What is a touch membrane?

A membrane switch is a sandwich of printed circuit layers that make contact when pressed. It wears out with use, so failures start with the buttons pressed most — Start, and the number 1 — while everything else keeps working. That pattern is how you tell a membrane fault from a board fault: boards tend to fail completely or behave erratically across all functions. On over-the-range microwaves, grease and steam creeping under the membrane can also hold a key down, producing a stuck-key error.