A short app note from Silicon Labs on burying pads to prevent snopping on keypads. Link here (PDF)
An individual’s financial matters are increasingly electronic in nature and decreasingly interpersonal. As financial institutions replace human interaction with electronic interfaces such as ATMs, the need to make electronic circuits tamper-proof becomes critical. A typical numeric keypad for financial transactions may contain up to hundred or more tamper prevention and detection features. Tamper detection circuits raise alarms and disable functionality, while tamper prevention features are designed to prevent intrusions and breaches. This application note addresses the elimination of copper pads on the accessible top surface of printed circuit boards (PCBs). Burying traces to internal layers of a PCB prohibits electrical contacts from snooping on copper elements within the PC board.