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Keeping your IC cool with upside-down thinking

There's no getting around it: many ICs need some form of heat sinking. Once you get into the regime where the unaided IC package can't be kept cool via airflow (whether forced or not), IC and package designers have two starting-point options. One is to have users put a heat sink on top of the package, along with a thermal pad (or even messy grease!) to maximize thermal conductivity; the other is to provide the package with a thermal slug underneath, which in turn uses the copper of the printed circuit board (PCB) as a heat sink. (Of course, there are even more advanced choices such as heat pipes, spreaders, or fluid cooling; we're not talking about those.)

Using a top-mount heat-sink is straightforward, but the thermal path from die to package top is not as effective as the bottom-side slug. But, while the second option is thermally superior and commonly used, it's a bit of a cheat and imposition on the PCB design and layout. In effect, the IC is saying “I'll be OK if you give me so many square inches of the board for cooling” but it also implies that nearby ICs cannot also put thermal demands on the PCB. In other words, there's a notion of real-estate demand and copper exclusivity that may not be practical.

That's why I was impressed when I saw what the Apex Precision Power team at Cirrus Logic did for thermal management on their latest motor-driver ICs, the Apex SA306-IHZ family. They designed the package so the user mounts it upside down on the PCB, in the middle of a square PCB cutout. Then a standard heat sink is attached to the bottom of the IC, which is on the top side of the PCB, and held down with two screws, as shown:


The people at Apex/Cirrus said that punching the square hole was no problem for the PCB fabricators, and the IC itself goes on the board using standard pick-and-place equipment, just as any IC does. So this simple upside-down approach neatly solves several problems: it provides more effective heat sinking, it uses standard heat sinks, it makes no thermal demands on the PCB copper, and it requires no special handling except for the screws that hold the heat sink in place, a fairly modest requirement.

This type of simple, clever solution to an ongoing problem is the kind of elegant engineering that impresses me, since it solves a problem with minimal or no burden on the user. Unfortunately, many design problems don't have this kind of nice, “we should have done this before” solution (at least, not that we have found yet), but when it does, it's a joy to see.

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