Design007 Magazine

Design007-June2019

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10 DESIGN007 MAGAZINE I JUNE 2019 The Bare (Board) Truth Feature Column by Mark Thompson, CID+, PROTOTRON CIRCUITS "All right! Don't rush me. I'm a-thinking… and my head hurts." —Yosemite Sam It's a fact: Great board design is the key to a great PCB. I'm even more certain of this after spending two days in a wonderful class pre- sented by Rick Hartley titled "Control of Noise, EMI, and Signal Integrity in High-speed Cir- cuits and PCBs." First, I would like to thank Rick for teach- ing his course. I thought I had a pretty decent grasp of controlled impedance and signal in- tegrity, but the gaps in my knowledge base can fill the Grand Canyon. This class helped me see how important the design truly is not only for functionality but also for minimizing revi- sions and passing EMI testing the first time. I deal with incoming designs for a living, but I had no idea how many things you should consider when doing a board layout, from part placement and organization to via location. I guess I am just accustomed to simply us- ing my field solver and answering fabrication- related questions to make a PCB producible. I thought I knew it all, from effective Dk and Er to layer "nesting" based on the layer interfac- es and prepreg selection for various controlled impedance structures. I assumed I knew what I needed to know from how the color and di- electric of a mask can affect controlled imped- ances to the use of ultra-thin dielectrics to in- crease inductance to pass EMI. Well, I was wrong! How far we have come from the days of slower-speed designs; their edge rates didn't matter very much because they were are basi- cally lumped elements. Those old designs are nothing like today's high-speed designs and all the bugaboos associated with distributed length line designs. Understanding the impact of nearby traces on controlled impedance lines and how magnetic fields work can make the Fabrication Starts With Solid Design Practices

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