go to pieces vs break down

go to pieces and break down both are verbs.

Word	Noun	Adjective	Verb	Adverb
go to pieces	No	No	Yes	No
break down	No	No	Yes	No

As verbs, break down is a hypernym of go to pieces; that is, break down is a word with a broader meaning than go to pieces:

Other hypernyms of go to pieces include lose it, snap.

go to pieces (verb)	break down (verb)
lose one's emotional or mental composure	collapse due to fatigue, an illness, or a sudden attack
	stop operating or functioning
	separate (substances) into constituent elements or parts
	make a mathematical, chemical, or grammatical analysis of; break down into components or essential features
	make ineffective
	cause to fall or collapse
	lose control of one's emotions
	fall apart