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 HDI PCB designs.
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 first and second order of the 6-layer board needs laser drilling, that is the HDI board.
The 6-layer first-order HDI board refers to blind holes: 1-2, 2-5, 5-6. That is, 1-2, 5-6 require laser drilling.
The 6-layer second-order HDI board refers to blind holes: 1-2, 2-3, 3-4, 4-5, 5-6. It requires 2 times laser drilling.
First drill a buried hole of 3-4, then laminate 2-5,
Then drill the 2-3, 4-5 laser holes for the first time, followed by the second laminate 1-6.
Then drill the 1-2, 5-6 laser holes for the second time.
Finally drill the through hole.
It can be seen that the second-order HDI board has undergone two lamination and two laser drillings.
Board Lamination Times:
First-order PCB: one lamination is enough, just like the most common circuit board.
Second-order PCB: Lamination twice. Take the example of eight-layer circuit board with blind/buried vias, laminate layer 2-7 with well-made blind/buried vias first, and then laminate layer 1 and 8 with well-made through holes.
Third-order PCB: its process is much more complicated. Laminating layer 3-6 first, then layer 2 and 7, finally layer 1 and 8. It requires 3 lamination times, so most PCB manufacturers can not make it.