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Teardown Compares Combo Sensors

A look inside three top combination sensors shows how vendors are taking very different routes to arrive at competitive products.

We did chip teardowns of three of the leading nine-axis inertial measurement units (IMUs) released in 2013: the BMX055 from Bosch Sensortec, STMicroelectronics' LSM9DS0, and the InvenSense MPU-9250. Each player's cost is very similar, despite their use of very different techniques.

Driven by the benefits of integrated devices and price decreases per function, the market for combo sensors is expected to grow from $446 million in 2013 to $1.97 billion in 2018, according to a report from Yole Development. At this growth rate, combo sensors will rise from 21% of the global inertial consumer market in 2013 to 66% by 2018.

The market acceptance of combo solutions has been extremely quick for six-axis IMUs and six-axis e-compass devices. The relatively new nine-axis solutions we examined will no doubt be followed by more innovative solutions. Combo sensors are becoming mainstream in consumer systems and also find use in cars, but smartphones remain the application that will drive the market for the next few years.

Prices are dropping sharply, with some IMUs sold to some large-volume customers for less than a dollar per unit in 2013. Prices for the nine-axis devices in this teardown are still relatively high, but cost pressures on them will rise as they reach volume production.

The three components use different packaging, with footprints that range from 4×4 mm (16 mm²) for the STMicroelectronics part to 3×3 mm (9 mm²) for the InvenSense device. STMicroelectronics and Bosch Sensortec use LGA packages, while InvenSense uses a QFN package.

The three packages vary widely in size.

The three packages vary widely in size.

Strong differences in internal structure emerged after we removed the epoxy resin. For example, STMicroelectronics and Bosch use five die, but InvenSense only uses two — one die for a six-axis accelerometer/gyroscope and one for a three-axis magnetometer.

Thus, each player varied widely in the silicon area they used, from 19 mm² for STMicroelectronics to 14 mm² for Bosch and just 8 mm² for InvenSense. More silicon die requires more wire bonding for connecting them. The STMicroelectronics IMU uses 76 wire bondings, compared to 25 for the InvenSense device.

The devices under the epoxy resin show even greater diversity.

The devices under the epoxy resin show even greater diversity.

Next Page: How STM shrinks its part

STMicroelectronics uses a single MEMS die for the six-axis accelerometer/gyroscope, shrinking the size of the six-axis function by more than 30% over its previous combo solution, which used two dice.

The component uses glass-fit bonding, but STMicroelectronics also uses gold-gold thermo-compression bonding for the sealing of some of its MEMS die, enabling a reduction of the sealing frame and thus shrinking the die size.

The accelero/gyro sensor in the STMicroelectronics LSM9DS0 nine-axis IMU.(Source: System Plus Consulting report from December 2013)

The accelero/gyro sensor in the STMicroelectronics LSM9DS0 nine-axis IMU.
(Source: System Plus Consulting report from December 2013)

Next page: Bosch does it all in-house

The Bosch component is the only nine-axis MEMS IMU with all its functions (accelerometer, gyroscope, and magnetometer) developed and manufactured by the same player. The BMX055 integrates the second generation of Bosch's geomagnetic sensor, with the three-axis support in a single die, versus three separates dice for the previous generation.

Bosch also uses an eutectic Al-Ge bonding for the capping of the MEMS gyroscope, instead of the classic “glass-frit” wafer bonding.

The three-axis magnetometer in the Bosch Sensortec BMX055 nine-axis MEMS IMU.(Source: System Plus Consulting report from December 2013)

The three-axis magnetometer in the Bosch Sensortec BMX055 nine-axis MEMS IMU.
(Source: System Plus Consulting report from December 2013)

Next page: InvenSense shaves size, costs

InvenSense latest nine-axis IMU integrates a new three-axis gyroscope, now using a single vibrating structure, versus three different structures for the previous generation. This new design results in 40% shrinkage in the three-axis gyro area.

A second benefit of this new design is that its Nasiri process has been changed. Cavities which were traditionally etched in the ASIC to allow MEMS structures to move are no longer used, resulting in a cost reduction.

The device also integrates a new three-axis magnetometer, which features an almost 40% size reduction from the previous generation.

The MEMS accelero/gyro sensor in the InvenSense MPU-9250 nine-axis IMU.(Source: System Plus Consulting report from March 2014)

The MEMS accelero/gyro sensor in the InvenSense MPU-9250 nine-axis IMU.
(Source: System Plus Consulting report from March 2014)

Next page: Similar costs, different supply chains

The three players use very different supply chains. Bosch and STMicroelectronics mainly rely on themselves for the manufacturing of their components. Bosch only relies on foundries for the manufacturing of its ASIC. ST purchases its magnetometer die from Honeywell.

By contrast, InvenSense, which is fabless, relies on many different players for the manufacturing and the assembly of its IMU. AKM supplies the magnetometer die, foundries make its six-axis accelerometer/gyroscope MEMS and ASIC, and OSATs handle package assembly. Only the design and test of the device are directly handled by InvenSense.

Despite the radically different approaches, the final costs of the nine-axis IMUs from the three players are very similar. Although InvenSense takes a clever approach to integration by using less silicon, it relies on many third parties, which add significant costs.

A full analysis of the three devices is available online.

A cost structure comparison of the three nine-axis devices.(Source: System Plus Consulting March 2014 report)

A cost structure comparison of the three nine-axis devices.
(Source: System Plus Consulting March 2014 report)

This article was originally published on EBN's sister publication EE Times.

1 comment on “Teardown Compares Combo Sensors

  1. Hailey Lynne McKeefry
    June 3, 2014

    It's interesting how close these three components are in terms of how the cost structure is broken out. Seeing is believing.

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