What type of screws are used for computer fans?

"Plastic thread cutting screws" is the technical name and are not classified other than by diameter and length.

Like wood screws they do not designate a thread per inch like other screws or bolts

I have a small drawer full of them I have salvaged from discarded items, plastic thread cutting screws are handy to have around since most everything is plastic these days.

In most stores and retailers you'll find them exactly under that description.
"PC fan screws", "Case fan screws" etc'.

If this is a more academic question, I'd love, too, know if they have a special type name, but I doubt it.