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48 The PCB Design Magazine • October 2016 How do we ensure that our high-speed digi- tal design performs to expectations, is stable given all possible diverse environments, and is reliable over the product's projected life cycle? One word: Impedance! For the perfect transfer of energy and to ben- efit from the highest possible bandwidth, the impedance of the driver must match the imped- ance of the transmission line and be constant along its entire length. Also, the power distribu- tion network (PDN) must provide low AC im- pedance up to the maximum bandwidth and deliver an undisrupted, minimal return path for high-current switching devices. These two seemingly unrelated concepts are controlled by the PCB stackup configuration including mate- rial selection and should be analyzed concur- rently. For a sufficiently large number of electron- ics products, failures are distributed in time as shown in Figure 1. This curve is called "the bathtub curve" and displays the typical reliabil- ity of diverse products regardless of their func- tionality. One would expect a product to fail after some years of service, but preferably long after the product becomes obsolete. Premature failures are of particular concern and are typi- cally the result of poor design practice or sub- standard manufacture. This column will focus on the design aspects. In a previous column, Intro to Board-Level Simulation and the PCB Design Process, I men- by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Rock Steady Design BEYOND DESIGN Figure 1: Product reliability life cycle—the bathtub curve.

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