Through the years of seeing posts on thermal paste application patterns and precise amount calculations, I haven't found definitive info about how much excessive thermal past would hurt thermal conductivity. The popular opinion seems to be that too much paste (like using the whole tube of it provided in the cooler box) is very bad for conductivity, and it's a common object of "look at this noob's work" pictures.
For a very simple example of how I see it, if you squeeze a large round blob of paste right in the center of the cpu and screw down the heatsink on it, it will spread out the excess around the cpu lid where it just sits harmlessly. Because nothing gets perfectly aligned in real life, there won't be a weird closed space in-between the heatsink and the outer rim of the cpu lid with pressurized thermal paste causing significant decrease in thermal conductivity. At least, that's the logical conclusion I've come to. But I haven't really done any precise scientific experiments myself, and I don't have the tools or experience to measure it exactly.
I've only replaced the heatsinks twice in my life, but I've never had cooling problems, and my temperature ranges seemed to correspond with what I've looked up to be the average normal range.
My impression is that using less than needed is a lot worse (and starts to become dangerous due to possible overheating) than adding more (where the amount limits itself and can't do any harm).
Am I correct in my thinking that excessive thermal paste can't do harm (provided you screw down the heatsink appropriately)?
2 Answers
This is true that excess of the thermal paste will be squeezed out, and the gap (which defines the thermal impedance) will be the same, assuming that the mounting force is high enough and viscosity of paste is low enough. It might be necessary to apply a small rotating motions to help the paste be squeezed out.
Obviously the entire surface area between a heat sink and CPU thermal slab should be covered entirely, which might not be true if there is no excess in the paste amount.
However, the above assumes that the paste is not conductive electrically, or the CPU top doesn't have electrically exposed pads/parts in the area where the excess of paste gets squeezed.
2To much thermal paste can hurt your CPU quite a bit. Surprisingly thermal paste is not a very good conductor - it's just better then air.
Ideally you want the tip of the metal to touch the CPU, and the paste to fill in the microscopic troughs. A thicker layer applied will still have a thicker later of paste even after heatsink is applied, and will thus conduct away less heat.
Note that most cpus will work with bad thermal coupling - they will typically underclock to generate less heat and protect themselves from damage.
The general rule is a pea size amount in the center of the CPU polished thinly over top of the CPU and then apply heatsink.
