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Advanced Imaging Magazine

Updated: January 12th, 2011 09:49 AM CDT

Taking a Look at Wafer-Level Packaging

Meeting consumer demand for increased features on camera phones and other mobile devices
An image sensor packaged at wafer level and provided with a ball grid array interface to simplify and reduce the cost of attachment to a printed circuit board.
Comparison of a conventional cell phone camera with an OptiML module from Tessera.
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By Barry Hochfelder

The wafer-level chip scale packaging of sensors has three configurations, depending upon how electrical I/Os of the image sensor are accessed (wire bonding from sensor to a substrate; edge connect and BGA; or Si via (TSV) and BGA. Finally, the integration segment is done with die attach equipment.

Assembly

The optical element, which eliminates such parts as plastic molds and flex-lead cables, has an X-Y dimension smaller than that of a sensor chip. As for height—usually a challenge for camera modules—Tessera says the Z dimension for the OptiML WLC’s VGA implementation comes in at 2.5 mm. Conventional modules range from 3.5 mm to more than 5 mm, according to the company.

“Once you’ve got the sensor and go into the flow, then you assemble the lenses in the lens barrel, allow for focus and put on the image sensor,” Bereziuk explains. “We saw the benefit of the economies of scale and advanced sensor alignment technology to eliminate manual focusing of the lens in the lens barrel.”

Once the lenses, whether two- or three-element, are aligned and bonded, they’re put on the glass surface of the image sensor wafer with very little manual intervention. “The final piece,” Bereziuk says, “is manufacture of the camera module with reflowable passive materials that can be used right on the circuit board. [The benefits are that it] eliminates contaminants and manual alignment and uses reflow materials.”

Nemotek’s Perdrigeat adds that another advantage of the glass is that it keeps its shape during heating, where plastic optics might move or lose their shape. The solution works not only for cell phone cameras, but PC cameras, gaming consoles and security cameras. The OptiML single-element VGA lens is designed to work with 2.2µm pixel size VGA image sensors. Again, the reflow compatibility allows soldering of the camera modules without damage.



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