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.