Understanding Smart Sensors - Nomads.usp

Packaging, Testing, and Reliability Implications of Smarter Sensors 231Additional protective materials for the die surface and the wire bondingconnections, such as a compliant thin conformal parylene coating or hydrostaticmethyl-silicone gel, are frequently used as the means to transmit pressureto the top surface of the sensor. Parylene deposition is a vacuum process inwhich the reactive vapor is passed over a room-temperature sensor and coats thesensor with the polymer [3]. The equipment used to perform the process isquite sophisticated, especially relative to gel coatings. The kind of media towhich parylene- and gel-protected sensors can be interfaced is limited by theproperties of the protective material.10.2.1 Increased Pin CountOne of the more difficult problems that must be solved when additional electroniccircuitry is integrated with or interfaced to the sensor is the requirementfor additional pinouts. Integrated circuits, including MCUs, have industryacceptedstandard packages that allow a large number of pinouts. To increasethe density in rapidly increasing surface-mounted applications, pin pitchesbelow 0.5 mm are being pursued. Tape-automated bonding (TAB) is one ofthe technologies being developed to address fine-pitch requirements. Sensors,on the other hand, usually are limited to eight or fewer pins. Furthermore, thepackaging varies considerably from manufacturer to manufacturer, with nostandard form factor (Figure 10.2). The requirements for a mechanical interfacefor pressure, force, flow, or liquid level cause additional packagingproblems.Additional circuitry, whether it is simple signal conditioning or moresophisticated approaches that include MCU capabilities, has an impact on sensorpackaging [4]. A “sensor-only” versus integrated sensor plus control circuitis one level of differentiating packaging requirements. However, increasedfunctionality through additional circuitry, either on the same chip or from aseparate chip included in the ultimate sensor package or module, affects the pincount, normally increasing the number of package pins. An exception occursfor simple amplification and temperature compensation circuits, where thenumber of pinouts is actually reduced from four to three for piezoresistivesensors.10.3 Hybrid PackagingHybrid packages, such as ceramic multichip packages, are routinely used duringthe research and development of semiconductors and MEMS devices. Theyallow researchers to probe selected portions of the die and verify expected