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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
    Commented Jul 17, 2012 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
    Commented Jul 17, 2012 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\$ Commented Jul 17, 2012 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\$
    – mbauman
    Commented Jul 17, 2012 at 18:55
  • \$\begingroup\$ If I recall correctly, the optimization I ended up making here was to ensure my blind vias didn't intersect eachothers layers — so I still had three via styles, but they were through, 1-2 and 3-4. This drastically reduced costs because the four layers could be constructed as a simple lamination of two boards with a single through-hole pass at the end. \$\endgroup\$
    – mbauman
    Commented Dec 14, 2023 at 18:27

1 Answer 1

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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\$
    – mbauman
    Commented Jul 17, 2012 at 19:07
  • \$\begingroup\$ Isn't via in pad supposed to be a last resort thing since they are difficult to manufacture with high yeild. \$\endgroup\$
    – quantum231
    Commented Dec 14, 2023 at 17:11
  • \$\begingroup\$ @quantum231 they add cost, but I've done products that sell 100,000 per year (still small potatoes compared to consumer stuff) and never heard that they caused a yield problem. \$\endgroup\$
    – The Photon
    Commented Dec 14, 2023 at 23:53

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