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Dust and Water Resistance Testing Methodology for Environmental Sensors
Keywords: Wearables, Sensors, Water Resistance Testing
Abstract – Wearable electronics have rapidly emerged as the next big wave for MEMS and sensors in consumer electronics after smartphones and tablets. What’s been typically seen only with medical electronics such as pulse/oximetry is now ubiquitous across the wearable market as standard MEMS content. In addition to pulse/oximetry, the fusion of temperature, pressure, and humidity sensors have further enhanced activity monitoring, environmental monitoring, etc. IDC (International Data Corporation) has predicted that ~189.9 million wearables will be worn by consumers by 2022. Just as ADAS (Automatic Driver Assistance System) helps to augment our driving experience, the wearable market is also experiencing a rapid growth in the number of sensor fusion applications helping users to both monitor their health and their external environment. Pressure and Humidity sensors are just a few of the many sensors that help to reinforce the total user experience from climate control to activity monitoring through detection of environmental changes. Careful consideration in the design of sensors utilizing an open cavity over the sensor element needs to be taken in order to guarantee in-situ and end of life performance. Primarily an understanding of the types of chemistries from our environment they will be exposed to. For ADAS (Automatic Driver Assistance System) systems requiring a condensation free environments the aid of a humidity sensor can achieve a closed loop control system to throttle a heater avoiding condensation on a lens surface obscuring the CCD element. For the wearable marketspace an open cavity humidity sensor requires a sampling of the environment in the form if water vapor but also must avoid collecting condensed moisture in the opening to avoid full saturation of humidity sensor element. In this case a gas permeable membrane is utilized that must resist liquid ingress at a specified depth of submersion while allowing gas to freely permeate. Alternatively for a temporary protective cover due to potential exposure to aggressive chemistries during manufacture a removable cover tape is applied to offer protection. The goal of the polyimide cover tape is to survive the reflow process and offer protection from chemistries which the assembly maybe subjected to. Some customers utilize conformal coating which may inhibit sensor performance therefore the pre-applied temporary cover tape can protect the sensor during processing and removed once the material has cured exposing the sensor element. In addition to the three testing techniques two formats for sensor element protection will be explored. Protection in the form of a temporary removable polyimide cover tape and a gas permeable but water resistant membrane will be analyzed thru dust, water spray, and submersion test techniques. For IP6X testing there is a need to verify dust ingress resistance by introducing a vacuum to the assembly in an effort to account for barometric pressure changes. Due to the small size of packaged electronics this is not practical on a device level but can be achieved thru the use of a fixture and laser drilled hole. This fixturing approach will be detailed in the presentation and compared to devices with and without laser drilled holes in an effort to create a supplemental standard the industry can use to properly evaluate the end of life robustness to environmental stresses. For IP66 testing a predefined flow rate of water will be sprayed onto the devices to determine if the protective covers are robust enough to survive high pressure spray. Again both a laser drilled hole vs. non-laser drilled hole will be compared for ease of inspection. In the case for the non-laser drilled hole the protective cover will need to be removed and examined for presence of water while the laser drilled hole can be inspected from the back side after the fixture is opened. Similarly for IP67 testing water resistance must be demonstrated but thru submersion of the devices to a depth of 1m. Consideration such as the types of liquid and their respective surface tension must be considered during this test in order to best represent the in-situ use conditions. Note that static depth pressure can also be converted to dynamic pressure from events like high platform diving or high velocity impact during water sports like wakeboarding. A table of properties will be shared ranging from bath soap, ocean water, DI water, tap water, etc. In summary, this paper explores the design challenges with balancing automotive and wearable customer-defined requirements with the tradeoffs in developing both a dust and water resistant environmental sensor solution. Various methodologies will be addressed and rationalized with reference to the International Protection Marking, IEC standard 60529 with a focus towards IP6X (Ingress Protection - Dust), IP66 (Ingress Protection - Water Spray), IP67 (Ingress Protection - Submersion). Keywords: Environmental sensor, Dust, high pressure spray, Surface Tension, MEMS, PTFE, low modulus encapsulant, Humidity Sensor, Pressure sensor, conformal coating, water resistance, IEC standard 60529, IP66, IP67, water proof, lidded cavity package, Laser Fabrication.
Steven Kummerl, Semiconductor Packaging Engineer
Texas Instruments
dallas, Texas
USA


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