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.
Many FPCB designer are not familiar with the manufacturer process and limitation. I always found design does not meet DFx requirement and insist on their design and finally bring lot of quality issue after mass production. 
The PTF uses silk-screen to print the silver and carbon combination conductive inks on PET substitute to reach lower cost purpose as compare with FPC (Flexible Printing Circuit). Unfortunately, the screen-printing still has some ability limits. Especially that it has problem for fine trace width and fine space. This article will discuss screen-printing on PET limitation.
Polymer Thick Film (PTF) flexible circuits are manufactured by screen-printing conductive ink onto flexible substrates such as PET (Polyethylene terephthalate). It can be single layer or multi-layers circuit by further screen-printing dielectric materials as insulating layer and applying through-hole technologies.