Alignment between layers Since there are more layers in a multilayer circuit board, the requirements for alignment between layers are also higher. Generally, the alignment tolerance between the layers is controlled at 75 microns. Due to the influence of factors such as size and temperature, the degree of difficulty in controlling the alignment between layers of a multilayer board will be huge. 2. Inner circuit production Multilayer circuit boards use unique materials such as high TG, high speed, high frequency, thick copper, the thin dielectric layer, etc. This puts forward high requirements for inner circuit production and pattern size control. For example, the integrity of impedance signal transmission increases the difficulty of internal circuit manufacturing. The line width and line spacing are small, the probability of open circuit and short circuit may increase; the increase of short circuit will reduce the pass rate; the number of thin lines signal layers will increase the probability of missing AOI detection in the inner layer; the inner core board is light and easy to wrinkle, Poor exposure and easy curling during etching; the unit size of multilayer PCB is more significant, and its product scrap cost is higher. 3. Pressing process Many inner core materials and semi-cured boards are superimposed, and defects such as slippage, delamination, resin voids, and bubble residues are prone to stamping production. In the design of the laminated structure, the heat resistance, pressure resistance, glue content, and dielectric thickness of the material should be fully considered, and a reasonable material pressing plan for the multilayer circuit board should be formulated. Due to many layers, the expansion and contraction control and the dimensional coefficient compensation cannot maintain consistency, and the thin interlayer insulation layer is likely to cause the interlayer reliability test to fail. 4. Drilling The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of multi-layer PCB drilling roughness, drilling burrs, and decontamination. Due to a large number of layers, the copper thickness and plate thickness have also increased, and tool breakage is likely to occur during drilling; if there are many BGAs, the narrow hole wall spacing can easily lead to CAF failure; the increase in plate thickness can also easily lead to the problem of diagonal drilling.