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I understand there are regular PTH vias drilled once all layers are stacked together then plated (e.g. the Thru 1:9 via on the left of the image below)

There are also Blind vias which are drilled into once all PCB layers are stacked together and the drill bit will drill slightly into the core (e.g. Blind via 1:4 in the image below)

There are also Buried vias which I image are drilled and plated once the relevant layers have been stacked (e.g. Buried via 3:6 and 2:5 in the image below)

Via type example image

What is the purpose of using a microvia? Is it even possible to manufacture the stack up below (ignoring the microvias for a moment, layers 4 & 5 are stacked followed by 3 & 4, then buried via 3:6 is drilled and plated, then layers 2 & 7 followed by 1 & 8 and blind via 1:4 and 5:8 are drilled (slightly into the core) and Thru 1:8 is drilled and these are plated but where in this process can buried via 3:6 and 2:5 be drilled and plated)?

In my mind, buried vias 2:5 and 3:6 prevent each other from being manufactured as you cannot completely stack up the layers required, drill through the board and plate it without covering up the layers required for the other buried via (if that makes sense?).

Is this when you would use a micro via? Or is the microvia used for different purposes?

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What is the purpose of using a microvia?

The main benefit is that it doesn't take up space on layers that it's not connected to. That can help dramatically in shrinking your total board area in a tight design.

Another benefit in high-speed designs is it doesn't have a big stub to create extra capacitance from the signal line to ground, which improves signal integrity.

Is it even possible to manufacture the stack up below

I doubt it. I don't see how you can have 2:5 buried vias and 3:6 buried vias in the same stack-up. Maybe if you made the 3:6 subassembly and drilled those vias, then built up to the 2:7 subassembly and used controlled depth drills to make the 2:5 vias.

In any case, it wouldn't be cheap with so many different drill operations on so many different partial assemblies.

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  • \$\begingroup\$ So if I needed to create a via from layer 4 to 5 for example, is it best to use a drilled via which is plated or a micro via? In either case, a stub is not created since the via does not exist on layers it is not needed on. SO in this case which would be better? Would one be more expensive to manufacure? \$\endgroup\$
    – MRB
    Commented Jun 14, 2023 at 14:01
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    \$\begingroup\$ The drilled via will probably be cheaper. Also, microvias are limited in aspect ratio, so with a 0.93 mm core, the minimum microvia diameter (depending on the tools your shop has, but 5:4 is a typical limit) could be as much as 1.1 or 1.2 mm, so the mechanical via can probably be smaller. \$\endgroup\$
    – The Photon
    Commented Jun 14, 2023 at 14:05
  • \$\begingroup\$ But in the case if a 2 to 5 via were required, with this stack up then it would need to be a microvia? \$\endgroup\$
    – MRB
    Commented Jun 14, 2023 at 14:08
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    \$\begingroup\$ @MRB, you could do controlled-depth drills on the 2-7 subassembly, or controlled-depth drills on the whole board (if you are okay with a stub from L1 to L2), or a stack of microvias drilled on each subassembly (but that is costly and has reliability concerns unless you have the room to make it a crank-shaft stack), or through vias with back-drilling to remove the plating from L6 to L8 (if you have the space to spare on L6-L8). \$\endgroup\$
    – The Photon
    Commented Jun 14, 2023 at 14:12
  • \$\begingroup\$ If you're working with a full-service shop, ask them which option has the lowest cost. \$\endgroup\$
    – The Photon
    Commented Jun 14, 2023 at 14:13

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