Understanding Staggered Via Transitions in Multilayer HDI Designs

Design and Layout
1

Dear community,

I’m trying to design staggered vias to transition signals from top to bottom in a 1+6+1 HDI stack-up.

Here’s the path I was initially thinking:

L1 to L2 → microvia
L2 to L3 → buried via
L3 to L4 → buried via
L4 to L5 → buried via
L5 to L6 → buried via
L6 to L7 → buried via
L7 to L8 → microvia

However, placing a via at every layer seems complex, and I’m not sure if it’s the most efficient method. I also don’t know if any design tools provide support to simplify or automate this kind of via chaining.

So I wanted to ask a few things:

Instead of going layer-by-layer, is it possible to go directly like:

  • In a 1+6+1 HDI stack-up, is it acceptable to use a microvia from L1 to L2, followed by a single buried via from L2 directly to a target inner layer like L4, L5, or even L7?
  • As through-hole vias aren’t practical for FBGA fan-out, I’m wondering if using L1–L2 (microvia), L2–L7 (core via), and L7–L8 (microvia) would be a valid alternative in a 1+6+1 HDI stack-up?

Any guidance or feedback would be greatly appreciated

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2

You’re totally right that going layer by layer with vias (L1 to L8) can get overly complex fast. The good news is you don’t need to go one layer at a time. In a 1+6+1 HDI stack-up, it’s pretty common to do something like:

  • L1–L2: microvia
  • L2–L7: buried via
  • L7–L8: microvia

That’s a solid and much more efficient way to transition signals from top to bottom without eating up unnecessary space or adding cost. It also avoids THs which as you mentioned aren’t ideal for BGA fanout.

Just make sure your fab house supports that buried span from L2 to L7 (most can but it’s always worth confirming).

So yes your simplified path is not just acceptable it’s a great way to go for HDI.

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3

Thanks for your response earlier — that really helped.

I had a quick follow-up: would a via from L2 to a deeper internal layer like L4, L5, or L7 be considered a skip via, since it connects non-adjacent layers?

Also, in a 1+6+1 HDI stack-up, are such skip buried vias typically supported, or would they require special fabrication processes?

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I think normally a micro-via goes from whatever layer is currently the outside to (at most) the copper layer on the same side of the nearest core. It might be able to go through multiple layers if they’re separated only by thin pre-preg.

A regular via goes from the current outside layer to the opposite currently outside layer, and there must be (at least) a core in between. Controlled-depth and backdrilling can loosen this all-the-way constraint, but going from layer 2 to anything probably requires an extra drill step performed before the final lamination (while layer 2 is still on the outside), and going to anything except the symmetric (1 before the bottom) layer is sufficiently unusual that you may need to pay custom prices.

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