Electronic – Why are DIMMs not equipped with a heat sink like a CPU


I know that a DIMM is composed of a set of chips that contain control logic managing the decode and prefetching memory operations. According to a product specification, I found that newer RAM works at a high clock rate (> 1Ghz) that is comparable to some CPUs. And that's what made me wonder why only the CPU is equipped with a heat sink, and not also the DIMM, besides a certain high clock rate (and thus the amount of heat needing to be cooled).

Best Answer

You're assuming that the power dissipation is directly related to the clock rate. That's true but there's more.

Suppose I have this chip A where only 10% of the chip area (die size) runs at the highest clock rate. Compared to a chip B of equal size where 100% of the circuits are running at the high clock rate, chip A would dissipate only about 1/10th of the power that chip B dissipates.

My point: not only the clock rate matters, also how much of the chip is actually running at that clock rate.

For a DRAM chips (PC DIMMs use DRAM) most of the area on the chip is DRAM cells (obviously) and these are run at a significantly lower speed than the external clock rate. The DRAM controller access the chips in parallel and in a sequence so that this lower speed is somewhat compensated for by parallelism.

On a CPU a much larger part of the circuits actually run on the maximum clock rate (depending on how busy the CPU is of course) so it is bound to dissipate a lot more power than a DRAM chip where only a small part of the chip is running very fast.