I've been recently reading how it's supposedly better to have a phone with a removable battery, because in case of a fall energy is dissipated by the popping back case.

Structural engineering time!

The back of a phone contributes to its strength and stiffness until it pops off. Actually, it won’t contribute until the phone has flexed enough to use up the play between the back cover and the phone, but that usually happens right away. A non-removable cover always contributes and it generally contributes more because it’s better connected to the rest of the device and is often made of a stronger and stiffer material. If they were the same thickness, a phone you couldn’t open would usually be stronger.

In practice, phones with removable covers are usually thicker and have an interior back cover. The double back wall makes a big difference. So does thickness, as we’ll see below.


Strength

Is how much force the phone can take before it starts to permanently deform – when the iPhone 6 Plus (or Sony Xperia Z Ultra) bend in someone’s pocket, it’s a strength issue. If phones are treated as homogeneous rectangular prisms, strength is proportional to the square of the thickness. Of course, we just said that they weren’t homogeneous prisms, but it’s a useful guideline.


Stiffness

Is (approximately) how resistant the phone is to non-permanent bending. The test where you apply a force to the phone’s back until the screen cracks is actually a stiffness test, because that kind of screen cracking is deformation-controlled. The screen’s own strength is involved, but not the phone’s. Just its stiffness. It’s proportional to the cube of the phone’s thickness, so going thinner is even worse when it comes to this particular test. The bending in the phone creates forces in the screen that are pit against the screen’s strength.

Longer phones will have higher forces on them (linearly proportional to the phone’s height, in the case of that screen cracking test), which hurts the iPhone a little more when it comes to in-pocket bending.


Impacts

Are more complicated. The ideal is probably for the phone’s edge to deform a bit like a crumple zone. Deformation can pop the back cover off – that’s why it looks like that helps. It doesn’t absorb much energy though. It only helps because it means that the phone is deforming and that is what absorbs energy. After that, the forces will go into the stiffest part of the phone and that’s often the screen. If there are imperfections, a small number of cracks will start at those and the screen fails. Without imperfections you have a greater chance that the glass will distribute the forces more evenly and not visibly crack.

- fuzzmz