I messed up. Bought an AMD A10-5800K APU and put it on an ASRock FM2A85X-ITX motherboard, then put that in an Apex MI-008 case.
Turns out that processor heats up to 75c straight off boot in that case, even with a fan installed on the side. Just not enough room for air. And I can't install anything besides the stock heatsink because there isn't room for a backplate on the motherboard.
Do you think downgrading to a 65w APU processor would be enough to fix this? Or am I looking at returning my case and getting a bigger one?
I am no expert but I would suggest you drop the CPU voltage to around 1V and the clock speed to 2GHz then slowly increase the clock speed until it becomes unstable. Its the other way round to elemein's suggestion but makes more sense in my head anyway. Another consideration is I don't know how much heat the GPU and CPU parts create or if you have similar options to adjust on the GPU side to a dedicated card. There no point droping 5W of CPU heat if the GPU creates 80% of the heat, I personally don't know what creates more heat but underclocking both will obviously be the better option. For more advice I suggest you start a new thread in the overclocking section with a title like "underclocking A10 to reduce heat".
Well, after fiddling with undervolting for awhile, I decided to just switch cases to the Cooler Master Elite 120. Same features pretty much in a slightly bigger case with better airflow, hoping that will resolve things.