Circuit Maker ---- the Difference Between First-order 6-layer PCB and Second-order 6-layer PCB

As we all know, there is through hole, blind via, buried via in printed circuit board as the above pic shows.

-- Through hole means the via running through top layer to bottom layer.

-- Buried hole means the via hiding in middle layers.

-- Blind hole means the via that can be seen only in top layer or bottom layer.

We use mechanically drilling to get through hole, and use laser drilling to get blind via and buried via.

Pic 1: To get first-order 6-layer PCB, firstly we use mechanically drilling to get through hole to get the through hole from L2 to L5. Secondly, we laminate L1 and L6 together, and then use laser drilling to get blind via between L1 and L2, L5 and L6.

Pic 2: To get second-order 6-layer PCB, firstly after laminating L2 and L5, we use laser drilling to get buried via between L2 and L3, L4 and L5, and use mechanically drilling to get through hole to get the through hole from L2 to L5. Secondly, we laminate L1 and L6 together, and then use laser drilling to get blind via between L1 and L2, L5 and L6.

We can find that first-order 6-layer PCB is laser drilled one time, second-order 6-layer PCB is laser drilled two times.

Therefore, the word, order here means the times of using laser drilling.

So now what is third-order HDI? Do you know?