FIG. 6.1 compares the diameter of borehole plug hole of 300um in common FR4 circuit board and 100um plug hole in PCB layered circuit board.Since the signal line is configured along the X and Y directions, a plug hole must be provided at the intersection of the X and Y directions to allow communication between the upper and lower wires.The bolt hole is arranged in a diagonal direction, which can reach the maximum number of bolt holes.Generally, density index of high density PCB is expressed by the density of plug hole.The number of plug holes per square inch is expressed in VPSG units.Figure 6.1 shows that the bolt hole density of FR4 circuit board is only 4VPSG, while that of PCB layered circuit board is as high as 20VPSG.In addition to the fact that the density of the planar circuit board of the additive circuit board is 3 times that of the general FR4 printed circuit board, the insulation thickness of the additive circuit board is only 40um, which is also thinner than that of the FR4 circuit board, so the density in the Z direction is 2 times that of the FR4 circuit board, so the density of the entire additive circuit board can be more than 10 times that of the general FR4 circuit board.As the density of the PCB is much higher than that of the FR4 printed circuit board, the yield of the PCB production will be greatly reduced if the precision required in the manufacturing process is not ensured.
Inspect the PCB layered circuit board
The traditional FR4 printed circuit board's glass fiber wireboard is composed of epoxy glass fiber cloth and copper foil, which are pressed together to form a conductive perforation between the upper and lower layers by means of mechanical drilling, and the circuit is formed by photoetching.Therefore, part of the process is mechanical processing, and part is chemical process.
In addition to a small number of perforation process, PCB laminated circuit board is basically completed by chemical process.As the density of the circuit is much higher than the traditional inspection method of the FR4 circuit board for quality control.The control of process error is very important for the multilayer circuit board, so how to choose the process control parameters and control process parameters is very important.However, since many process parameters can not be directly checked or observed, how to monitor these process parameters is one of the key points to determine whether the production technology of layering circuit board is mature or not.