Today's optical effects can be explained by the principle of optical three primary colors. When white light is refracted through a diamond mirror, it is refracted into a variety of color transition chromatograms. In the visible light range of 400 ~ 800um, the colors can be divided into red, orange, yellow, green, blue, and blue , Visible spectrum of purple. The human eye is like a three-color receiver, which is particularly sensitive to the three colors of red (620um), green (555um), and blue (470um), while other colors can be adjusted in different proportions through the three colors of red, green, and blue. The same, most monochromatic light can also be decomposed into three primary colors of red, green and blue.
According to the principle of the three primary colors, the three primary colors of red, green, and blue are independent of each other. No one primary color can be synthesized from other colors, and the three primary colors can be added and mixed in different proportions to form a mixed color called an additive mixed color. It can be seen from Figure 1 that the rules are: red + green = yellow, red + blue = purple, blue + green = green, red + blue + green = white, so white sunlight can be refracted by water vapor to show the colors of the dazzling rainbow .
It can be understood from the above principles that white LEDs can also be mixed out in this way, so there are usually 3 ways to deploy white LEDs.
Method 1 is a multi-chip light mixing technology, which fixes three chips of red, blue, and green or two chips of blue and yellow light in the same package, and then adjusts the light output of each chip to control the light mixing ratio by adjusting the current of each chip To achieve the goal of mixing white light. Among them, the red, blue, and green multi-chip mixed light technology presents the best color saturation and color rendering (Color RendiTIon), but it must also overcome the problems of chip light decay, excessive heat source concentration and heat dissipation packaging. If any of the chips fail early, the light source for the white light cannot be obtained.
Method 2 is to excite uniformly mixed blue, green, and red phosphors with ultraviolet LEDs, so that they excite a certain proportion of the three primary colors to mix and output white. The three-wavelength white light-emitting diode has the advantages of high color rendering, but has the disadvantages of insufficient luminous efficiency and uneven light mixing.
Method 3 Around the blue LED = glue filled with YAG (Yttrium Aluminum Garnet) phosphor, and use a blue LED with a wavelength of 400 ~ 530nm to emit light to excite the yellow YAG phosphor to produce yellow light, but At the same time, it also mixes with the original blue light, and then forms blue and yellow mixed two-wavelength white light.
Since the third method is simple and low in cost, it is often packaged in this way, but such LEDs will have high color temperature and unevenness and a weak red spectrum, resulting in poor color rendering.
Since white LEDs with blue phosphors that excite yellow phosphors have these problems, technology has been introduced to solve this problem, and controlling the uniformity of fluorescent glue can be done in two ways. First discuss the colloidal mixing technology, which can be used to uniformly mix the phosphor powder in the optical glue by the vacuum stirring method to improve the uniformity of light distribution. In addition, it can also improve the uniform coating technology of phosphor powder. There are many coating methods today. First, the phosphor powder is sprayed on the wafer by the nozzle, but the problem of uneven spraying needs to be solved. In addition, it can also be attached to the chip using fluorescent adhesive tape.
This method can solve the problem of the thickness and distribution of fluorescent glue, but there will be problems of fitting alignment and gold wire breakage. The two technologies mentioned above require the use of higher-precision machines and technologies, so the introduction threshold is higher. For those who are interested in entering the packaging field, considering the machine cost and technical threshold, most of them are the simplest Dispensing technology for packaging, but dispensing will face uneven coating thickness and different distribution range.
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