What else changed? We think the metal dome switch may have. Let's look closer.

The dome is like a really tiny jam lid or Snapple cap—you press down and it springs back up.

If anything changes about the dome—if it's cracked or deformed—the key may behave erratically. Likewise, if the prongs break or bend, the key stops working.

There are myriad possible reasons for this switch to crack or wear out—manufacturing defects, plain old fatigue, prolonged heat, moisture, outgassing from other components, and corrosion are all common culprits.

It's entirely possible that several of these factors are contributing to switch failures, which could explain why Apple is having such a hard time untangling the problem. Fourth time's the charm?

These switches are also magnetized from the factory. Best guess as to their composition: ferritic stainless steel, with a thin polymer coating on the bottom. The difference in surface finish from the 2018 version (left) to the 2019 (right) indicates Apple may be using a revised heat treatment, or alloy, or possibly both.