IMO Deadlifts are definitely the best exercise for building a thick back. I do 4 sets of 4-6reps with over 400lbs once per week and I never have any problems, anymore. I've learnt that to avoid injury it is crucial to create and maintain a "block" throughout the entire set, by this I mean take a deep breath, tighten your core and never round your back during the movement. You can take another breath when you're in the standing position but I don't recommend it if possible because this compromises the "block" that you're trying to maintain. Also remember that if you find that you start rounding your back at all then you're going too heavy!
If you're methodical with the execution of your deadlifts it can be your best friend when it comes to bodybuilding but it can also be the opposite if your sloppy.