"One of the major features Intel has claimed with for its new X-Series of CPUs (both the Skylake-X and Kaby Lake-X) is that they’re all unlocked, with higher TDP ratings and a top-end platform to pair with the chips, dubbed X299. However, according to overclocker der8auer, there’s a serious set of problems with every X299 board he’s tested to date.
The first issue is that at least some X299 boards are only shipping with a single 8-pin power connector for the CPU. An overclocked Skylake-X pushing above 4.5GHz can pull more than 300W. According to his tests, the 8-pin cable temperature on his open testbed is hitting 65C in an air conditioned room.
That’s significant, because most people don’t run their PCs in open testbeds, may not have air conditioning, and may not have proper cooling at the bottom of the chassis (I confess, until this video, I’d never actually considered how hot an 8-pin power cable gets under load.) He suspects that in a closed-case configuration an overclocked Skylake-X with just one 8-pin cable could be hitting 90C-95C and recommends against any Skylake-X motherboard limited to just one 8-pin connector.
But the larger problem is the VRM design. Every motherboard der8auer tested — and he mentions three by name: the Gigabyte Aorus X299 Gaming 3 motherboard, the Asus Prime X299-A, and the MSI X299 Pro Gaming Carbon — were tested with the same CPU, a Skylake-X overclocked to 4.6GHz at 1.25v. According to der8auer, this chip is known to hit 5GHz and was chosen for these tests precisely because its behavior had been well-characterized. The cores were tested in the non-AVX version of Prime95 for 10 minutes and temperature readings were taken from the front and back of the motherboard."
Note that der8auer isn’t certain his temperature probe was in the proper spot on the back of the Asus motherboard, as the gap between front and back temperatures is usually larger than what he observed. But either way, these temperatures show extremely high VRM heatsink temps and it has a direct impact on the CPU’s ability to hold its overclock. As the VRM temps rise, the CPU starts underclocking to 1.2GHz. The MSI board’s better temperatures are apparently explained by more aggressive throttling when the CPU is under load.