Color Logical Analysis Approach for LED Testing in Manufacturing

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Figure 7: Green color detected several times

Color waveform has information that indicates whether the LED under test has any alignment issues. Luminosity changes if LED is slightly misaligned. This luminosity change may indicate some alignment issues. If luminosity is within a desired range, it is reasonable to assume the LED under test is properly aligned. Figure 8 is an example for a LED in proper alignment and slight misalignment positions. The color is the same but the luminosity is different.

Figure 8: Alignment versus luminosity

There are more than PCOLA features that can be tested by the color waveform. Below are some examples that cover some functionality of PCBAs under test.

One common electronic product design is that an LED (power-on indicator) is used to indicate whether the product is powered on. During production test, the LED is expected to have a sequence of OFF, ON, OFF as the PCBA under test goes through from unpowered, to powered-on, and to unpowered again. If the sequence of OFF-ON-OFF is synchronized with the actual operation of unpowered-powered-unpowered procedure, it will be reasonable to assume that the power-on indicator works properly. Figure 9 is a color waveform collected from a PC monitor board’s power-on indicator. The indicator is a blue LED, and its light is dim.

Figure 9: Color waveform of a power-on indicator

A PCBA under test has active and standby states. A multi-color LED is used as an indicator. One color indicates one state. A standby state shall have a lower brightness than an active mode. A color waveform can be used to capture such behavior when the PCBA under test goes through active and standby states.

Figure 10 shows state changes captured by a color waveform.

Figure 10: Color waveform of state change between active and standby

A flashing LED is often used to indicate a “busy” state. It is difficult to identify flashing activity if only color is measured, but, luminosity measurement varies. Figure 11 shows a color waveform for a flashing LED. When the color is correct, and the luminosity shows a high / low change sequence, it is reasonable to assume that the LED is flashing, and the PCBA under test is “busy”.

Figure 11: Color waveform of a flashing LED

The above are some examples for testing some features by analyzing color waveform. Color waveform data is collected while the DUT is under test by running non-LED designated test. Analysis of color waveform is conducted in parallel by the controller. Therefore, for those LEDs that are ON during non-LED designated tests, zero test development effort and zero extra test execution time are achieved.

Conclusion

This paper proposed a color waveform approach to relieve efforts to develop complicated test programs to safely turn LEDs ON/OFF, and to increase LED coverage without increasing test execution time. It is easy to use in production to achieve quick-to-production goals. However, there are limitations such as those LEDs that happen to be ON/OFF while the non-LED designated tests are running. For those LEDs that are never ON, this remains a topic for further investigations.

References

[1] “Test Coverage: What Does It Mean when a Board Test Passes?”, Kathy Hird, Ken Parker, Bill Follis, Proceedings International Test Conference 2002.

[2] “LED Test Case Study”, Yang Hua, Technique Report, Agilent Technologies, 2012.

Acknowledgements

This paper received strong support from Chris Cain, R&D manager. Kwan-wee Lee helped to setup the system for experiments. Hwee-yng Yeo helped in editing the paper. Their support is much appreciated.

Editor's Note: This paper was first published in the 2014 IPC APEX EXPO technical conference proceedings. Reprinted with kind permission from the authors Zhi-Min Shi and Hua Yang, Keysight Technologies (formerly Agilent Technologies’ Electronic Measurement Group).

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