CPU board vs circuit board

CPU board and circuit board both are nouns.

Word	Noun	Adjective	Verb	Adverb
CPU board	Yes	No	No	No
circuit board	Yes	No	No	No

As nouns, circuit board is a hypernym of CPU board; that is, circuit board is a word with a broader meaning than CPU board:

CPU board: the main circuit board for a computer
circuit board: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities

Other hypernyms of CPU board include add-in, board, card, circuit card, plug-in.

CPU board (noun)	circuit board (noun)
the main circuit board for a computer	a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities