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Design007-Jan2021

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10 DESIGN007 MAGAZINE I JANUARY 2021 The idea for the January issue topic came about in a familiar way: Several survey respon- dents named stackup design as one of their biggest continuing challenges. Here are just a few of the comments we received about stackup problems: • Choosing too many layers for the stackup • Last-minute stackup changes • EDA tool's stackup functions not intuitive • Not understanding stackup limitations Stackup design may be the single most impor- tant piece of the puzzle in PCB design. Over the years, I've heard design- ers say, "If you get the stackup right, the design is almost done." Get the stackup right and you're on your way. With proper commu- nication between the fabricator and OEM, coupled with today's CAD tools' stackup func- tions, stackup design shouldn't be such a problem. But here's the thing: There's very lit- tle agreement about stackup design best practices. There are hundreds of tradeoffs that must be made, and each decision can cause havoc somewhere else in the stackup. Something as seemingly simple as changing copper deposi- tion to avoid bowing and twisting can cause trouble with controlled impedance require- ments. If you're using a North American fabricator for your prototypes and an offshore fabrica- tor for volume, you already know that some offshore fabricators can't match your proto- type company's "sweet spot" for via and trace sizes. This month, you'll read a few horror sto- ries about stackups that weren't optimized for their fabricator's processes. Much like opinions, everyone has a differ- ent method for doing stackup. Even though conventional wisdom says that your stackup should be finished at the beginning of the design cycle, not all design- ers follow this dictum. Some d e s i g n e r s w o r k w i t h their fabricator to create the best stackup, while others expect the fabricator to design the stackup for them. Many designers just assume that the CAM depart- ment is going to make a lot of changes to their stackup, and if they don't hear about these changes, no news is good news, right? In the end, it all comes down to the simple question of who owns the layer stackup: the OEM, or the fabricator? Many OEMs choose to The Shaughnessy Report by Andy Shaughnessy, I-CONNECT007 How Does Your Stackup Stack Up?

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