If you apply too little Thermal Paste it may not spread enough to cover the CPU cores.
Here is an example of applying too little Thermal Paste.
The coverage area falls a bit short. It does not quite reach the edge.
However, this will still do the job.
Note: It is better to apply a little less than the ideal amount of Thermal Paste, rather than applying too much which may cause it to spill over the sides.
