Left iPhone 6s on heating pad accidentally

Hi, I fell asleep on the couch last night and my iPhone 6s ended up on the heating pad. I don't know how long the phone was on there, but when I woke up, I noticed and turned the phone off right away. It's been off for about 15 minutes and it's still hot. I was able to view the lock screen in order to turn it off, so I'm hopeful that it will still work, but I'm wondering if there will be any long term effects and when is it okay to turn the phone back on? Thanks so much.