Let's continue to introduce the next structure: “2-N-2” structure.

"2-N-2" refers to two layers of blind vias, which is called second-order, where N refers to inner layers.

The second-order stacking has staggered via and stacked via, with different levels of difficulty, cost, and processes. The stacked via is shown in the cover diagram, which is relatively cost-effective and has good reliability.

The staggered via diagram is as follows:

From the diagram, it can be seen that this stacking requires another blind via to be drilled on the blind via, so the previous blind via needs to be filled with copper or a material that can conduct electricity and withstand laser exposure, which involves the electroplating technique of HDI PCB.