Understanding Smart Sensors - Nomads.usp

Packaging, Testing, and Reliability Implications of Smarter Sensors 245raises the need for an additional type of testing. If a pressure sensor is combinedwith an MCU, one of the more difficult testing situations occurs. Testing forthe pressure sensor must be able to apply the physical medium being tested tothe device and measure the response. Measuring the response to a physicalstimulus is not a standard test for the semiconductor industry, especially undermultiple temperatures. Standard testers can test the digital and analog portionsof the MCU. The application of a physical stimulus and the ability to heat andcool the device under test rapidly and accurately drive the need for a modifiedand unique tester. Because the testers are one of a kind and not available asstandard equipment, the cost of the tester is a large part of the final unit’s cost.Not only are the testers expensive, but the throughput is limited. Limitedthroughput will cause increased cost to each part due to increased depreciationcosts allocated to each device. The cost is further increased by the need for multipasstesting. For calibration, each part may require testing at least three differenttemperatures, to determine the transducer’s output characteristics overtemperature. Those values are then used to derive the compensation algorithmthat is loaded into the on-chip EPROM. To complete the cycle, the device isagain tested over temperature to confirm the accuracy. Therefore, not only is aspecial tester required, but it becomes a bottleneck because it must be usedtwice to complete each device—once to measure the characteristics and a secondtime to verify the result.Finding ways to reduce the cost of testing is one of the key issues to makea low-cost integrated sensor and MCU a reality. Ideas that could prove promisinginclude a thorough characterization of the design, limiting the operatingtemperature, limiting the accuracy, programming the MCU to take data duringtesting, and loading the test and compensation algorithm into the MCUprior to testing.10.7 SummaryAs smarter sensing technology is developed, microcontroller or ASIC capabilitieswill be included with the sensor. That will occur either as a monolithicstructure or as multichip components; in either case, an increased number ofpins will be added to the other sensor packaging requirements. However, all thejustification for future cost effectiveness of semiconductor-based sensors ispredicated on those sensors following the learning curve similar to that of semiconductors.For sensors to approach the progress made in semiconductors,plastic packaging must be developed. Furthermore, packaging standards mustbe standardized for test equipment (handlers) and second sources. Sensor packagingdraws heavily from both semiconductor and hybrid packages, but new