Square chip with pads on the underside, no visible leads. The RP2040 is a QFN-56.
How it looks
Real image: CORE3588, rendered by KiCad from our board file
CORE3588 top side, the eFuse chip U1 from straight above.
1U1, a QFN-style chip (VQFN-10, 2 x 2 mm). The grey tabs at its edges are the ends of metal pads that sit flat underneath it.
2Pin 1 marker, so the machine and you know which way round it goes.
Real image: CORE3588, rendered by KiCad from our board file (angled 3D view)
KiCad 3D render of the same area, tilted.
1The same U1, seen at an angle: a flat block sitting on the board, with no legs sticking out at all. Compare the chips with bent legs at the top of the picture.
How it works
Where it fits
Smaller than SOIC for the same pin count, bigger and easier than BGA. Its tiny pads favour a flat finish like ENIG. The RP2040 is a QFN-56; on CORE3588 the eFuse U1 is a 10-pin QFN.