Ingress Evaluation
Rate-based measurements of gas accumulation determine the leak integrity of sub-millimeter electronic housings. A micro cavity pressure rise identifies the failure of a hermetic seal by monitoring the change in internal atmosphere over time. This phenomenon occurs when a differential pressure exists between the external environment and the small internal volume of a sensor or micro-electromechanical system.
Calculation Basis
Volume and time constants dictate the rate of the change. A micro cavity pressure rise follows a predictable curve described by the Howl-Mann equation.
Testing Procedure
Optical or thermal methods detect the change in gas concentration. One common technique for assessing micro cavity pressure rise involves the use of a helium mass spectrometer after a bombing phase. The device is placed in a high-pressure chamber filled with tracer gas for a specific duration.
After removal, the sensor measures the amount of gas escaping the cavity. This value allows for the calculation of the equivalent standard leak rate. Sensitivity decreases as the internal volume approaches zero because the total gas mass becomes difficult to detect.
Quality Threshold
Acceptable limits depend on the intended mission life of the electronics. The micro cavity pressure rise must remain below the point where moisture triggers corrosion.
