Existing white LED technology is manufactured using a combination of a blue LED and a yellow phosphor. White light is perceived when blue light from the LED is mixed with the yellow light that is emitted from the phosphor.
In such a device, it is important that the color of the light output is uniform, and that the colors are consistent among different devices. A conventional phosphor-dispensing method is typically used to fabricate this device, where phosphor in epoxy slurry is directly dispensed on top of the LED die.
The difficulty of accurately dispensing a consistent amount of phosphor during the manufacturing process is widely acknowledged. Furthermore, two other phenomena take place after the dispensing process. First, the phosphor tends to settle down over time prior to curing (See Figure 1). Second, the encapsulate materials will also shrink down, leading to different casting height before it is fully cured. These two changes cause significant interaction effects between the LED and phosphor. The higher the casting height, the higher the amount of phosphor, which causes the white light to be more yellowish since more blue light is being converted by the phosphor. In the case in which more phosphor settles to the bottom, a bluish white light is seen (See Figure 2). The end result is a non-uniform color output from one device to another, leading to a wide color spread along the axes of the chromaticity chart in the white LED.
Figure 1: LED cross-section showing Phosphor sedimentation.
Figure 2: Illustration on white light output as a result of different casting (different amount of phosphor).
The film-based method helps minimize the color spread of a white LED, thus improving manufacturing yield and reducing product cost. In the prior method, the color spread spans as much as 0.04 along the axes of the chromaticity chart, while the film-based method is able to reduce the spread to less than 0.02. (Please refer to the experimental results in Figure 9).
Efficient light collection, mixing, and transmission is ensured by placing the LED and phosphor film within a cavity. The phosphor layer is less affected by the heat generated by the LED by avoiding direct contact, thus preserving the emission efficiency of the phosphor material. Phosphor tends to lose conversion efficiency with increased temperature, of course, and therefore the more distant the phosphor film from the LED, the less an undesirable heating effect is experienced.
Furthermore, by embedding the phosphor film inside a cavity, it is protected from other elements in the environment that are known to adversely affect phosphor, such as moisture.
Yet another advantage of embedding the phosphor film between two encapsulate layers is to ensure there is no mismatch in the coefficient of thermal expansion, which can give rise to de-lamination between the dissimilar layers.
Experimental approach and discussion
Phosphor in polymer binder (silicone) is drawn across a substrate by using a Doctor Blade to produce phosphor film. Carrier tape was used as a substrate in this experiment. Selecting the correct silicone to produce a continuous film with good surface properties is one of the important steps to produce phosphor film with consistent film thickness.
Three types of silicone were evaluated to identify the best silicone to create continuous film with good surface properties: no voids or bubbles. Silicone A was used in this evaluation as it created excellent films with the desired surface properties. Figure 3 shows the quality of phosphor film using different types of silicone.
Figure 3: Quality of phosphor film after curing with different types of silicone.
Next, the highest possible fill-phosphor ratio is mixed with silicone to avoid phosphor settling. Full films with the thickness described in Table 1 were fabricated. The films were scanned in a 1cm raster with Avago’s Blue Moonstone device (Figure 4), and Correlated Color Temperature (CCT) was measured. The results revealed that phosphor film with CCT of either 3000 K or 4100 K was non-uniform due to small film thickness. Phosphor film with CCT of 6500 K or 9300 K was more uniform due to larger film thickness. The optimum film thickness of 180 μm was then used to produce the phosphor film for the subsequent evaluations.
Correlated Color Temperature
Phosphor Ratio
Film Thickness
Film Uniformity
6500K
60%
180μm
Uniform
9300K
60%
180μm
Uniform
3000K
40%
100μm
Not Uniform
4100K
40%
75μm
Not Uniform
Table 1: Optimum film thickness and phosphor ratio for the required correlated color temperature (CCT).
Figure 4: Isometric view of Avago’s Moonstone Power LED Star package (ASMT-Mx09).
The phosphor film with carrier tape was first fixed on an adhesive board and then diced with the laser (Figure 5).
Figure 5: The converter layer is processed on a glass carrier and cut with the laser.
Figure 6 illustrates a successful model of the experiment. An LED is placed inside a cavity and a wire bond is made from one terminal of the LED to a terminal (not shown) at the base of the cavity. A first layer of encapsulant is placed above the LED. The phosphor film is placed such that one side is in contact with the first encapsulant. Then a second encapsulant is placed such that the other side of the phosphor film is in contact with it. As a result, the phosphor film received nearly all the blue light and converted at least a portion of the blue light to yellow light. As illustrated in Figure 7, the walls of the cavity acted as a reflector and channeled the combination of blue and yellow light in the direction desired to further improve color mixing, thus enhancing the uniformity of the emitted white light.
Figure 6: Illustration of phosphor film assembly in Avago's Moostone LED package.
Figure 7: Illustration on the cavity of wall acts as reflector to shape the ray light to desired direction.
With the proposed assembly method, the phosphor film is placed in close proximity to the die, allowing the blue light emitted by the blue LED to see a consistent phosphor thickness. Thus, the conversion of blue to yellow is consistent and uniform. The uniform ratio of blue to yellow light yields an end result of more consistent perceived white light. As shown in Figure 8, the current practice of ‘binning’ white LEDs is simply a work-around to manage the variation in white color and tint that are the result of today’s manufacturing processes. The inefficient binning process creates poor yields to the manufacturer, as LEDS that don’t fit well in the desired color bins are discarded. To date, there have been no solutions to address the supply chain risk waste produced by this method. Results shown in Figure 9 reveal that contemporary color spread spans as much as 0.04 along the axes of the chromaticity chart, while the phosphor-film method is able to reduce the spread to less than 0.02.
Figure 8: White LEDs Color Binning
Figure 9: Comparison of Color Spread with Dispensing vs Phosphor Film
Conclusion
The color consistency of white LEDs may be enhanced by employing a film where the phosphor material is uniformly incorporated in GaInNbased white LEDs. Incorporating the phosphor into a film provides an accurate and consistent amount. A consistent color converting effect is achieved as the blue light ‘sees’ a consistent layer of phosphor; consequently, the ratio of blue light and yellow light is maintained, leading to the perception of a consistent white light. This technology enables the manufacturer to control the color temperature and minimize production variance. The color consistency improvement of phosphor film conversion is 50 percent compared with conventional phosphor-based white LEDs.
Phosphor film is proven to be feasible to achieve narrow color binning. However, further enhancements of the homogeneity are required as it is limited by the current process.
Acknowledgment
The author would like to thank Margaret Tan Kheng Leng for providing the support and information needed to write this article.
References
[1]. E. F. Schubert, Light-emitting Diodes, Cambridge, 2003.
