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52 DESIGN007 MAGAZINE I JULY 2021 Article by Anaya Vardya AMERICAN STANDARD CIRCUITS Introduction One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. is article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer's perspective) and the design decisions that will impact product reliability. DFM Design for manufacturing (DFM) is defined as the practice of designing printed circuit boards that meet not only the capabilities of the customer's assembly manufacturing pro- cess, but also the capabilities of the board fab- rication process at the lowest possible cost. While not a substitute to early design engage- ment with the PCB fabricator, this series of articles will provide guidelines that will help to "design for success." Controlled Impedance PCBs that contain controlled impedance lines require specific constructions and tighter manufacturing process controls. e fabrica- tor needs to tailor the construction for PCBs requiring impedance to precisely match the required nominal impedance values. e fab- rication drawing should specify the required nominal impedance and tolerance and the fab- ricator will create a construction to meet the intended design impedance requirements. Primary Impedance Factors • Trace width • Copper thickness • Dielectric spacing • Indicated "Reference Only." See Impedance Requirements • Overall PCB thickness • Material requirements Top fabricators use some form of impedance modeling soware to determine the specific PCB construction required to produce the specified impedance. e PCB drawing should only specify the nominal impedance, toler- ance, and nominal line width. is will allow for the creation of the most cost-effective PCB material construction. DFM 101: PCB Controlled Impedance

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