Understanding Smart Sensors - Nomads.usp

236 Understanding Smart Sensorswelded to the header, and pressure sensor port attachment is accomplishedthrough solder, brazed, or welded techniques [5]. Such packages are also usedfor accelerometers, optical devices, and other sensors.10.4.1 Plastic PackagingA plastic chip carrier package that uses leadframe technology has been used formanufacturing pressure sensors for over a decade. A single mold forms both thebody and the back of the chip carrier. The patented unibody package provideslower cost, fewer process steps, higher pressure range capability, and greatermedia compatibility compared to earlier versions made of separate body andmetal back plate [5]. The leadframe assembly technique allows easy handling ofseveral devices at one time and the automation of assembly operations such asdie bond, wire bond, and gel-filling operations. Automation allows tightprocess controls to be implemented and still provides high throughput.One of the more difficult problems for sensors that interface to harshenvironments is media compatibility. Figure 10.6 is one approach to achievinga low-cost interface.10.4.2 Surface-Mount PackagingThe packaging shift to SMT (see Figure 10.1) is also affecting sensors. That canmean specifically designed surface-mounted sensor packages or, in more complexsensors, a printed circuit board (PCB) assembly that uses SMT exclusively.An example of the extent of surface-mount packaging in a sensor is demonstratedby the GPS sensor shown in Figure 10.7. That 2-in by 3.25-in by 0.5-inPCB has five surface-mounted ICs, including a 32-bit MCU [9].The single-board radar sensor in Figure 10.8 shows several surfacemountedcomponents [10]. As volume increases in those applications,increased integration and possibly higher density MCMs will allow smallerSiliconPressureSolder or glassSiliconSolderSolderable surfacePlasticFigure 10.6 Increased media compatibility pressure sensor packaging.