Color Logical Analysis Approach for LED Testing in Manufacturing

Estimated reading time: 13 minutes

In order to address the challenges, we want to see whether there is any chance that the LED is turned ON during a non-LED designated test stage (non-LED designated test is a test that is not specifically designed for LED test purpose). If there is, it would present an opportunity to save effort to develop the LED test program. If the LED is monitored during the non-LED designated test stage, the monitored information about the LED may provide information on what LED tests are required at the board level. If the monitored LED data is processed in parallel with the non-LED designated test, the LED test execution time can be saved. This is the motivation to select some PCB boards in high volume production for further study.

Table 1: Summary of LED ON observation

Four high volume PCB boards were studied to observe whether LED is ON during the running time for the non-LED designated test. Then, the LEDs are sorted into four categories: always ON along with power-on, ON occurring at certain times during boot/BIST, ON occurring at some times during functional test, and no-ON state.

Table 1 summarizes observations of LED ON during non-LED designated test for four PCB boards in the study. Each PCB under test had different numbers of LEDs (20, 17, 0, and 10). The number of ON LEDs for each PCB varied, 8 out of 20 (80%), 12 out of 17 (70%), 0 out of 0 (100%), and 10 out of 10 (100%). It indicates that quite high percentages of LEDs have ON occurring at certain times during non-LED designated test stages. This provides an opportunity to test the LEDs without special test development efforts designated to test these LEDs, and no extra test execution time to cover these LEDs if the LED is monitored and data is processed in parallel with non-LED designated test execution.

PMCMF-CLA Approach

The PMCMF-CLA approach looks at the opportunity of LED ON during PCB power-on and functional test, and parallel computing capability of the modern CPU. Each LED has one sensor to monitor its color and luminosity, as illustrated by Figure 3. These sensors continuously measure and transmit color and luminosity measurement to the controller for processing.

Figure 3: Parallel multi-channel, multi-function architecture

The controller has multi-tasking capabilities. While it executes power-on test and functional test, it also collects LED color and luminosity data transmitted by the LED sensors. At the same time, the controller processes data according to what the LED feature is to be tested. This parallel data handling capability helps to test LEDs without adding extra test time during non-LED designated tests.

Figure 4: A color waveform sample

Each sample contains information about color, luminosity, and the time stamp of the sample, which can be represented by a 3-tuple,. A color waveform is built on a collection of 3-tuple that is sorted on sampled time (TimeStamp). A bar chart is used to represent color in wavelengths (nanometers). A line chart is used to represent brightness (luminosity and its unit value is relative). Figure 4 is a sample of a color waveform. The bar chart has four colors due to four different color LEDs being used to walk through the sensor. For ease of understanding, a small square mark is labeled on each bar to indicate a detected color. A yellow line chart represents brightness. The higher the value, the brighter the LED is.

Figure 5: Color and Luminosity detected in a waveform

Color waveforms can be used to help test LEDs which is discussed in the next section.

Features tested by analyzing color waveform

This section discusses LED features that can be tested by analyzing color waveform. First, LED’s PCOLA [1] is discussed, then more features are explored.

Accordingly fundamental properties of a device on a PCB are Presence, Correct, Orientation, Live and Alignment (PCOLA) [1]. These properties form the starting point to investigate whether color waveform can help LED test.
Color waveform can test whether the LED is present. A color sensor is assigned to monitor one LED. A color waveform is created for the monitored LED. If a color and luminosity are detected in the color waveform, it is reasonable to assume that the LED under test is present.

Color waveform can be used to test whether the LED mounted on the PCB is correct and as specified. Usually, it is expected that a correct LED means a correct color and proper range of luminosity value. For a single-color LED, it is reasonable to assume that the LED under test is correct if a desired color and luminosity are detected. Figure 5 is an example for a single color LED. For a multi-color LED, multiple colors are expected to appear in the color waveform. Therefore, a sequence of colors needs to be detected. If several colors are detected in a color waveform, it is reasonable to assume the LED is the correct LED. Color waveforms in Figure 6 have red and blue color detected. It indicates that the LED under test is a multi-color LED (red and blue).

Figure 6: Red and Blue detected in a color waveform

The color waveform cannot tell whether the LED under test has the right orientation. If the LED is placed in a wrong orientation, it probably will not be turned ON. ‘No ON’ could due to multiple reasons. Therefore, no conclusion can be made for the question of whether the LED is in the correct orientation.

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