So, sometimes people complain about the relatively weak thermal interface material on the new Intel processors. Especially compared to their HEDT counterparts, which have a solder based TIM, it seems that it's a low-balling approach to use plain thermal paste on a CPU that still costs over USD $400. Well, luckily we have Der8auer who takes the time to explain the problems with solder and even tries to solder TIM on the 6700K Skylake. This is a good read, I promise!
The gist of the article comes down to this: micro-cracks in the solder interface puts the CPU at higher risk of damage earlier in its total lifespan than conventional thermal paste does. In addition, mining the materials required for preparing the CPU for solder thermal interface (like gold and indium) is expensive and ecologically not that great. Regular ol' thermal paste may perform worse, all things considered it's a better choice especially for small sized processor dies.
Please have a look at Overclocking.guide for the full article and scientific background information. Amazing research!