Don’t Keep Your iPhone in Hot Locations
Leaving your iPhone to swelter in the heat is asking for major trouble. Don’t store your iPhone in places where the temperatures are especially high, or where it wouldn’t have a chance to cool down. Keeping your iPhone in your car is an obvious no-no, but fortunately, that’s not as common these days since we tend to keep our smartphones with us.
Also avoid keeping your phone in a bag or box if it’s going to be sitting out in the sun for a while. If you sleep if your iPhone under your pillow, try to break that habit, too.
On a related note, your iPhone does gain heat when it’s charging, so try to keep it in a cool spot while charging, and try to avoid using your phone too much until charging is done.