Can an iPhone screen crack if it fell on its back ?

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Yes, it can. Regardless of where the telephone falls, the screen can in any case break. I strongly suggest using telephone cases or having texture cushioning. But if you are fortunate enough phone will not crack.

depending on luck or not then yes. Falling on the back should give a lower chance of this but its not 100%.

Absolutely. No matter where the phone falls, the screen can still crack.

I highly recommend using phone cases or having fabric padding.

It doesn't seem likely but I guess if it hit in just the right place it might.

More likely the back glass would crack if it's a newer model. That's honestly the worse of the two to have break since the screen can be replaced but not the back glass, as it's part of the phone enclosure. If you took a broken back glass screen to Apple, they'd say you have to replace the whole phone.

Yes, the front screen can crack, no matter what part of the phone makes contact with a hard surface when it's been dropped.