
manufacture exceed the 1:1 ratio that is going to give reliable plating,
secondly when you do the maths, is it cheaper to buy another 15 drilling
machines to do the controlled depth drilling (1 high stack, twice on the
drill bed) compared with using capacity at bond and plating?.
Thirdly, the accuracy you can achieve with the mechanical depth drilling
and the shape of the point of the drill bit, means some very careful
design to ensure that the hole will contact the inner layer, and not
either just miss it or go too far and contact the layer below.
Finally, our customers require that the top of the blind via hole is
capped and flush with the surface. This obviously is not possible with
mechanically depth drilled holes.

Having said all of that, we recognise the benefits of micro-blind via
holes for redistribution layers for BGAs etc, and the bonus in tracking
density by reduction in pad size associated with the micro blind vias,
and so we have an ESI laser, and are working with a number of
photodielectric suppliers to produce sequential build up circuits, so
we can offer micro blind vias that are depth drilled.

Dougal Stewart
Product Development manager
Exacta Circuits Ltd
Scotland

A Forward Circuits Group Company

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