Wrist worn watches and other devices employ integrated circuits (ICs) to perform various functions while minimizing power usage and size. Though small and efficient, these ICs are susceptible to voltages applied to the circuits as an electro static discharge (ESD). When electro static charge accumulate in a user who wears an electronic device, the high voltage that is created by the static charge may discharge through the integrated circuit or other susceptible portion of the electrical system in the device to a nearby metal object that is at the earth ground potential. By discharging into the circuit, the applied voltage can alter the state of the circuit or cause damage to the circuit. This can be troublesome for wrist worn watches and other devices which have external parts connected to an internal IC or other susceptible circuit.
In most generic analog only or analog/digital wrist watches that have a metal or plastic body, the side push buttons and the crown dial are notoriously vulnerable to electrostatic discharge. The air gap between these parts and the electronic circuits within the watch body presents a relatively low impedance path to high voltages created by static charge. To avoid ESD damage to the electronic circuit, most existing watches employ a metal module clamp mechanically connected to the watch case back. The clamp provides an easy path for EDS discharge current to flow between the case back and the crown dial and between the case back and the side push buttons. For instance, the metal shaft of the crown dial can be physically connected to the module clamp using a spring fastener so that the ESD at the crown dial will go directly to the watch back and not affect any electronic component. The side push button has a controlled air gap by design between it and the clamp. The clamp physically protects the circuitry from the nearby side push buttons when ESD occurs at the push buttons. Therefore, when ESD occurs at the push buttons, the charge will arc through the air gap to the clamp and then to the watch case back. No circuitry will thus be damaged. With this configuration, a static charge won't affect the electronic system.
When a wrist watch has sensors in exposed metal form such as those used for EKG monitoring, body fat and skin temperature measurements and so forth, the ESD protection requires special considerations. The conventional way of connecting the watch module clamp to watch case back directly to bypass ESD can not be applied.