Which is more a relative pronoun than being subordinate conjunction, but yes we can use it as a subordinate conjunction.![](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcShHKZHSWUER9vQzp_t6kRwysUEyizCJNMvlkf7_bawsjCw_3ar2cS56x4&s=10)
It's used as the subject of dependent clause, but subordinating conjunctions are followed by the subject of their clause.
Example:
I see the litter which came yesterday.