WLCSP
This package is very small and cheap to produce. It also has benefits due to the lack of bond wires. Bond wires increase parasitic inductance, and can be a problem for high speed designs.

The downside is that they are very tiny!

The last 5 layers of the Skywater PDK define the layers necessary to build the WLCSP package.
Course feedback
A great course to really learn about Analog ASICs and electronics in general, great material and support community.
Diego Satizabal (analog course)