Different Order of HDI PCB Boards
The fabrication process of first-order HDI PCB is relatively simple and well controlled.
While the second-order HDI PCB is complex due to the issue of alignment, punch and copper problem. There are a variety of second-order design of HDI PCB, one is the staggered position of the various stages which needs to connect the next-layer through the wire in the middle layer connectivity, the practice is equivalent to double first-order HDI PCB. The second is to overlap two first-order holes, thus second-order PCB is achieved by the superposition method, and the processing is also similar to the double first-order, but there are many technical points to be particularly controlled.
The third is to punch directly from the outer layer to the third layer (or N-2 layer), the process is much different from the previous, and the drilling process is more difficult.
The third-order HDI PCB use the analogy method like the second-order ones.
Board Pressing Times
First-order PCB: one pressing is enough, just like the most common circuit board.
Second-order PCB: press twice. Take the example of eight-layer circuit board with blind/buried vias, press layer 2-7 with well-made blind/buried vias first, and then press layer 1 and 8 with well-made through holes.
Third-order PCB: its process is much more complicated. Press layer 3-6 first, the next is layer 2 and 7, the last is layer 1 and 8. It requires 3 pressing times, thus generally, most PCB manufacturers are able to do it.