David,
My legs are kind of short, so let me open wide and see if I can't
get one inserted up to the kneecap.
We've faced this set of tradeoffs several times, and IMHO the short
answer is that layers are much less expensive than moving to BoBoB
(Blind or Buried or Both). Although you may be able to save a layer
pair, and *possibly* a gnd/pwr reference structure to go with it,
the number of fabrication steps and checks multiply so quickly that
gains in materials related costs (layers) are quickly eclipsed.
The real use of this technology, again in my humble opinion, is in
order to make possible a product that can't be made otherwise, within
given constraints. The constraints we've had force us to complex
structures include X/Y dimensions, the Z dimension, and that nefarious
4th dimension, TIME.
Interestingly, the design time saved by using the technology avail-
able can easily be consumed by the handoff, engineering, and planning
of the much more complicated process. If you go with the complicated
approach, make sure you know when that n-week turn *starts*, and that
you know exactly what data is needed and how to make it.
Generally, costs for BoBoB are measured not in dollars but in gold
bars. Having said that, let me explain how I've come to progress
through the technology maze on a variety of complex products.
For the 1st 3 dimensions, without looking at the exact logic, I've
usually found it most comfortable to pack one side to the maximum
reasonable density (yeah, right, define that!), and attempt to place
the secondary side such that you overlap to the highest extent.
That is, rather than using 60%/60%, use 90%/30%. When you get to
90%/70%, you're in the likely range of not being able to pin-escape,
and this is the time to look for the "next idea, please".
Speaking very empirically, if you can get through pin-escaping then
you can probably succeed in routing with 3 non-attach layer pairs.
Notice I didn't say how long this will take, and also that I used the
word *probably*. As always, much depends on the specific circuit/
product/components etc. Of course, there are plenty of PWB's that
won't require this many layers, but this discussion is geared towards
the ones that do. If you lack enough Z-axis space for the number of
layers that you believe you need, a buried via layer-pair goes some
distance toward making things possible, as you can now use the space
under the SMD lands. This effectively places one or more partially
finished two-layer board in the middle of your finished board.
When you can't achieve complete pin-escapes, the next step cost-wise
would be blind vias one-layer-in from either or both sides. This
minimizes the increase in fabrication steps, the effect is that of
including semi-finished two-layer boards on the outside of your
overall board. Sometimes this approach can save some layers, and
may be the angle that best reflects your original question (remember
your original question? I don't!).
If you were really far off on the pin-escape step, bringing them
down one layer can lead to the feeling that you've simply moved the
problem in one layer, which you have (it's a type of sinking in
the pit of the stomach). If you moved the problem without solving
it, then you need the more expensive deeper blind vias. This is
effectively placing semi-finished multilayer boards on the outside
of your finished board. Once you've signed on for this, you've
pretty well blown the wad and a buried via layer-pair in the middle
will keep those blind vias from being lonely. This is how wedding
receptions get big. We use this technology when the density hits
105%/105%, and try like crazy not to need two depths of blind vias
combined with multiple buried via combinations.
Now I've definitely run on far too long, and must go before I dis-
appear into my own mouth with a loud metallic clank. But before I
sign off I should warn you that blind and buried have to be mixed
carefully, they can't overlap (with apologies to Ralph Hersey!) :
I I
I I
U I U
U U U
U U U
U I U
I I
I I blInd/bUried
not possible possible
Good luck,
Jeff Seeger Applied CAD Knowledge Inc
Yankee Technical Officer Tyngsboro, MA 01879
[log in to unmask] 508 649 9800
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