Smartphones will warm up from the moment you switch them on; this is unavoidable. But phone overheating can be a serious issue, damaging devices and affecting performance. What causes a smartphone to heat up, what causes it to overheat, and how can it be avoided?

WHY DO SMARTPHONES HEAT UP?
The answer lies in a basic property of physics: movement generates heat. Our smartphones have to physically move things around to work at all, so they have to generate heat. The amount of heat your smartphone produces is largely proportional to the amount of electricity moving through it. If you’re playing a game that demands a lot from your central and graphics processing units, both housed in a central system-on-a-chip (SoC), they will become warm, as they require more power to carry out their tasks.

Your phone becoming warm is no reason to worry, they’re designed to work that way, but if it becomes unpleasantly hot, you probably have cause for concern.
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http://www.techgeez.com/2016/07/why-smartphones-heat-up-and-overheat.html?m=1