The membrane keyboards, which began appearing on calculators and appliances in the early 1970's, typically contain three sheets of polyester, a flexible plastic. Each sheet is slightly thicker than a piece of paper. Circuits are printed on the top and bottom sheets using a silver-containing ink that conducts electricity. The middle slice is a spacer with a hole at each key location. When a particular spot is pressed, the top and bottom sheets make contact through the hole in the spacer, completing the circuit.

The circuits are printed with a silk-screening process, which is commonly used to print signs and some fabrics. The top and bottom circuits are printed side by side on the same sheet of polyester, which is then folded over. Rolls of keyboards thousands of feet long can thus be mass-produced.

''It's almost like automated cookie-making,'' said Mr. Larson of Oak Switch. The next step was to go beyond calculator and appliance keyboards and develop one comfortable for typing. That was not easy, for keyboard design is an exacting science.

The keys, for instance, must move, or ''travel,'' the proper distance. Yet the contact should actually be activated when the key is only part way down, with the rest being ''overtravel.'' When it gets to the bottom, the key should not ''bounce'' or the letter will be registered twice. A good keyboard should have multikey ''rollover,'' which means that several keys can be struck almost simultaneously without jamming.

The keys should be neither too hard to push down nor too easy. The top of the keys cannot be too shiny, or there will be glare, nor too rough, or they will hurt the fingers. The keys cannot wiggle too much and must be sloped and shaped for comfort. In Europe it is becoming standard that the so-called home row of keys should be no more than 1.2 inches above the top of the table.

Oak Switch uses a plunger and two springs - one to return the key after it is depressed and another to depress the membrane. The net result is to allow the top of the key to move .15 inch in order to move the membrane only .005 inch.

Chomerics, which demonstrated the flatness of its membranes by putting a working keyboard on the cover of its 1978 annual report, took a different approach. It uses a plunger and only one spring and has also inflated the membrane switch by putting a tiny air bubble between the top and bottom polyester slices. The air bubble allows the key to have greater movement.