Peter,

I just use micro vias to get the dogbones on the second layer then use a
bottum up blind via to that layer, when the aspect ratio is below 10:1. This
provides the test pads on the bottom side. When the aspect ratio gets
absurd, I do the double micro via trick with buried vias connecting both
sides. You already knew that but I needed to say it for some strange reason.

I've been filling, sequentially laminating and plating over the top for many
years. Still have some dimples but minor stuff. Works well and have you all
noticed how prices have come down significantly? Why do you suppose that is?

Earl

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