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Design007-Sept2020

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14 DESIGN007 MAGAZINE I SEPTEMBER 2020 me through, and I use those even now to then. Fundamentals don't go away. Shaughnessy: What do you see when you look at the industry now? It seems like there's a lot more focus on thermal. Do you see that happening? Jouppi: The focus on thermal has always been there for the indus- tries in which I worked. The mechanical engi- neering thermal side worked for a long time on getting a component database that was univer- sal. There were thermal models, a number of different modeling structures for components that people tried to get as a standard struc- ture to work from, and a lot of work was done there, but I didn't see everyone buying into it to where it became a commonly used method for all companies. It would have been nice to have that standardized in such a way that it would address both earth and space environments. The problem that falls out of that is a lack of understanding of what the thermal resis- tance values really mean. There are a lot of the thermal resistance values defined in the JEDEC standards. They've done a really good job, and those people realized the importance of each specific thermal resistance associated with the component, such as the component junction-to-case, junction-to-board, junction- to-ambient, etc. They realized that, depending on how you were mounting a component to the board—as well as how many copper planes were in the board—you'd get a different tem- perature response for the heat transfer. They came up with these different models for look- ing at different board constructions, which was the same kind of concept that I was trying to introduce with IPC-2152 released in 2009. That leads to something else that I really would like people to pay attention to—trace heating. I wanted to concentrate on the pow- er that's dissipated in the conductors. A lot of people do not account for the power losses in the electrical conductors, and I've been pushing that everyone should know the conductor loss- es. There are a couple of reasons why managing power dissipation in traces is important. There's a design rule that deals with paral- lel conductors. When you have a bunch of traces laying up next to each other—they can be side-by- side or through the stackup—and you have multiple traces that are conducting current, there's a rule for summing the currents and summing the cross-sectional ar- eas to size the traces properly. That rule is very difficult to assess in most designs, and the re- sult is under-sizing traces. The point is that the charts—both IPC-2152 and the old charts, meaning IPC-2221 and its predecessors—the trace temperatures in those charts represent a single trace being powered up all by itself. When you have parallel con- ductors, if you look at a really dense board and you have a lot of current running, you have traces running all over the place, and it's next to impossible to employ the design rule of siz- ing parallel conductors properly. To avoid that problem, you can manage the power losses in the traces and map that power into the board for thermal analysis. You have the best of all worlds. You also begin to further understand if more detailed thermal modeling is needed in certain areas of the design. Shaughnessy: Are the EDA tools companies starting to pay more attention to thermal? We see FloTHERM from Mentor, Johannes Adam has an Altium interface for his TRM thermal management tool, and there are a few others. Jouppi: Thermal has evolved a lot over the years. Attention to thermal issues has always been there; it just takes time to develop the tools with a set of resources. The cool thing about this kind of design tool, especially for the big companies, is that they can put the resourc- es wherever they want, and they can get the tools out relatively fast. The key is to have a community that is asking for it. I've worked directly with some big and small companies, asking them to develop certain tools, Mike Jouppi

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