HDI: High Density Interconnection, short for high-density interconnection, non mechanically drilled, with micro blind hole loops below 6mil, and wiring width between inner and outer layers. The production method of multiplayer board with a line gap of less than 4mil and a pad diameter of no more than 0.35mm is called HDI board.

Blind hole: Short for Blind via, which enables the connection and conduction between the inner and outer layers. Buried hole: Short for Buried via, it realizes the connection between the inner layer and the inner layer, with a large blind hole.

Blind holes and buried holes are small holes with a diameter of 0.05mm~0.15mm. The methods of hole formation for buried blind holes include laser hole formation, plasma etching hole formation, and photo induced hole formation, usually using laser hole formation. Laser hole formation is also divided into CO2 and YAG ultraviolet laser machine (UV).

The first and second orders in the 6-layer board refer to HDI boards, which require laser drilling.

6-layer first order HDI board finger blind holes: 1-2, 2-5, 5-6 Namely, 1-2 and 5-6 require laser drilling.

6-layer second-order HDI board finger blind holes: 1-2, 2-3, 3-4, 4-5, 5-6 Two laser drilling operations are required. First drill 3-4 buried holes, then press 2-5, then drill 2-3, 4-5 laser holes for the first time, then press 1-6 for the second time, and then drill 1-2, 5-6 laser holes for the second time. Finally, the hole was drilled through. From this, it can be seen that the second-order HDI board has undergone two presses and two laser drilling operations.

In addition, second-order HDI boards are also divided into: staggered second-order HDI boards and stacked second-order HDI boards. Staggered second-order HDI boards refer to blind holes 1-2 and 2-3 that are staggered, while stacked second-order HDI boards refer to blind holes 1-2 and 2-3 stacked together. For example, blind holes 1-3, 3-4, 4-6, and so on, are the same for third and fourth orders.