Analog Devices uses New Strategies to Build the ADXL346
Contributed by: St.J. Dixon-Warren
Analog Devices recently launched the digital ADXL346 3 mm x 3 mm x 0.95 mm thick, three-axis accelerometer. This device competes directly in the consumer electronics market with the Bosch SMB380, the STMicroelectronics LIS331DL, the Freescale MMA7660 and the Kionix KXSD9, which are also available in small footprint 3 mm x 3 mm packages. Chipworks analysis suggests that the launch of this new device was made feasible by Analog Devices new MEMS foundry strategy.
Since the introduction of the ADXL50 in 1991, Analog has built their MEMS inertial sensors using their iMEMS technology, which cleverly integrated the micromechanical structures and ASIC circuitry on a single die. Both the 3 mm x 5 mm ADXL345, which was discussed recently in a posting on the MEMS Industry Group Blog, and 3 mm x 3 mm ADXL346 contain a separate MEMS and ASIC dies, and they thus represent a marked change in Analog Devices MEMS strategy. This new strategy allows Analog to separate the MEMS chip design from the ASIC design. They can choose to have the ASIC fabricated at a foundry. Chipworks’ analysis has found that the ADXL346 contains the same MEMS die as the ADXL345. Analog Devices has used some clever engineering to fit the two dies into the smaller footprint ADXL346.
Figure 1 ADXL345 Package Top (3 mm x 5 mm)
Figure 2 ADXL346 Package Top (3 mm x 3 mm)
The corresponding plan-view x-ray photographs are shown in Figure 3 and Figure 4. The footprint for the MEMS and ASIC dies inside the ADXL345 is 2.8 mm x 3.3 mm, which is too large to fit inside the 3 mm x 3 mm ADXL346 package.
Figure 3 ADXL345 Package X-Ray
Figure 4 ADXL346 Package X-Ray
Two strategies have been used to fit the MEMS and ASIC dies inside the ADXL346. First, Analog has designed a new, smaller ASIC die for the ADXL346. The ADXL345 uses the 1.37 mm x 2.17 mm XL345C ASIC, while the ADXL346 uses the 56% smaller 0.86 mm x 1.94 mm XL346 ASIC. As mentioned, both parts use the same 1.25 mm x 1.47 mm MEMS die. These parameters are summarized in Table 1.
Shrinking the ASIC appears to have been insufficient to fit the two dies into the 3 mm x 3 mm ADXL346 package, and hence Analog has also rotated the MEMS die 90° with respect to the ASIC inside the ADXL346 package and then wire bonded the MEMS die to the package substrate, rather than directly to the ASIC. The interconnection to the XL346 ASIC is through wiring in the package substrate. Only a ground wire to the MEMS die lid connects directly to the ASIC. Chipworks has not seen this strategy used previously. The Bosch SMB380, the Freescale MMA7660 and the Kionix KXSD9 have the MEMS and ASIC mounted side-by-side with direct wire bond connections from the MEMS to the ASIC. The STMicroelectronics LIS331DL stacks the ASIC over the hermetic lid of the MEMS die (this strategy requires thinning the die to achieve a 1 mm thickness).
The rapid appearance of the smaller footprint ADXL346 a few months after the launch of the ADLX345, shows the benefit of Analog Devices new strategy. They were able to rapidly shrink the area of the ASIC, while keeping the MEMS die the same, thus achieving the smaller 3 mm x 3 mm package size required for the consumer electronics market. It is worth noting that Analog was an early to the market, in 2006, with a three-axis accelerometer, the ADXL330, which was fabricated with their integrated iMEMS process and packaged in a 4 mm x 4 mm x 1.2 mm thick package. It would appear that the complexity of their integrated iMEMS process meant that they could not easily achieve a 3 mm x 3 mm x sub-1 mm thick package without changing their MEMS strategy.
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ADXL346
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ADXL345
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Manufacturer
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Analog Devices
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Analog Devices
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Part number
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ADXL346
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ADXL345
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Type
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3-Axis,
±2 g/±4 g/±8 g/±16 g Ultralow Power Digital Accelerometer
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Analog Devices
3-axis ±2g/±4/±8/±16 g Digital Accelerometer
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Date code
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Likely 901
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Likely 905
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Package markings
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46X
5901
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45XC
#905
6601
PHIL
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Package type
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16 lead LGA package
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14 lead LGA package
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Package dimensions
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3 mm × 3 mm × 0.95 mm
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3 mm x 5 mm x 1 mm
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MEMS Die markings
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None
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None
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MEMS Die size (full die)
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1.25 mm x 1.47 mm (1.84 mm2)
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1.23 mm x 1.46 mm (1.84 mm2)
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ASIC Die markings
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XL346
ADI (M) 08
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<ADI logo> (M) 08
XL345C
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ASIC Die size (die edge seal)
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0.86 mm x 1.94 mm (1.67 mm2)
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1.37 mm x 2.17 mm (2.97 mm2)
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Package Efficiency
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39%
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32%
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Table 1 ADLX345 versus ADXL346 Comparison
Chipworks Reference Reports
- EXR-0910-802 Analog Devices ADXL346 3-Axis Accelerometer MEMS Exploratory Report
- MPR-0907-802 Analog Devices ADXL345 Digital Accelerometer MEMS Process Review
- MPR-0708-801 Bosch SMB380 Accelerometer MEMS Process Review Report
- EXR-0905-802 Freescale MMA7660FC 3-Axix Accelerometer MEMS Exploratory Report
- EXR-0902-811 Kionix KXSD9 3-axis Ultra low power digital accelerometer MEMS Exploratory Report
- MPR-0809-802 STMicroelectronics LIS331DL Accelerometer MEMS Process Review
- PPR-0602-801 Analog Devices ADXL330 3-axis Accelerometer Process Review Report (MEMS)