Digging Into Ivy Bridge's Overclocking Issues
Why do Ivy Bridge processors behave this way? How come 32 nm Sandy Bridge-based processors stay cooler, even at higher clock rates and voltage settings? Is this Intel’s Ford Edsel moment? We don't necessarily think so, but let's take a look at the factors that cause us to be concerned.
More Heat Output Per Mm2
The Ivy Bridge-based Core i7-3770K sports 1.4 billion transistors on a die area of 160 mm2. The Sandy Bridge die measures 216 mm2 and contains 995 million transistors. While we're comparing, we should also keep in mind that the HD Graphics 4000 engine occupies about one-third of the largest Ivy Bridge die. On Sandy Bridge, HD Graphics 3000 is closer to one-fourth of the package. In total, the CPU's area shrank by roughly 40%.
Now, let's approximate the surface area consumed by everything except graphics resources:
- Ivy Bridge: ~105 mm2
- Sandy Bridge: ~160 mm2
The Sandy Bridge-based Core i7-2600K has a TDP of 95 W, whereas the Ivy Bridge-based Core i7-3770K’s TDP is 77 W. Suddenly, it becomes a clearer that overclocking causes the smaller Ivy Bridge die to emit as much heat per square millimeter as Sandy Bridge. This naturally affects how much higher than Sandy Bridge we should have expected Ivy Bridge to scale.
New Transistor Technology
Another variable we've seen presented are the new tri-gate transistors. Intel claims that, in a typical use case, they use up to 50% less energy compared to a traditional transistor, as their three-dimensional structure of one horizontal gate together with two vertical ones effectively triples the field effect of the gate, which should reduce leakage current drastically.
We want to emphasize the phrase “typical use case” here because the more you overclock, the further away from typical you get. While we can confirm the claimed power reduction for typical use cases (just wait until we get to the benchmarks), at this point, it is not entirely clear how these transistors perform at substantially increased clock rates. Maybe they're just not optimized for the speeds an overclocked chip pushes yet. It might be necessary to wait for Ivy Bridge's successor, Haswell, to see if the limits of 22 nm manufacturing can be pushed harder.
Increased density and new transistor technology can certainly give rise to more heat. But we've seen these things before, and each time, cooling and packaging technology manage to cope with the resulting output. What could be holding Ivy Bridge back? It could be the fact that Intel decided to use thermal paste instead of the usual fluxless solder (you'll need to have a solid grasp of Japanese to understand the original article, or use Google Translate) between the CPU die and heat spreader.
Using a box knife (we do not recommend doing this at home), the author of the linked story pried the heat spreader away from the chip and was able to replace the cheap paste Intel used, trying out both OCZ Freeze Extreme and Coollaboratory Liquid Pro thermal pastes. The OCZ offering allowed for 1.55 V at 4.9 GHz, while the Coollaboratory material ramped up to 5.0 GHz, operating stably. This was accomplished even with air cooling, although the author did not use a stock heat sink, opting for a Thermalright Silver Arrow SB-E instead (Ed.: this information was pulled from the original Impress PC Watch site with Google's rather shaky translation of Japanese). If there is a smoking gun in this equation, we think this is it, especially considering that the researchers at Impress PC Watch managed 20% more efficient cooling.