Could be. Nobody here can possible tell from an internet post. Make an appointment with Apple or the nearest Apple Authorized service provider and let them evaluate the problem.
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iPhone 6 started to separate then completely separated and stopped working. Had to replace battery because old battery exploded. Owner is elderly person who doesn't know much about how to operate most of the phone's programs. I only mention this because our carrier suggested the problem occurs when the phone is older and has excessive use. 3 years should not be considered old and excessive use is just reaching. Replace your battery before yours explodes is my recommendation.
Actually, three years can be quite old for a lithium polymer battery. Over that time, the number of discharge/charge cycles may be very high, and if so, then a battery may easily be subject to age/use degradation and swelling. So 3 years is perhaps a bit short, but it would depend on the cycle count moreso. Or if not used at all for long times, lithium batteries also degrade more rapidly (I.e. they don’t do well unused sitting for months or years).
Older lithium batteries or batteries with a high cycle count are prone to swelling from their degraded chemical components.
Question: iphone 6 screen is popping out is it due to battery swelling?