Heat Sink from this bundle
As others already stated, the heat sink is suboptimal. It has a flat contact surface with a bumpy coating. The CPU is slightly higher (measured ~0.25mm) than the RAM chips. Due to this, there’s a small gap between the heat sink and RAM chips.
I’m waiting for thermal conductive pads atm. They are not as good as thermal paste, but should bridge the gap between heat sink and RAM. Additionally, I’ll remove the bumpy coating from the contact surface with fine sand paper.
Raspberry Pi PoE+ hat
I’ve ordered the Raspberry Pi PoE+ hat, as it is listed as compatible with the Rock Pi X. It is and works. Though, the hat comes with 9mm through-hole spacers. Due to this, you’ve two problems. First, you can’t mount it additionally on the heat sink. You need different spacers (no through-hole, needs screw on one end). Second problem, the spacers are too short. If you use them, one SMD part touches the HDMI port. I’m waiting for 10mm spacers and hope that they are sufficient.
Cheers, Daniel