Mind Chasers Inc.
Mind Chasers Inc.

Assembly: Pattern Recognition's Performance Depends on Search Window Size and Minimization of Error

An analysis of pattern recognition's dependence on search window size and error minimization during a pick & place operation with a 7 mm2 QFN device is discussed.

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A Pattern Recognition System (PRS) combined with an upward looking camera can be utilized on a pick & place (PNP) machine to recognize a device (e.g., IC, connector, resistor, etc.) on a nozzle before placement. The PRS is used to correct both the device's linear and rotational position before placement. This system depends on a nozzle picking a device from a movable tape and bringing the device to the camera for inspection & adjustments before placement.

Reliable operation requires that the device be brought fully within the finite PRS search window. However, multiple sources of error can accumulate and cause a PRS detection failure due to the device being outside the PRS search window.

Sources of error are listed below:

  • Feeder drift (variability of tape pocket position after each feed)
  • Device position within tape pocket. In our example, the pocket width is 7.3 mm, so the 7 mm2 package may be offset up to 0.15 mm during the pick operation.
  • Nozzle positional accuracy / repeatability for both pick operation and return to PRS window
  • Movement of device on nozzle during flight of nozzle from tape to PRS window

If these sources of error accumulate to position the device outside of the PRS's search window, then the PRS detection and correction operation will most likely fail or produce undesirable results.

Another potential source of error during placement is an incorrect calculation of positional offset by the PRS system. We have found that the PRS positional detection accuracy can suffer as the device moves away from the center of the PRS search window. Therefore, this source of error may limit the size of the search window used since this error may lead to the device being placed sufficiently off center on the PCB that it does not auto correct during reflow. This will depend on the type of package being placed and the lead or ball pitch

The figures shown below utilize a 7 mm2 QFN package with thermal pad. The search window in the horizontal (X) direction is approximately 10.56 mm (set to the maximum the system allows).

The first figure shows a PRS training operation with the device manually centered. As might be expected, this produces an ideal score with no offset detected.

Figure 1. Initial training with device manually centered

This second figure shows the device manually moved 64 mils (thousandths of an inch) in the X direction, but the PRS system calculates an offset of 60 mils, which would be a source of error during placement.

Figure 2. Device is on the edge of search window

This last figure shows the device moved an additional mil (65 mils total) and is now apparently outside the PRS search window since the system reports "Pattern Not Found".

Figure 3. Device is outside search window and not detected

As the last figure shows, if a device is positioned above a PRS camera outside the system's search window, the PRS operation may fail. Therefore, it is vital to ensure that all sources of error combined are less than the linear difference between the search window edge and device edge. In this example, the margin for error is ~ 1.45 mm, and it is expected that this margin is sufficient for using PRS with this 7 mm2 package and the required placement accuracy of this device.

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