Understanding Smart Sensors - Nomads.usp

232 Understanding Smart Sensorsfunctionality, especially if the entire die is not performing to predicted performancelevels. Hybrid packages are also used for production sensors.10.3.1 Ceramic Packaging and Ceramic SubstratesCeramic packages, such as the ceramic DIP (CERDIP), utilize a leadframe thatis attached to the ceramic base through a glass layer. After die and wire bonding,a ceramic top is glass-sealed to the base. The same technique is used forother form-factor ceramic packages, including the ceramic flat pack. Ceramicpackages are usually used for high-reliability applications and are much moreexpensive than other semiconductor packaging techniques. They are very usefulin the development phase of a sensor because the silicon die does not have to beencapsulated. That allows various test points on the die to be easily probed andmeasured in packaged form [5].Ceramic technology is used in hybrid assembly techniques for sensors.The ceramic substrate, usually an aluminum oxide, provides a firm mountingplatform for the sensor die. Stress isolation can be obtained by utilizing a compliantsilicone for the die attachment. The ceramic substrate allows laser trimmingof thick-film resistors deposited on the ceramic surface to provide thecalibration for a signal-conditioned sensor.10.3.2 Multichip ModulesThe extensive research that is being performed for packaging multichip microelectronicsis also being evaluated and adapted for manufacturing combined sensor(s)and microelectronics. Figure 10.3 shows potential packaging techniquesthat could possibly be used for sensors [4]. These approaches include (a) conventionalchip-and-wire, (b) flip-chip, and (c) TAB. Chip-and-wire is the standarddie-on-substrate packaging technique. Flip-chip packaging is discussed inSection 10.4.3. TAB packaging eliminates wiring bonding from the die to alead by directly attaching a lead to the top of the die. Any one of these methodsis a potential candidate for MCM packaging.For MCMS with one or more sensing elements and silicon ICs, silicon isone of the substrate materials that could be used. The approach of a silicon circuitboard to achieve wafer-level, hybrid integration may be the best interimsolution for a combined sensor and MCU. However, there are several varietiesof MCMs, depending mainly on the substrate technology: MCM-L, MCM-C,MCM-D, and MCM-Si are common classifications [6].An MCM-L (the -L suffix stands for laminate) uses advanced printedcircuit board technology, copper conductors, and plastic laminate–baseddielectrics. It is essentially a chip-on-board (COB) technology, as shown in