The Effect of Innerlayer Processing on Conductor Height

摘要

Innerlayer conductor height is often well controlled and slightly thinner than the nominal expected value. The nominal thickness for half-ounce copper, for example, is 0.7 mils, but conductors in finished circuit boards oftentimes measure approximately 0.6 mils in height. The precisions of the manufacturing processes that have been developed to produce electro-deposited foil allow venders to provide foils on the low side of the thickness specification. Further, fabrication steps such as the cleaning processes that are employed to enable the photoresist to adhere to the copper, and the oxide treatments that enhance the copper-to-epoxy bond in the finished board remove some copper from the surface. Together, these factors account for the deviation from nominal copper thickness. The finished thickness of innerlayer copper traces is usually of secondary importance when compared to other parameters such as conductor width and the clearance between conductors'. Unless taken to the extreme, thinning of innerlayers will have a small impact on the capability of the conductors to carry direct current or low frequency signals. Even in higher frequency designs, the thickness of the trace has a second-order effect on controlled impedance. It is perhaps for these reasons along with time and cost factors that fabricators reprocess innerlayers when conditions permit rather than scrap them. Due to increased functionality within a smaller area and tighter impedance controls required in very high frequency applications, today's electronic circuits are more demanding then ever to fabricate. As conductors become narrower, tolerances must get tighter to achieve the desired functional requirements. OEMs and designers of printed circuit boards should be aware of the impact of innerlayer copper thickness variations on their products. To ensure signal integrity and product quality, the time has come to rethink the practice of reprocessing innerlayers.