Generally all spot welding electrodes are made from a copper alloy. During the resistance welding operations if their tensile strength is exceeded they will bend or collapse and fail from the mechanical force being exerted on them. RWMA Fig 18.3 in the RWMA Manual 4th Edition, shows the relationship of electrode weld face size, applied force and maximum capability.
This data is related to force applied and has nothing to do with mushrooming which is the normal failure mode for a spot welding electrode from repeated heat cycling under force. Always be sure that your electrode size is adequate for the force being used in the welding operation.
Resistane Welding Manual 4th Edition