Thermal pads:
Thickness & hardness (compress ability) -- is critical
The heatsink Must make contact with the GPU die, thermal paste Must cover the entire GPU die.
Unlike the CPU, that is NOT a heat spreader on the GPU, that is the actual die (chip)
You must keep the heatsink flat/level with the GPU die & not let it get crooked as you want full contact.
Hard pads will not let the GPU make contact, they will not compress
Thick pads ... prevent contact
Thin pads will never get contact or any compression & will not transfer heat.
The best testing it a trial assembly to make sure contact is being made.
(ANSWER to) - What are the thermal pad sizes for my 30 series graphics card?