Why is my controller not turning on after disassembly/reassembly?

I have taken apart two controllers to replace the thumbsticks. Both times after putting the controllers back together, they will no longer turn on or charge. I have double checked and plugged in/out all ribbon connections and the battery connections. Also, when the battery is plugged in, the circuitry directly to the left of the battery is very hot (hot enough to make you pull your finger away after touching it).

Any idea on what is causing this? Everything worked fine before replacing the thumbsticks.