If the conjunction is a coordinator, then comma is needed provided what it links are independent clauses.
If words, phrases or subordinate clauses are linked, then comma is not needed.
conjunctive adverbs are also conjunctions: however, for instance.
They are not strong enough to link two independent clauses only separated by a comma --Semi colon wanted.
subordinating conjunctions:
compare the punctuation
Provided it will not rain, we go to the city centre to do some shopping.
We go to the city centre to do some shopping provided it will not rain