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Original Authors: Ari Kuukkala , Afore
Edited by Cyth Systems
Chip-scale-packaging (CSP) technologies are gaining popularity for microelectromechanical systems (MEMS), but engineers with traditional test systems are challenged by the small size of the sensor and constant pressure to reduce costs. They need solutions that can test and calibrate the sensor with real stimuli. For example, the test solution for a low-g accelerometer typically must be able to tilt the sensor to known angles to measure against Earth’s gravity.
The Afore KRONOS wafer-level test handler, with integrated PXI-based tester electronics, enables the testing of low-g accelerometers and gyroscopes with real stimuli on dicing tape. With this new approach, sensor manufacturers can remove multiple pick-and-place processes, increase throughput, and reduce their cost of test.
Afore has been developing advanced systems for sensor test, including special wafer prober test solutions for MEMS, since 1995. Advanced packaging technologies like wafer-level CSP have been around for more than 10 years, but they have only recently become suitable for high-volume manufacturing.
CSP can be smaller and cheaper to manufacture compared to traditional plastic or ceramic packages. This enables new use cases in health monitoring and other wearable devices. Still, sensors must be calibrated, and small dimensions create challenges for this.
Traditional pick-and-place handlers put sensors on trays after packaging and singulation. A machine takes the sensors from these trays and places them on the stimulus unit, which, for accelerometers, is tilted to different orientations. The sensors are connected through contact sockets and cables to automated test equipment (ATE), which measures sensor outputs and writes correction parameters to the sensor memory as directed by the test program.
When dealing with such small sensors (1 mm x 1 mm), these traditional machines and approaches don’t work well. Mechanical handling could easily damage the sensors, and the airflow makes them fly out of the trays.
To address these problems, we at Afore developed a system that avoids any pick-and-place operations. Instead, it directly performs accelerometer and gyroscope tests and calibrations on a film frame that is available directly after the sawing process. This approach removes additional steps within the sensor manufacturing process and the overhead of trays and the time consumed by multiple pick-and-place processes.
Our test cell solution shown in the top image is built around a wafer prober combined with a rate table featuring two rotating axes that enable 6 degrees of freedom (6DOF). The tester is integrated on top of the wafer, which eliminates moving and, thereby, the wearing of cables. Rotating axes via sliprings enable communicating with and supplying power to the tester and prober. The axes also make infinite rotation possible without wearing any cables.
Left: A 200 mm Sensor Wafer With About 15000 Sensors Placed on a Film Frame
Right: Comparison of Manufacturing Processes
This solution gives our customers in the semiconductor industry the most effective way to test CSP motion sensors. It helps them cut manufacturing and investment costs and improve yield without compromising test accuracy. In the long term, this approach can lower the price of sensors to a level where customers can afford to embed them everywhere. This would further fuel the Internet of Things (IoT) trend and help, for example, provide better patient care, increase our achievements in sports, and improve our health.
Realizing Our Solution With the PXI Platform
Because the tester in an Afore KRONOS system is on top of the wafer prober and rotating with the other system, we needed a high-end and accurate but small form-factor test solution. The size of our test platform was critical, so we chose PXI. Its compact size, wide range of instruments, and industrial specifications with great mean time between failures gave us everything we needed to develop our high-end test solution. The modularity of the PXI platform allowed us to customize our solution with the right instrumentation based on our customers’ needs and preferences while keeping everything small and robust enough to rotate our prober.
These days sensors typically include digital communication like I2C or SPI. They also need programmable power supplies such as source measure units (SMUs) for diode test and sometimes even frequency measurements. With NI and the PXI platform, we know we have ways to provide all these capabilities without having to redesign our solution.
For example, a sensor has one ground pin and five active pins. We could implement digital communication, power and current measurements, and current sinking with an NI PXI Pattern Digital Instrument (PXIe-6570) and SMUs. Because we needed to store all the data from a particular device based on x- and y-coordinates, we used TestStand software with the NI TestStand Semiconductor Module™ for test flow management. This also allowed us to configure pins and control a multisite factor of 32 without touching any LabVIEW code. In addition, we could create Standard Test Data Format (STDF) result files.
Ari Kuukkala, Afore
Edited by Cyth Systems