Why metal dome keypad must design dimple on it?

Keypad contact pad with fiber contaminationforeign dust contaminate inside metal dome and make contact fail.

Recently my company phased in the metal dome as rubber keypad contact solution. This metal dome solution had been implemented to many cell phone keypad solution also. Better key press feeling, longer life and good reliability is the benefit to use metal dome. As data show  the metal dome keypad can sustain 2,000,000 times of finger press verse 300,000 times of poly-dome.

In the beginning we removed the dimple from the metal dome in order to reach higher key press life. Because of the dimple will damage the contact pad the PCB and obtain verdigris from the copper layer base of PCB. The verdigris will finally disconnect the metal dome and contact pad of PCB and result in keypad malfunction.

After several times product trial run, we found we never get the key press life improve but got the trouble of keypad insensitive pressing issue. The keypad press feeling is good but no function while press key slightly. We must give more force to press the keypad then active the key function. The worst is that sometimes the insensitive keypad happens after a period of usage.

We did investigation on the insensitive keypad and found some tiny dust or fiber under metal dome or on the PCB contact area will bring this kind of defect. the foreign partials will stop the metal dome contact to the PCB while slightly press. The metal dome will contact to PCB while give more force press down the keypad and squelch the partials.

So, how can we do? add the dimple back to metal dome? Sealing the metal dome on PCB under clear room environmental?

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