The human eye is highly sensitive to variations in light color and brightness, particularly when it comes to subtle differences in wavelength. For instance, a change of more than 1 nm in the wavelength of 585 nm light can be easily detected by the human eye, while for red light at 650 nm, even a 3 nm shift is noticeable. This sensitivity makes color and brightness uniformity critical in LED applications, especially in displays where visual consistency is essential.
In the early days, LEDs were primarily used as indicators or simple display elements, often functioning as single units. At that time, the control over their wavelength and brightness was relatively low. However, as LED efficiency and brightness improved, their applications expanded significantly. When LEDs are used in arrays or large displays, the human eye's high sensitivity to color and brightness becomes a challenge. If LEDs are not properly sorted, unevenness in color or brightness can lead to an uncomfortable viewing experience, which is undesirable both for manufacturers and consumers.
To address this, LEDs are typically tested and sorted based on key parameters such as dominant wavelength, luminous intensity, color temperature, operating voltage, and reverse breakdown voltage. This process is crucial in LED manufacturing, but it also represents a major bottleneck in production capacity and cost. As demand increases, sorting methods have evolved to meet higher standards, with modern sorters capable of categorizing LEDs into up to 72 bins. Despite these advancements, the need for even greater precision continues to grow.
There are two main approaches to sorting LEDs: chip-based testing and packaged LED testing. Chip-level sorting is particularly challenging due to the small size of LED chips, ranging from 9mil to 14mil (0.22–0.35 mm). Testing requires microprobes and precise mechanical systems, making the equipment expensive and limiting test speeds. Current chip sorters can handle around 10,000 chips per hour, with monthly capacities reaching about 5KK. While some systems use a two-step process—testing first and then sorting—the risk of misalignment during intermediate steps can reduce reliability.
On the other hand, testing and sorting packaged LEDs is more straightforward. These devices can be evaluated for wavelength, brightness, viewing angle, and voltage, allowing them to be divided into different bins. Modern sorting machines can process up to 18,000 LEDs per hour, with monthly capacities reaching around 9KK. However, the increasing demands for tighter tolerances—such as wavelength accuracy within ±1 nm or even ±0.5 nm—place additional pressure on chip manufacturers, who must now ensure higher levels of uniformity before selling their products.
While sorting LEDs after packaging is currently the most economical approach, it still faces challenges due to the vast variety of LED types and customer requirements. To truly reduce costs and improve efficiency, the focus must return to the epitaxial growth process. Achieving consistent wavelengths and brightness across the entire wafer is the ultimate goal, as it would minimize the need for extensive sorting. Until this is fully realized, improving the speed and cost-effectiveness of chip sorting remains a critical step in advancing LED technology.
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