The Dragonboard 845 design left me feeling very disappointed. Especially when considering the extremely high price, combined with how much potential the platform has that is, frankly, wasted.
Unfortunately, you aren’t going to achieve adequate cooling with that design. Its a PoP configuration, which is supposed to implement cooling through the copper in the PCB. In a good design, like the Dragonboards 410 and 820, you can just stick the board to a big heatsink with a thermal pad. In fact, if you look at the 410’s board, you can even see a big gold square on the bottom – that surface is there specifically for cooling.
With the 845, you have the CPU barricaded in. On one side you have the RAM blocking heat flow, and there are so many EM covers over the board, that you will never get heat out of it that way. Putting a heatsink on the RAM will help… a little. But with a CPU that powerful, you are going to find yourself quite disappointed no matter what you do.