I sometimes need to explain my approach to thermal design. In many cases, as many readers may know, thermal design takes a back seat to electrical and even mechanical design, and consists mainly of figuring out which heat sink to slap on a hot chip. The "thermal solution" bails out the lack of a good thermal design process. But at a certain point, this approach fails, and much scrambling and money-spending results. So here are my thoughts on a good thermal design process.

You can think of the process as being somewhat hierarchical; if that term brings to mind  strict inflexibility, maybe "from the outside in" is the better way to think about it. Generally speaking, there are three scopes of analysis:

• System air flow (air is the ultimate heat sink, after all -- yes, even for liquid cooled systems!); this covers total air flow requirements, number and size of fans, vents, and so on
• Local air flow: does the air circulation make it to critical components? are there any airless hot spots?
• Component level heat balance: what thermal paths exist, and which can be enhanced, so that the adequate air flow from the previous scope can carry away the heat generated by the component.

The reason the "outside in" strategy is so important is that most often, if the global air flow is inadequate, no component level enhancement can solve the thermal problem. Lack of air flow on the system level is just insurmountable, and needs to be fixed before any other strategy can work. This is true of both forced and natural convection systems; in the fan-less case, the size of the system becomes a critical design parameter, much to the distaste of the folks who want to have their electronic devices be as small as possible.

Another reason the "outside in" strategy is important is that very often, acoustic noise is a criterion for the final product. Making a system quiet after the fact -- well, if you've seen it done, I'd like to know how!

To summarize, the "outside in" thermal design strategy takes care of the biggest bottleneck -- the "ultimate heat sink" --  first. Once that is taken care of, the other issues are usually solvable.