Update 2/6/2019: Intel confirmed to us that some Core i5-9400F processors do come with pTIM (polymer TIM - a.k.a. thermal grease), but that can vary based on the location of manufacturing.
Sources close to the matter also tell us that the company will transition the Core i5-9400F and Core i5-9400 models to solder TIM (sTIM) entirely in the near future. Naturally, that means there will be no need to differentiate between the two products on the packaging. All "K" series models in the Core i9 and i7 series will continue to come with sTIM, as well. We also learned that the company will use pTIM for all Core i3 "F" series models indefinitely.
On another note, our resident overclocking expert Allen "Splave" Golibersuch has a Core i9-9900KF in hand for liquid nitrogen overclocking. As you can see from the images above, in contrast to the Core i5-9400F pictured below, this processor comes with the standard 9th-gen integrated heat spreader design.
Intel's new "F" series processors have spurred plenty of criticism from the enthusiast community because even though the processors come with a disabled graphics unit, Intel still assigns the same recommended price points as their fully-operational counterparts. Now it turns out the company is also discarding Solder TIM (sTIM) on the new models, instead returning to its oft-maligned thermal grease.
Much to the delight of enthusiasts, Intel's ninth-gen processors marked the return of solder interface material (sTIM), but prolific chip-detective momomo_US uncovered photos of a delidded Core i5-9400F processor without sTIM.
Recent motherboard firmware updates posted to MSI's website have confirmed support for two Core i5-9400F steppings. According to the recent findings, the P0 stepping denotes an eight-core die with sTIM and has two cores disabled, while the U0 stepping (pictured above) has a smaller six-core die that comes with standard thermal grease. The U0 stepping also comes with a thinner PCB and the same IHS (integrated heat spreader) design found on the eighth-gen chips, as you can see in the image below.
Solder TIM improves the thermal transfer efficiency between the die and heat spreader, which lowers operating temperature and often unlocks a higher overclocking ceiling. Lower temperatures also help during stock operation–they typically enable longer turbo boost durations. sTIM is more expensive than the standard thermal paste that Intel has used for several generations of its processors, so a return to the thermal grease is a cost-cutting measure that Intel may deem acceptable given the locked multiplier on the chips.
Intel has also long preached the perils of solder TIM, which can lead to shorter lifespans due to microcracks in the solder TIM that form during heat-induced expansion and contraction. That's more of a concern with smaller die, such as the six-core die on the -9400F.
These same effects also can break the solder bumps that connect the substrate and the PCB, leading to premature chip death. Intel has never officially disclosed solder-induced chip failure rates (to the authors' knowledge), but the increase in the failure rates is thought to be within a single percentage point. Of course, when you punch out north of a million die per day like Intel does, even a 1% increase in failure rates equates to a huge hit to the bottom line.
But enthusiasts still love solder TIM for its superior cooling effects, and AMD's adoption of the material has made it a must-have for enthusiast-class chips.
Whether Intel will stick with solder for the other F-series processors, such as the unlocked Core i9-9900KF, is an open question. It also remains to be seen if Intel will bring the P0 stepping to market (all known retail models carry the U0 stepping), or differentiate between the two different steppings on retail boxes. Currently, that doesn't appear to be the case. We've pinged Intel for more details and will update as necessary.