Why is Ivy Bridge so hot and bothered?

If you read our Ivy Bridge coverage carefully, you'll know that we observed some rather high temperatures when overclocking the Core i7-3770K. With a single-fan air tower, our chip ran at a reasonable 50-60°C when clocked to 4.4-4.5GHz at its default voltage. However, when we pushed to 4.9GHz on 1.35V, the temperature soared past 100°C. Other reviews have observed similarly scorching temperatures, so it's not just our sample.

As we noted earlier this week, the Core i7-3770K's power consumption increases dramatically when it's pushed to 4.9GHz—as it should, given the proportional relationship between CPU power consumption, frequency, and the square of the voltage. The additional power is dissipated as heat, which contributes to those high temperatures.

The question is: why does Ivy appear to reach higher temps when overclocked than Sandy Bridge? Is it just the additional power draw being turned into more heat? Is it a thermal density issue, since Ivy's heat output is concentrated into a much smaller die area than Sandy's?  Or is it something else?

Overclockers.com points out there's another factor to consider: the interface between the CPU die and heat spreader. The site claims Ivy Bridge employs thermal paste between the die and heat spreader, which differs from the fluxless solder that lurks under the lid of Sandy Bridge CPUs. It's possible that a less efficient heat transfer interface could be leading to higher temperatures for Ivy.

Curious, we asked Intel about the interface between the Ivy Bridge die and the heat spreader. Intel has confirmed to TR that Ivy uses a "different package thermal technology" than Sandy Bridge. The firm stopped short of answering our questions about why the change was made and how the thermal transfer properties of the two materials compare. However, Intel claims the combination of the new interface material and Ivy's higher thermal density is responsible for the higher temperatures users are observing with overclocked CPUs.

Intel also points out Ivy Bridge has a higher TjMAX specification, which governs when the CPU starts throttling in order to protect itself from heat damage. The cut-off for the Core i7-3770K is 105°C, while the 2600K starts throttling at 100°C.

Intrigued, we decided to perform some basic testing, comparing overclocked Ivy Bridge Core i7-3770K and Sandy Bridge Core i7-2600K chips on the same motherboard with the same cooler.

To keep things even, we aimed for similar levels of power consumption for the two CPUs, which should translate directly into similar levels of heat to be dissipated. Fortunately, we were able to reach rough power parity at equal clock speeds. At 4.9GHz, the Ivy-based system draws 236W at the wall socket, and the Sandy rig pulls 231W. The CPUs require similar voltages, as well. Ivy needs 1.368V to hit 4.9GHz, while Sandy takes 1.381V, according to CPU-Z. (For what it's worth, CPU-Z reports the Core i7-3770K's default voltage as 1.024V, while the 2600K registers 1.240V. We need to increase Ivy's voltage by about a third to hit 4.9GHz, but Sandy demands only a 12% boost.)

Turns out the difference in temperatures between the two was quite a bit wider than the 5°C difference between the thermal throttling thresholds. Our Ivy CPU flirts with 100°C at those settings, while the Sandy Bridge chip is currently running at about 80°C in the same test system.

It appears that, all other things being equal, Ivy Bridge runs hotter. That almost certainly means the cooling is less efficient.  We can't say with certainty whether the problem is primarily the difference in thermal interface materials or in power density—or perhaps equal parts each—but it does look like Ivy Bridge chips present a new sort of challenge for would-be overclockers. Those wishing to push toward the 5GHz mark might be better served by sticking with their older 32-nm processors, at least for the time being.

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