Factory Imposed Limits: Heat Spreaders, TIM & The IVR
One reason for the cooling problem is Intel's use of inadequate (but arguably much cheaper) thermal paste instead of indium-based solder. Although we can debate the durability of solder over time, particularly as it relates to CPUs with small dies, we have seen processors of different sizes operate stably and error-free over many years with solder between the die and heat spreader.
Moreover, thermal pastes have their own long-term stability issues. Over time, the oils in these materials separate from the solids, introducing air gaps between the surfaces and increasing thermal resistance. This effect is different in all pastes, but it can't be prevented completely.
Why is this such a big deal to us? The following curve from our Core i9-7900X review, which we generated with a very high-end cooling solution, shows clearly that waste heat is dissipated poorly and inadequately with paste between the die and heat spreader. What actually worked effectively for a 91W chip like the Core i7-7700K now leads to a thermal bottleneck.
In the end, the following graph represents the glaring temperature differences between the heat spreader on top and cores underneath. We were shocked in our launch story and remain so today.
Although we're using some of the highest-end and most expensive cooling hardware available, we still measure up to a whopping 71 Kelvin difference between the cores' reported temperature and the heat spreader's top. Obviously, a more mainstream closed-loop liquid cooler under full load would look quite silly.
Observation #2: The dissipation of waste heat is hindered by the CPU's construction and Intel's deliberate decision to use thermal paste between the heat spreader and die. Regardless of how much pressure you use or how cold you can get your heat sink, you'll never realize Skylake-X's potential the way it's currently configured. Intel applies a thermal brake, favoring longevity and sacrificing performance.
Now, you might be tempted to remove the heat spreader and replace Intel's thermal paste with something better. But that's simply not a realistic course of action for most enthusiasts. It takes a special tool, a steady hand, and some prior practice. Of course, the process also obliterates your warranty.
It'd be even more extreme to leave the die exposed and use a good torque screwdriver to minimize the possibility of mechanical damage from a non-uniform/excessive load. That's still a risky move, though.
In the end, de-lidding is one solution to this cooling bottleneck, though it lacks mass appeal. A certain contingent of enthusiasts will try their hands at it regardless, and we can only caution that you consider the risks first.
Skylake-X's Integrated Voltage Regulators
Intel's Haswell and Broadwell designs employed a Fully Integrated Voltage Regulator, incorporating power delivery onto the package/die. FIVR was to simplify motherboard layouts by consolidating five platform-based voltage regulators down to just one. But the implementation created some issues for overclockers, too.
Skylake-S did away with FIVR. Now, Skylake-X re-incorporates integrated voltage regulation, though its IVR is linear, rather than switching.
What's this all about? Well, the motherboard's external voltage converters do not deliver the Vcore, as in Kaby Lake-X, but rather an intermediate voltage (VCCIN, or eventual CPU input voltage) as input for Skylake-X's IVR. If you take a look at the picture below, you can see the point for measuring VCCIN for Skylake-X or Vcore for Kaby Lake-X. The CPU determines which voltage is delivered from the VRM, and it can be between 1.6V and up to a maximum of 2.55V.
Anecdotally, it was this hybrid approach that led to so many CPU deaths in the run-up to Intel's launch, as folks switched from Skylake-X at 1.8V to Kaby Lake-X and applied far too high of a voltage.
Based on the lower intermediate voltage VCCIN, delivered by the VRM (which does the biggest part of the voltage regulation job), the IVR generates voltage for the cores (Vcore) and all needed sub-voltages for the last-level cache, mesh topology, the I/O (VCCIO), the system agents (VCCSA), and the PIROM (VCC33).
This intermediate voltage VCCIN is controlled by the CPU via the SVID (Serial Voltage ID) bus, and the R35201 controller also supports Intel's latest VR13.0 PWM. This VID-based voltage is similar to the former loadline of the Vcc of older CPUs.
Skylake-X's Maximums & Extreme Overclocking
Intel specifies a TDP of only 140W for existing Skylake-X CPUs. The maximum current is an incredible 190A (peak, for <2ms), but also cut down to 73A for the Thermal Design Current. The Thermal Design values (for wattage and current) are defined by Intel to clarify the VRM load and cooling requirements under a constant load. The maximum package power is set to 297W. Tests with higher values cause the motherboard to shut down at 365W.
Observation #3: Power consumption for VCCIN beyond 300W has nothing to do with realistic overclocking, since the CPU is loaded beyond its thermal spec well before that point. For long-term stability, a maximum value of up to 250W is more realistic (even if it's still quite high).
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