1. Difficulties in making inner circuit
Multilayer pcb line has high speed, thick copper, high frequency, high Tg value of a variety of special requirements, internal wiring and graphic size control requirements are increasingly high. For example, for ARM development board, there are many impedance signal lines in the inner layer. To ensure the integrity of impedance increases the difficulty of production of inner layer lines.
There are many signal lines in the inner layer, and the width and spacing of the lines are basically about 4mil or less. Multi - core plate layer thin production is easy to wrinkle, these factors will increase the production of inner layer.
Suggestion: the line width and distance should be designed above 3.5/3.5mil (most factories have no difficulty in production).
For example, the six-layer board is recommended to be designed with a false eight-layer structure, which can meet the impedance requirements of 50ohm, 90ohm and 100ohm of the inner 4-6mil line width.