Multilayer PCB’s can be constructed with varying thickness between each layer. For example, a 4-layer PCB of 1.6 mm thickness with 35 microns copper on internal layers can be constructed in many different ways. See 2 examples below:
While the overall thicknesses and circuit continuity are identical , the impedance value for trace on L1 will be different for each structure. This will cause impedance sensitive boards to stop functioning.
It is always advisable that all gerber inputs be specified with clear stackup. Any impedance requirements should also be specified.
HI-Q prefers that customer plans the stack up / trace width for impedance traces ever before the designs are routed in a CAD systems. This will avoid rework after the design is released to production. This could also be used for signal integrity analysis by the designer.