Internet of Things: Are new test strategies needed?

The Internet of Things (loT) seems to be on a good growth trajectory. Novel sensor devices for the end nodes, combinations thereof are launched every month.

Successful test strategies need to ensure that the new sensors devices will be tested and calibrated efficiently. Sensor test typically includes providing a defined stimulus signal while the device is being tested. Today classical gravity, pick & place or turret sensor handling with a stimulus box and their economics are deployed. Whereas for some application areas they still seem to fulfill the requirements, the limits need to be considered. What will be the alternatives?

Sensor Test Requirements

Many sensors – many of them being MEMS based – require a thorough calibration and test prior to being assembled into the end node device. Sensor test typically includes providing a defined stimulus signal – e.g. acceleration, turn, magnetic field, sound, moisture, gas – while the devices are being tested not only to validate the device function but also to calibrate its output signals. Test needs to be done under challenging conditions: e.g. acceleration as high as 100g is 10 times that what electronics in a fighter jet are typically exposed to.

Over the past two decades handler manufactures have provided test handlers, with a stimulus module. Some sensor handling equipment has also been built for specific stimuli and using traditional pick & place concepts.

It is clear though that standard gravity, pick & place and turret handlers were designed for different applications – other package types, test times, tester constraints, temperature requirements etc.

Unique Features of Sensors Influence Test

Sensor devices have very unique features much different from the “average” IC:

  • low number of IOs (6-14)
  • small package outline – 4mmx4mm until a few years ago, now 2mmx2mm or smaller, the first wafer-level sensors are emerging
  • “active areas” on the package – pressure, microphone or optical ports
  • test time can be long, e.g. >30secs for pressure sensors, or short, e.g. >5secs for low g accelerometers
  • tester resources are hardly a limit and test parallelism of 256x can be achived

Particularly consumer applications sensors have other important attributes:

  • they need to be low cost to enable a high volume market penetration
  • if they ramp, they can ramp steeply – but many devices do not make it to volume
  • product life cycles are short (2-3 years)
  • the next product generation often requires an altered or totally different stimulus

Implications on Test Equipment

While testers can be reconfigured and re-used, it is not uncommon that test handlers cannot be upgraded to meet the next generation requirements and are essentially obsoleted after a period much shorter than the depreciation time. This trend is often exacerbated by device package outlines shrinking beyond the practical capability of the handling concept initially chosen for the first generation sensor device.

MEMS/sensor manufacturers will need to analyze the appropriate application areas and limits of “classical” gravity, pick & place or turret sensor handling with a stimulus box and their economics. Test equipment suppliers need to provide solutions that support alternatives including sensor calibration by electrical stimulus, wafer level stimulus, strip testing and carrier based testing.

Upcoming sensor fusion into single packages (“combos”) and the integration with radios and uControllers as driven by the loT will add further challenges and increase the need and value of a well-defined test strategy.

Xcerra – Complete Test Cells for MEMS/sensors

Xcerra combines industry-leading Multitest MEMS concepts with the long-term experience of the LTX-Credence ATE team. Xcerra is in a unique position to offer full turn-key, one-stop shop MEMS test cells comprising ATE, test interfaces, test handler and MEMS stimulus. Moreover with the Xcerra inhouse design and engineering capabilities, optimized MEMS test solutions can be offered that leverage the unprecedented ability for advanced harmonization and integration.