Furthermore, it appears new 9th Gen Core chips like the Core i9-9900K will run quite hot. A Gigabyte Z390 OC guide recommends the use of a custom water-cooling and to adjust the TjMAX Temperature to 110°C:
The quote references their guide to achieve a stable 5 GHz overclock on all cores on the Core i9-9900K, which was cooled via a custom watercooled setup and a Vcore ranging from 1.3-1.4 V. GIGABYTE's internal testing thus indicates that these higher end, unlocked 9000-series CPUs will run incredibly hot if you wish to push them, and the soldered IHS may not be as effective in cooling these dense processors as we may have hoped.
Via: TPU