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 recommends these processors sell at the same 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 much-maligned thermal grease.
Much to the delight of enthusiasts, Intel's ninth-gen processors marked the return of solder interface material (sTIM), but prolific tweeter momomo_US uncovered photos of a delidded Core i5-9400F processor without sTIM.
Recent postings to MSI's website have confirmed firmware support for two Core i5-9400F steppings. According to the recent findings, the P0 stepping denotes an eight-core die with sTIM, 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, 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–it typically enables 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, which Intel may deem as acceptable given the locked multiplier on the chips.
Whether or not Intel will stick with solder for the other F-series processors, such as the unlocked Core i9-9900KF, remains an open question. It also remains to be seen if Intel will differentiate between the two different stepping on retail boxes, though currently it doesn't appear that is the case. We've pinged Intel for more details and will update as necessary.