Understanding Smart Sensors - Nomads.usp

242 Understanding Smart SensorsThe newest approach for establishing media compatibility for sensors usesin situ monitoring [15]. A typical three-step approach for verifying media compatibilityis to (1) characterize, (2) expose to environment, and (3) retest forconformity to specification. That requires removing the sensor from the testenvironment and also changes the exposure of the sensor through the additionof oxygen prior to retest. New procedures have been developed that test theorganic compounds used in the sensor’s packaging, the silicon sensing element,and the integrated circuitry in the sensor, without removing the sensor fromthe test chamber. New sensor materials and modified packaging designs havebeen shown to survive in a variety of harsh media common to applications inautomobiles and trucks. Previously a comparable confidence level would havebeen possible only after achieving millions of unit hours in the actual applicationor experiencing a high rate of warranty returns.10.5.1 The Physics of FailureThe sensor packaging problems specific to the application must be thoroughlyaddressed to ensure acceptable performance over the sensor’s expected lifetime.An approach has been proposed to improve the reliability and reduce the costof sensors [3]. Identifying minimum expectations and critical application factorsthat limit device lifetime are part of a methodology known as the physics offailure approach to reliability testing. The physics of failure approach involvesanalyzing the potential failure mechanisms and modes. Once those are understood,application-specific packaging development can proceed, the proper testconditions can be established, and critical parameters can be measured andmonitored [16].One key failure mechanism for sensors that operate in harsh media applications(e.g., pressure sensors) is corrosion, specifically, galvanic corrosioncaused by dissimilar metals in electrical contact in an aqueous solution. Using aphysics of failure approach, one can start to ask the appropriate questions.Which environmental factors contribute to the failure mechanism? What acceleratesit? What should be done to the sensor package to prevent corrosionor minimize its impact over the sensor’s lifetime? What is the expected sensorlifetime?The answers to those questions allow simulations to be performed andcosts to be analyzed before lengthy and potentially expensive testing is initiated.The testing will ultimately prove the acceptability of a particular proposal.Unless the proper mechanisms are known, overdesigning can add unnecessarycost to the packaging, such as a passivation layer, a hard die attachment, or anynonstandard process. Furthermore, the design tradeoffs that result may notprove beneficial. For example, adding a hard die attach to the pressure sensor to