USB 3.1 to Ethernet bridging — what the function means for the BOM
The LAN7801T-I/9JX is a single-chip USB 3.1 to Ethernet bridge controller — it presents a USB 3.1 SuperSpeed device interface on one side and a 10/100/1000 Ethernet MAC/PHY on the other, so a host with a USB port gains a wired network connection without an internal PCIe or MII bus. This is not a USB hub or a PHY-only part; it is a full bridge that handles the USB protocol stack and the Ethernet framing in one package, which means the host CPU offloads the network traffic processing.
Package and rework — 64-VFQFN with exposed pad
The 64-VFQFN exposed-pad package (Microchip calls it 64-SQFN, 9x9 mm) has a large centre thermal pad that must be soldered to a PCB ground plane for both thermal relief and electrical ground. Will it survive the hot air? Yes, but the exposed pad needs a preheat soak at 150°C to avoid warping the package during removal.