Apple uses CNC technology to make stainless steel into the middle frame of iPhone 4s, and the antenna of iPhone 4S is embedded in the middle frame. Starting from the iPhone 5, Apple chose aluminum alloy material to build the back shell of the iPhone, which is also used to make products such as Apple Watch and Mac.

IPhone 8 rendering

However, there have been many problems in apples using aluminum alloy as the back shell material. For example, the iPhone 6 released on 20 14, due to the insufficient strength of aluminum alloy, appeared the "Bai Wan Gate". On the iPhone 6s, although the 7000 series aluminum alloy with higher strength was used, the "oxidation door" appeared afterwards. If the iPhone 8 is made of stainless steel, the process will be greatly improved.