Membrane key switch with anti-inversion feature

A membrane switch of the matrix type including a plurality of contact sites and a plurality of layers with means for clamping said layers mechanically together, comprising:

• a first layer having at least one first conductive trace thereon defining at least one first contact point for each contact site of said matrix switch;
• a flexible second layer having at least one second conductive trace thereon defining at least one second contact point for each said contact site of said matrix switch;
• a spacer means between said first and second layers to hold said first and second contact points normally apart;
• a third layer comprised of a flexible membrane material, said layer including a curvilinear dimple disposed over said first and second contact points of said first and second layers, said dimple having an upward and inverted position relative to said third layer, such that by pressure to said dimple to move said dimple from its upward to its downward position, said dimple effects closure of said contact site by pressing said second layer against said first layer to electrically engage said at least one second contact point on said second layer with said at least one contact point on said first layer;
• and
• means for stiffening said material of said third layer in the plane of said material parallel to said first and second layers and around the periphery of each said dimple, said stiffening means extending radially outwardly from the center of each said dimple, thus precluding lifting of said third layer of material and locking of said dimple in the downward position upon effecting closure at a contact site.