You are correct, a putty or gel would conform better than the elastomer, I selected the elastomer because the thermal conductivity was low enough and this was a “easy” to implement method. There are lots of “better” methods, for instance a vapour chamber design, but they require a lot of engineering to implement.
As a matter of interest, the 820 already has a gell between the memory chip on top of the SOC and the shield. Unfortunately it doesn’t gain a lot for two reasons, 1) there is a small air gap between the SOC and the memory (as we have already pointed out) and 2) the metal of the shield is so thin it doesn’t really do a very good job of spreading heat, so we added the black sticker that is on top of the shield. The black sticker is actually a carbon nano-tube thermal conductor (developed for use in cell phones) which does a quite good job of spreading the heat. After all that it is only a 10% gain in thermal limited processing.
The silicon elastomer on the bottom is quite good at conforming to the “rough” shape of the components under the board (the hundreds of resistors, and capacitors). The balls under the SOC are really good thermal conductors, and the 10 layers of copper inside the PCB do a good job of spreading the heat over a larger surface area (better than the black sticker), so all you need to do is get the heat away from the PCB.
There are two ways to get the heat away from the PCB.
- the most effective would be forcing air across the surface of the PCB (a fan mounted at the side of the board blowing air across the board). This has several downsides, low reliability of the moving parts, the power consumption of the fan, and the noise. proper ducting would make this by far the best possible cooling with the least cost. This is why all graphics cards have fans, you need to send the heat somewhere.
- thermal conductivity from the heat source to the heatsink (the elastomer pads), and a heat sink designed for free-air dissipation. If you look at the heatsink on a graphics card you will see the fins are really close together, these are for forced air applications. The heat sink I selected has the fins far apart, this is designed for convection cooling. The advantage of this solution is it is still cheap, and ‘good enough’ while avoiding the noise of a fan.