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68 DESIGN007 MAGAZINE I JANUARY 2021 The multilayer PCB is the most critical com- ponent of an electronics assembly. If it fails, your system fails! The PCB is so fundamental that we often forget that it is a component, and like all components, must be chosen based on specifications to achieve the best possible per- formance of the product. Stackup planning involves careful selection of materials and transmission line param- eters to avoid impedance discontinuities, sig- nal coupling, unintentional return paths, high AC impedance and excessive electromagnetic emissions. Materials used for the fabrication of multilayer PCBs absorb high frequencies and reduce edge rates thus putting the mate- rials selection process under tighter scrutiny. Ensuring that your board's stackup and imped- ances are correctly configured is a good basis for stable product performance. In the late 1980s, my colleagues and I were designing relatively high-speed broadband communications products (Figure 1). We knew that the impedance of the digital transmission lines had to be between 50–60 ohms but had no way of determining the exact value. Then, in April 1990, the IPC-D-317 standard was released. This was the first guideline for Elec- tronic Packaging Utilizing High-Speed Tech- niques that incorporated transmission line equations. Problem solved? However, punching the numbers of the lengthy equations into my scientific calculator seemed to always result in different values. As frustrating as this was, I tolerated it for years until finally in 1995 I had the idea to simplify the process by creating an online multilayer impedance calculator that everyone could use. This tool was based on the IPC-D-317 standard (now IPC-2251) closed-loop equations which are, in reality, just approximations—but it was the "best guess" for its time. Field solvers first appeared about five years later but were too expensive for the average punter. Free online tools are great but to realize some return, in 2010 I imported the equations to a Windows application, coined the term "stackup planner" and started to build on its func- tionality. Later, a 2D Bound- ary Element Method (BEM) Field Solver was integrated into the iCD Stackup Planner to achieve industry-leading accuracy. iCD then continued to develop applications with high-speed design features specifically for PCB designers. These included the iCD PDN Stackup Planning: Three Decades of Innovation Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Designing a multilayer PCB using Daisy, Board Master, Unix software.

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