Cognex vision system reduces MEMS inspection time

Cognex CDC-200 cameras

A MEMS part-inspection tool developed by DWFritz Automation has helped a consumer-electronics manufacturer reduce MEMS inspection time by 90 per cent.

Microelectromechanical systems (MEMS) need to be positioned in a plastic housing to submicron tolerances to align mechanical components on the die with interfacing components on the housing.

A traditional manual inspection with a microscope takes more than five minutes and cannot consistently hold the tolerances required in this application.

DWFritz Automation, from Oregon in the US, developed a MEMS part-inspection tool that uses two Cognex CDC-200 cameras and a Cognex MVS-8100 frame grabber to perform more than 500 measurements across nine manufacturing steps with a cycle time of no more than 30 seconds per step.

This includes loading, unloading and moving the part during the inspections.

The automated vision-based inspection system holds the required submicron tolerances and the flexibility of the system makes it easy to accommodate design changes and process improvements.

A consumer electronics manufacturer recently introduced a product that takes advantage of MEMS.

The MEMS device requires extremely accurate in-process inspection at each stage of a complex manufacturing process.

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.

Another challenge was the inability of human inspectors to consistently make the precision measurements required in this application.

The manufacturer was concerned that the use of manual inspection would result in product failures in the field, which could hurt the reputation of the product.

Yet this application was particularly complex to automate because the flexibility of the plastic housing made it difficult to fixture repeatably.

The consumer electronics manufacturer worked with DWFritz Automation to develop an automated precision inspection system that provides consistent and repeatable inspections at 15 times the speed of a human inspector.

DWFritz specialises in designing and building sophisticated and customised turnkey automation systems that utilise intelligent machine vision, advanced robotics and micron-level precision technologies.

These systems usually perform inspection after a manufacturing operation to immediately identify process problems and avoid investing additional resources in assemblies that do not meet specifications.

Components are manually loaded into custom-built nests installed on a Primatics 0.02-micron X-Y stage.

Using custom computer touch-screens and a standard computer keyboard, the operator selects a recipe and then activates the cycle-start command.

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 light curtain ensures operator safety by instantly stopping all machine motion if any object breaks the curtain while the machine is operating.

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 1280 x 1024 pixels at a frame rate of up to 26Hz, and uses a global shutter.

The CDC-200 camera is designed for use with the Cognex MVS-8100D frame grabber.

The 8100D features a full library of proven vision software tools, including Cognex Patmax technology for locating objects despite changes in angle, scale or appearance.

Available in one, two and three-channel configurations, the 8100D enables interchangeability of any combination of up to three cameras, eliminating the need to purchase multiple frame grabbers for different camera types.

Asynchronous acquisition support enables each camera to accept triggers and acquire images independently of one another.

A key challenge was repeatably fixturing a part that is framed by a deflectable plastic material.

DWFritz developed a sophisticated mechanical design using a six-point kinematic constraint fixture and highly repeatable low-friction clamping force.

The part rests on a vacuum bed that draws it against three carbide support balls to maintain a constant plane.

Registration pins on either side of the part limit the movement to the X-axis.

Then a snugger riding on a low-friction slide assembly engages a notch on the part and applies a force that holds the part against the back of the fixture to fix the part in the X-axis.

The force is low, which eliminates permanent plastic deformation, and repeatable, which eliminates error from varying elastic deformation.

The part can be removed from the fixture, replaced and re-measured with 0.2-micron repeatability.

The assembly includes a diced die that is glued to a ceramic substrate mounted to a plastic housing.

Then a flex circuit is also attached to the housing.

The next inspection verifies and measures the adhesive on the ceramic prior to attachment of the die.

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.

The vision inspection is performed in a couple of seconds.

The rest of the 30-second cycle time is consumed by loading, moving and unloading the part.

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.

Patmax can locate objects with high accuracy despite variations in part appearance caused by process variations, reflective surfaces, partial occlusion, nonlinear changes in lighting or uneven image formation.

The high-resolution camera then captures an image used to determine the offset and rotation of the fiducial to submicron tolerances relative to a golden standard measured by the National Institute of Standards and Technology (NIST).

The software controlling the inspection machine then compares the offsets to the tolerances to determine whether or not the assembly passes inspection.

The vision system is calibrated in relation to the fixture.

The Galil motion controller drives the stage so that the vision system can see each of the different fiducials.

A laser displacement sensor then measures the height of the die.

The sensor has a resolution of 0.01 microns, an accuracy of +/-0.03 per cent and a 50kHz sampling speed.

The next step in the manufacturing process is installing a flex circuit on the substrate.

Flex circuits allow the board to conform to a desired shape and are manufactured using identical components used for rigid printed circuit boards.

The flex circuit has fiducials that are inspected in exactly the same manner as the MEMS die.

Once the flex circuit has been inspected, the next step is bonding the wires between the flex circuit and MEMS dies.

The wire bonds are then encapsulated.

The assembly is then inspected again to measure the encapsulation using the same methods used to measure the adhesive in the first inspection step.

The precision inspection machine inspects more than 500 measurements across nine different process steps at a rate of 120 parts per hour, enabling 100 per cent inspection with only one operator.

The kinematic fixture provides better than 0.2-micron repeatability when removing and replacing parts.

The inspection system measures large and small features on the same part and correlates measurements between high- and low-magnification cameras.

The Cognex cameras and frame grabber provide the high levels of resolution needed to ensure that submicron tolerances are met on every unit delivered to the consumer.

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