The Skylake-X Mess Explored: Thermal Paste And Runaway Power

Skylake-X: The Current State Of Its Problems

In the wake of Skylake-X's introduction and disappointing results from our overclocking attempts, we put a lot of thought into the power and thermal issues plaguing Intel's highest-end desktop CPUs. These roadblocks boil down to a couple of salient points that we'd like to explore in as much depth as possible:

(1) Skylake-X at its stock settings can barely be cooled during normal operation. This is due to its power consumption being extremely high in some situations, and its thermal paste keeping waste heat from being dissipated effectively.(2) There’s barely any room for enthusiasts to overclock. Also, many motherboards limit Skylake-X CPUs further due to poor design choices, such as insufficient VRM cooling. Those looking for high overclocks need not apply.

Test Equipment & Setup

In an effort to suss out both points, we decided to grab one of the simpler LGA 2066 motherboards out there, build a bench table capable of supporting vertical operation, and start running Core i9-7900X through more tests.

Our experiments went two directions. First, we examined thermal sensor readings and where they were reporting heat. Second, we compared our infrared thermal measurements around the motherboard's LGA interface and VRMs to double-check the sensors' plausibility. This also allowed us to document the warm-up phase and how heat spread via time-lapse videos.

Finally, we’re interested to know if and how other on-board components are affected by the processor-imposed hot-spots.

We’re using the most current version of our motherboard’s BIOS to guarantee reliable sensor readings, along with stable operation. The new beta version of HWiNFO (v5.53-3190) was chosen for the same reasons.

The motherboard's CPU power supply employs a total of 5+1 phases, realized by an International Rectifier IR35201 dual-loop buck controller. It officially supports Intel’s VR12.5 Rev 1.5, and also apparently VR13. Kudos if you counted more regulator circuits; doubling of five phases allows two circuits per phase, reducing each VRM's load and spreading hot-spots out more evenly.

Each circuit has its own 60A IR3555 PowIRstage. These highly integrated chips combine the necessary gate drivers, high- and low-side MOSFETs, and Schottky diode in one package. In contrast with most MOSFETs, the IR3555 is able to read analog values for the built-in temperature sensor. So, how is it possible to also determine the temperature of hot-spots on the PCB without an IR camera handy?

MSI uses Nuvoton's NCT6795D Super I/O chip, which is able to collect and report a wide variety of sensor readings. One of these readings comes from a thermistor (see picture below) placed among the PowIRstage chips. This is why we chose the spot right underneath this thermistor, on the motherboard's back side, as the location for our video-based measurements.

Additionally, we'll check temperatures on the regulator circuits’ chokes and capacitors, as well as board temperatures all the way to the CPU.

Frequency Throttling & Emergency Shutdown

It’s important to understand that motherboard manufacturers deliberately add certain safety mechanisms to their designs. One example from our test platform is that a Skylake-X processor’s clock rate throttles to exactly 1.2 GHz if the thermistor reports a temperature of 105°C or more (see the MOS line in the image below). That frequency is maintained until the temperature drops under 90°C. Only then does it restore the processor’s full speed.

Even though the board material’s flashpoint (FR4) is significantly higher than 105°C, the recommended maximum temperatures for continued operation is between 95 and 105°C. Otherwise, the motherboard might suffer from dry-out, bending, or hairline fractures in the conductor paths. This safety-consciousness is a welcome trend, to be sure.

Enthusiasts using Intel’s Extreme Tuning Utility (XTU) can find this setting under Thermal Throttling: Yes, in yellow. But what about other settings, such as Motherboard VR Throttling?

First, a bit of background. Without the corresponding MOSFETs with temperature sensor output (mostly as voltage) the IR35201 buck controller provides its own temperature readings. Long ago, it was supposedly possible to read voltage converter temperatures as VRM1 and VRM2 for graphics cards with certain PWM controllers. However, the temperature values weren’t determined by temperature sensors, but by the chip measuring itself, because the MOSFETs being used didn't have sensors inside.

In our case, we get the reported values from within the PowIRstage. After all, the values under VR T1 and VR T2 are significantly higher than we'd expect.

The PWM controller can only guarantee a stable and safe power supply if all components stays within its technical specifications. This means that a maximum temperature setting is necessary. Here, that's 125°C. At and above 125°C, XTU’s Motherboard VR Throttling: Yes setting turns yellow and the CPU’s frequency throttles to 1.2 GHz. At 135°C, the motherboard simply shuts down to avoid hardware damage.

The CPU protects itself as well. It estimates the temperatures for its cores and package based on readings from different integrated digital temperature sensors (DTS). The precision of those estimates increases as the sensors get hotter. Under 40°C, their measurements are meaningless. However, they're very accurate above 80°C, which is where it counts. If the core or package temperature gets too hot, throttling ensues.

The package temperature includes the integrated voltage regulator’s leakage currents. The IVR is responsible for providing different voltages to subsystems within the CPU. High overclocks and manual voltage increases can cause the temperature limit to be exceeded unexpectedly. Tools might not be able to reliably capture this effect, which means that the CPU might throttle without any reason that would be visible to the user.

Observation #1: It’s well-known that the CPU might throttle its clock rate due to its core or package temperatures being too high. However, the Super I/O chip might also throttle it due to VRM temperatures being too high. Finally, the PWM controller can also cause throttling if it gets too hot, since this could result in a dangerously unstable power supply. Moreover, it’s an urban legend that the PWM controller can report VRM temperatures.

The Test System

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