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Vision leads to quicker MEMS inspection

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A consumer electronics manufacturer has introduced a machine vision-based inspection solution that reduces the inspection time of products containing micro-electromechanical systems (MEMS) by up to 90 per cent. Micro-electromechanical systems (MEMS) need to be positioned in a plastic housing to sub-micron tolerances to align mechanical components on the die with interfacing components on the housing.

DWFritz Automation, Portland, Oregon, which specialises in designing and building custom automation systems, developed a MEMS part inspection tool using two Cognex CDC-200 cameras and a Cognex MVS-8100 frame grabber. The tool can perform over 500 measurements across nine manufacturing steps with a cycle time of no more than 30 seconds per step.

Manual inspection is often used with MEMS, but the high production volume of this application would have required approximately 15 human inspectors to keep pace. 'If the manufacturer had been forced to use manual inspection methods, it might not have even made sense to bring this product to market,' said John Watt, director of marketing and business development for DWFritz. 'The combination of an accurate fixture, high resolution cameras and powerful vision tools made it possible to inspect the part in a fraction of the time. The result was a successful product that meets the highest quality standards.'

Components are manually loaded into custom-built nests installed on a Primatics 0.02µm X-Y stage. As the nest is moved into the inspection area, the part identification is automatically captured by an onboard barcode reader. A Cognex CDC-200 camera with a low resolution lens is used to detect the location of features on the assembly. Another CDC-200 camera with a high resolution lens precisely measures those features.

A key challenge was fixturing a part that is framed by a deflectable plastic material in a repeatable way. DWFritz developed a sophisticated mechanical design, whereby the part can be removed from the fixture, replaced and re-measured with 0.2µm repeatability.

The assembly includes a diced die that is glued to a ceramic substrate that is mounted to a plastic housing. The low resolution camera is used to inspect the adhesive since high accuracy measurements are not required. Hundreds of line finder and caliper functions from Cognex's VisionPro tool library are used to measure the amount of adhesive on the substrate to be sure it is within specification. The VisionPro caliper tool is used to locate edges or edge pairs despite noise in the image background.

If the substrate adhesive passes inspection, then a diced die is placed onto the substrate. The Primatics stage moves the fiducials one by one to the centre of the image of the second high-resolution camera. The VisionPro PatMax tool is used to locate the fiducials on the die using the low resolution camera. The high resolution camera then captures an image used to determine the offset and rotation of the fiducial to sub-micron tolerances relative to a golden standard measured by the National Institute of Standards and Technology (NIST).

Cognex CDC series cameras use CMOS active pixel sensor technology to convert light energy directly to digital image data. The camera converts the image data to a digital stream, split into data packets according to Cognex’s CogLink camera communications protocol, and sends the digital stream over the CogLink high-speed serial bus to a Cognex MVS-8100D frame grabber. The MVS-8100D reassembles the packets of image data and makes the captured camera image available to Cognex software running on the host PC. The CDC-200 camera captures image sizes up to 1,280 x 1,024 pixels at a frame rate of up to 26Hz, and uses a global shutter.

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