The apu CPU and south bridge (apu1) or apu2/3 CPU (single chip) are passively cooled by heat conduction to the bottom of the enclosure. This assumes use of our enclosure. Third party enclosures should work if they are made from aluminium, and have a board standoff height of 5 mm.
Based on our measurements, at full load the CPU runs a few degrees cooler in the black (CASE1D2BLKU) or red (CASE1D2REDU) enclosures, compared to the plain anodized enclosure (CASE1DU).
There is a reason why heat sinks are usually black - better emissivity.
Spec for the thermal conductive pad:
Thickness: 0.5 mm
Thermal conductivity: 6 W/mK or better.
Note: Due to the high power density of the CPU, pads with lower thermal conductivity will not work well.