The inner core and outer core layer of the keypad contact pads were group into one certain connection,
The keypad filter IC also been used in the circuit for keypad ESD and EMI protection.
Just refer to the simple solution below to trace any particular keys that got a problem.
Nokia 1680c,1681 Keypad Connection Tracks
You may clean first all the corresponding keypad keys contact pads on the PCB board including the keypad membrane before proceeding into troubleshooting.
Just simply trace first each of the corresponding connection line paths for possible cut-off or open circuit. Just mend or apply a tiny jumper wire to reconnect the open line paths.
If seems that all of each corresponding connection were both okay, the problem may possibly lies on the keypad filter IC (glass or crystal IC). Replacing it is the only option to fix the keypad problem on Nokia 1680c or 1681c.