Non-conductive thermal paste has the advantage that it does not conduct electricity and won’t kill your hardware if you accidentally get a drop on a critical component. However, it’s not as effective at heat transfer as conductive TIMs like liquid metal.
Most mainstream thermal compounds are made of two materials: the bonding compound, which could be various types of silicones (silicone grease), urethanes, acrylates, solvent-based systems and so on; and the filler material that offers good thermal conductivity. Silver, aluminum, copper, zinc oxide and boron nitride are commonly used as fillers.
The best non-conductive thermal pastes combine the properties of both the bonding compound and the filler material to produce a high-performance product. Depending on the product and manufacturer, they will provide an electrical resistivity of 1013 O*cm and a thermal contact conductance between copper surfaces of roughness 15 m and higher.
You can determine if a certain non-conductive thermal paste is really non-conductive by holding your multimeter’s chords at two points where the product was applied and measuring the resistance between those two points. If the product is electrically conductive, it will register a high value between these points and will be prone to short circuiting your hardware.
Liquid metal TIMs are extremely good at heat transfer, but they’re also highly electrically conductive and can fry your computer if you get any on a sensitive component. Moreover, applying them requires focus and attention, since getting the wrong kind of TIM on your CPU or GPU will lead to unintended consequences.