Understanding Smart Sensors - Nomads.usp

Packaging, Testing, and Reliability Implications of Smarter Sensors 229Substrate17%Package5%Package14%Die30%Die14%(a)ASIC24%Assemblyand test64% Assemblyand test32%(b)Figure 10.1 Cost of the elements of (a) a pressure sensor in a DIP package compared to(b) an accelerometer in a surface-mount package. (After: [1].)sawing or scribe-and-break techniques. Good sensor dice are placed in carriersthat allow automatic pick and place machines to transfer dice from the carrierto the final package, where a die bond attaches the sensor firmly to the package.Wire bonds connect the electrical contacts on the die surface to the leads of thepackage, which allow the sensor to interface to external components. The packageis then sealed, if it is metal or ceramic, or encapsulated, if it is molded plastic.Lead plating and trim (or singulation) occur next and are followed bymarking and final test operations.The actual design of the package must take into account sensitive areas ofthe semiconductor device and the sensor’s specific function. The following list[2] includes the characteristics that affect sensor packaging. The sensitivity ofthe semiconductor to light must be minimized in packaging for an accelerometerbut optimized for a photodiode. Similarly, during design the package’s sensitivityto stress must be taken into account to prevent stress from affecting theoffset and sensitivity in a stress-sensitive pressure sensor. For smart sensors, thekey item in the list is the fact that integration level affects the sensor’s package.• Wafer thickness and wafer stack (e.g., single, silicon-silicon, silicon-glass);• Dimensions;• Environmental sensitivity/requirement for physical interface;• Physical vulnerability/stress sensitivity;• Heat generation;• Heat sensitivity;• Light sensitivity;