My device uses a silicone keypad to detect a key press rather than a physical button. 

After setting up, it works smoothly, even without applying much pressure to the keypad. 

However, after a while ( say 2 months), you will need to apply much pressure on the key pad before a key is detected. It continues like this for a while, and then no keys can be detected again.

So we open, clean the PCB keypad traces with "methylated Spirit". And it works back as new. Sometime, we see black residue on the keypad PCB traces, which appears to come off the conductor of the silicone keypad. We wipe this off and everything is back to normal. 

My question is how to avoid this problem. 

![enter image description here](https://i.stack.imgur.com/eC7un.jpg)![enter image description here](https://i.stack.imgur.com/uVGb7.jpg)