These pictures show the keypad assembly of the HP-21 calculator. Corrosion from leaked Nickel cadmium battery fluid is very evident.
The switches for each key assembled onto a glass-fiber printed circuit board.
The electrical connections on the PCB are all gold plated. The flexible thin pieces of metal are what caused the switch to close when the key tops on the keypad is pressed.
Note the small thin piece of plastic. This is intended to protect the key switch from dust or spill drinks that may seep in from the key tops. It sits between the keypads and the PCB switches. However it does not seals the key switches 100%. This is because tiny holes have to make to allow the heat stakes from the faceplate to pass through to reach the PCB. The heat stakes are used to securely fastened the key switch PCB to the faceplate where the calculator key tops are placed.
This is the faceplate with key tops removed.
On hindsight, using the heat stakes may have prevented a more complete sealing of the key switch PCB against dust and liquid spillage. The heat stakes replaces screws that are needed to fasten the key switch PCB onto the faceplate and hold them against the key tops.
I believe that the corrosive liquid from the battery did not actually eat away the gold plating but rather the copper below the gold plating via the thin edges where the copper meets the PCB board itself. Without the copper backing the gold plating just fall off.
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