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I'm designing a 4-layer PCB for a very small adapter from a BGA on one side to several discrete SMT connectors on the other. Due to space constraints, I cannot use through holes for some of the interior connections; they must be routed through blind vias.

After some discussion with the PCB fab company I'm starting to understand a bit better why blind and buried vias cost so much money and how these complicated boards are constructed. I also appreciate that microvias add cost to the board. Is there any way to route a signal from one side of the board to the other without at least one set of microvias?

My current design has three via styles:

                 Layer 1-2
----------+  +------+  +--------------------
          |  |      |  |
----------|  |------+  +------+  +----------
          |  |                |  |
----------|  |----------------|  |----------
          |  |                |  |
----------+  +----------------+  +----------
        Through            Layer 2-4

So the trouble comes with having both blind vias terminating on the same layer from either side of the board. They can't drill those separately before the layers are laminated together, and so the top one must be bored out afterwards and filled in (at high precision and cost, as I understand it). But I need these signals the whole way through the board! Are there any tricks to working around this? Or are my design specs simply too challenging for my modest budget?

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    \$\begingroup\$ An alternate way to save space is to use via-in-pad. If you aren't doing that already, it might save you enough space you won't need one of your two blind via types. \$\endgroup\$ – The Photon Jul 17 '12 at 3:31
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    \$\begingroup\$ "Is there any way to route a signal from one side of the board to the other without at least one set of microvias?" Yes, use a through via. \$\endgroup\$ – The Photon Jul 17 '12 at 3:32
  • \$\begingroup\$ YOu can ask for equations for cost factors in estimating board costs but Blinds vias ought to be avoided. I had formulae for estimating this on a spreadsheet in one minute, but that was 10yrs ago and is no good anymore. Overall design of power ground planes and shielding using other methods such as micro-vias needs more info to evaluate design as whole for cost-reduction DFC DFT, DFM suggestions. Any good process engineer ought to know these. I did at one time. Process capability of Fab shop is huge part of the equation and if they are already making alot of other high volume pwbs for U. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 17 '12 at 4:35
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    \$\begingroup\$ @ThePhoton - you've answered most of my question in your two comments. You should submit an answer. \$\endgroup\$ – Matt B. Jul 17 '12 at 18:55
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An alternate way to save space is to use via-in-pad. If you aren't doing that already, it might save you enough space you won't need one of your two blind via types.

This was meant to be a flippant comment, since it technically answers the question, but doesn't really provide any new information. But the OP seems to think it's useful:

Q: Is there any way to route a signal from one side of the board to the other without at least one set of microvias?

A: Yes, use a through via.

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    \$\begingroup\$ Thanks. Your 'flippant' comment is very useful as it answers the most basic part of my question: is it possible? \$\endgroup\$ – Matt B. Jul 17 '12 at 19:07

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