Here is one I have recently been hit with. My current incarnation as an engineer is with and RF component manufacturer. We recently hired a new design engineer, he uses embedded inductors in the pcb fab. The prototypes, produced with a board mill, worked fine. The pilot production boards had a parametric problem. We traced it to a reduction in the inductor trace width due to the etch factor (currently unspecified on the fab drawing.) Due to size constraints the inductor uses .004" traces with .004" separation. So here is the question; how do I specify the acceptable variation on a specific feature of the PCB, and what is reasonable (i.e. how tight can I spec. trace variance without driving the cost out of sight).

Thanks,

Dan