Even the internals of the RT-AC66U is very much identical with the RT-N66U. However, there is no micro-SD card slot on the board as seen on the N66U. Even with the large heatsink, the router runs fairly hot. We've measured the router's temperature during its idle state to be a lurkwarm 55'C.
Underneath the two RF shielding cans is the business end of the router.
To the left of the main Broadcom BCM4706 600MHz MIPS32 74K CPU is a pair of 128 MB Samsung RAMs (256MB). The chipset to the bottom right corner is the Broadcom BCM53125 which handles gigabit switching for 7-ports.
Over to the right we find the two wireless radios – a Broadcom BCM4360 (3×3 802.11ac radio transceiver) and Broadcom BCM4331 (3×3 dual-band 802.11n radio transceiver). Judging from the configuration and settings, it is most likely that the BCM433 handles the 802.11n @ 2.4GHz band while the BCM4360 handles both 802.11ac and 802.11n @ 5GHz band.
There is a large aluminum plate covering the bottom side of the board, with no thermal pads between the plate and the board. The heat is most likely channeled from the board through the plate via the brass standoffs. We think the plate also acts as an RF shielding.
On the left side of the board are a couple of DC-DC switching regulators.
Interestingly there are two flash NAND chips on the AC66U. A 128MB Samsung chip sits in the center and the small chip on its left is small Macronix 2MB.