Quick question about sperm

So this weekend my boyfriend and I were messing around. (Sorry TMI) I was giving him a hand job and approximately 15-30 min later I was fingering myself with the hand that I was using to give him a hand job to arouse him. I had his pre cum on my hand when I was giving the hand job but will I get pregant even though it had dried on my fingers? He told me that once sperm dries then they are dead. Is that true? How long does it take for sperm to die?