Alan,

There are multiple ways to handle this problem. The most brute force is to design the keyboard interface so that it works with some series R in the interface lines. Then supplement this with C to GND on the MCU side of the series R. Then also equip each signal line with either a pair of Shottky clamp diodes to VCC and GND. (There are multiple vendors that make small package clamp diode units for USB ports and such that can be applied directly to the signal lines). Clamp diodes will be more effective if they are behind some series R from the location of the ESD events. In lieu of clamp diodes you may choose to use low voltage TVS devices. When using these make sure to return the GND side of these directly to the GND reference plane of the ESD entry point so they shunt the ESD event energy as close to the entry point as possible.

There are keypad technologies that I have had applied on some projects that I worked on that provide good ESD protection. These apply to the type of keypads that are made out of mylar or poly flex type materials in layers to make key switches. This type of keyswitch has layers with screen printed traces of conductive material that contact together through an intermediate sheet that has holes in it at the key switch locations. These can be made hardened against ESD events by adding additional layers with a screen printed grid of conductive lines that are connected into GND or FRAME around the keypad area. I have used ones where these grid layers are printed of a conductive carbon material and others that used a silver ink material.

Michael Karas