Why is the logic board heating up so fast?

I replaced the housing frame on an iphone 8 plus. After turning it back on. The logic board gets hot as it turns on. It reaches 107° F in a matter of 15 seconds. It can reach 115° F. The issue is above where the sim card goes. Most likely the A11 chip. But I’m not sure. The phone was recently reset and has no apps besides the built in ones. So the phone isn’t pushing any hard apps. I’ve tried two batteries. The only flex cables connected are the battery and screen. What causes the overheating? What do I have to do to fix it?