Understanding Smart Sensors - Nomads.usp

228 Understanding Smart Sensors10.2 Semiconductor Packaging Applied to SensorsMany of today’s sensor packages resemble semiconductor packages of the1980s or even the 1970s. The semiconductor industry has made significantprogress in high-density plastic encapsulated packages. The increased use ofsurface-mount technology (SMT) is among the more important changes. Toachieve increased functionality without increased silicon complexity, availablesilicon technologies are being combined at the package level in packages basedon semiconductor, not module manufacturer, assembly techniques. These multichipmodules (MCMs) are being evaluated for several applications, includingautomotive.As the use of surface-mount technology increases, a decline is occurring inthe use of the previously popular dual in-line plastic (DIP) package. Otherthrough-hole packages like single in-line plastic (SIP) and pin grid array (PGA)will also not increase. New SMT approaches, like ball grid array (BGA) andmicroBGA packages are the focus of current packaging development. Forfuture, highly integrated components, packaging techniques must take intoaccount more complex, system-level requirements, as well as SMT assemblyrequirements. The ability of the sensor industry to adapt to the newer semiconductorpackaging approaches to sensors will determine the acceptance of smartsensor technology and the future growth of the industry.The packaging, assembly, and testing contribution to the cost of a wellestablishedmicromachined pressure in a DIP package is shown in Figure10.1(a) [1]; the sensor chip is only 14% of the total cost. For more complexpackaging, such as a pressure sensor, in which media isolation is required, thecost of packaging, assembly, and testing can climb to 95% of the total cost.The type of sensor, the amount of circuitry, and the application canchange the cost significantly. An accelerometer has a separate control chip andg-cell in a surface-mount package. The die cost in that case is 30% for the g-celland 24% for the ASIC, as shown in Figure 10.1(b). However, the greatest challengewith the accelerometer for automotive air bag applications is inexpensivetesting [1].Sensor packages have basic requirements that are similar to those of semiconductordevices. The variety of harsh sensor applications makes packagingmore difficult than packaging for a semiconductor device. However, the basicpackage operations occur in similar order.A completed sensor wafer has a final processing step that prepares it forpackaging [2]. That step could include thinning the wafer and attaching abackside metal such as a gold-silicon eutectic. Sensors tested at the wafer levelthat do not meet minimum specifications are identified as rejected units by anink dot. Sensors are then separated into individual dice from the wafer by