The other posters explained it all. If no delays, you probably are fine. If you have a 1.5 hour delay and must clear customs before rechecking luggage, things can become tricky. Travelling hand luggage only helps a lot here. But then I had this experience of a 2.5 hour difference in Manchester between my arrival and departure on separate tickets, and due to a major delay of the first flight things totally messed up. I had sheer luck that the 2 tickets were with the same alliance and the crew on the delayed flight arranged a rebooking of my next flight, while they had all the right to leave me on my own. Narrow escape hence, and I side with Often1 that in such cases building in an overnight is the safer bet.