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104 DESIGN007 MAGAZINE I SEPTEMBER 2020 "I tell ya, I don't get no respect," was an opening line made famous by one of the fun- niest comedians of the last century, Rodney Dangerfield. And while decidedly not funny in the world of electronics, thermal engineering has, unfortunately, often been treated with less respect than it deserved. Dealing with the heat generated by electronics was often not given full consideration until after the design was completed and prototyped, and the problem manifests as a failure. It's a simple fact that where there are elec- trons flowing (superconductors aside), there is heat. "How much heat?" is the salient ques- tion, as well as, "Will it be a problem?" The next question is how to deal with it. To the first point, heat in electronics is almost always a problem. The reason is that there is an inverse relationship between heat exposure and the reliability of electronic devices. Integrated cir- cuit transistors are vulnerable to failure due to diffusion of metals through insulators caus- ing shorts. In short form, the higher the heat, the lower the reliability. Thus, keeping devices cool is a vital objective. There is another reason to keep things cool, however, which is to mitigate the mechanical strain that is manifested when devices having vastly different coefficients of thermal expan- sion (CTE) are intimately joined, such as a BGA soldered to a PCB. It is a well-known fact that solder joint failure is a leading cause of assembly failure. There used to be a saying employed by ther- mal engineers that helped succinctly frame both the challenge and the solution. "It all goes back to air," and that has remained true since it was first observed and uttered, perhaps as early as the first vacuum tube amplifier. The challenge that has remained ever since is how to get the heat generated by electronics "back to air." There are multiple ways that heat can be man- aged. At the earliest steps, the choice of tech- nology is important. To provide some perspec- tive, the world's first electronic computer, the ENIAC, had some 30 separate computing units Thermal Management: Electronic Technology's Rodney Dangerfield Flexible Thinking by Joe Fjelstad, VERDANT ELECTRONICS