The human eye has a very high resolution of light color and brightness, and is particularly sensitive to differences in color and changes in color. The sensitivity of the human eye to different wavelengths of light is different. For example, for a light with a wavelength of 585 nm, the human eye can feel it when the color change is greater than 1 nm, while the red light with a wavelength of 650 nm can be perceived by the human eye when the color change is 3 nm.
In the early days, since the LED was mainly used as an indicator or display lamp, and generally appeared as a single device, the requirements for wavelength sorting and brightness control were not high. However, with the continuous improvement of the efficiency and brightness of LED, its application range is becoming wider and wider. When the LED is used as an array display and display device, due to the sensitivity of the human eye to the wavelength and brightness of the color, the non-selected LED will have a non-uniform phenomenon, thereby affecting people's visual effects. Uneven wavelengths and uneven brightness can give people an uncomfortable feeling. This is not the LED display manufacturers are willing to see, but also people can not accept.
LEDs are usually tested and sorted according to several key parameters such as the main wavelength, luminous intensity, light on, color temperature, operating voltage, and reverse breakdown voltage. LED testing and sorting is an essential step in the LED manufacturing process. At present, it is the bottleneck of production capacity of many LED chips and package manufacturers, and it is also an important component of LED chip production and packaging costs.
1, LED sorting method
LED sorting has two methods: one is the chip-based test sorting, and the other is the test sorting of packaged LEDs.
(1) Test Sorting of Chips
LED chip sorting is very difficult, mainly because the LED chip size is generally very small, from 9mil to 14mil (0.22-0.35nm). Such a small chip requires a microprobe to complete the test. The sorting process requires an accurate mechanical and image recognition system, which makes the cost of the device very high and the test speed is limited. Today's LED chip testing and sorting machines cost about 1 million yuan RMB per unit, and its testing speed is around 10,000 per hour. If calculated according to 25 days per month, the capacity of each sorter is 5 KK per month.
Currently, there are two methods for testing the chip's sorting:
One way is to test the sorting done by the same machine. Its advantages are reliable, but its speed is slow and its productivity is low;
Another method is that testing and sorting are done by two machines. The test equipment records the location and parameters of each chip, and then passes this data to the sorting equipment. The advantage of quick sorting is that it is fast. However, the disadvantage is that the reliability is relatively low and it is easy to make mistakes, because there is usually a process of substrate thinning and chip separation between the two steps of testing and sorting. During this process, the epitaxial wafer may be broken and partially broken. Incomplete fragmentation or partial incompleteness makes the actual chip distribution inconsistent with the data stored in the sorter, making it difficult to sort.
The key to solving the bottleneck problem of chip test sorting fundamentally is to improve the uniformity of the epitaxial wafers. If an epitaxial wafer has a wavelength within 2 nm and the change in brightness is within +15%, all the chips on the wafer can be classified as a bin, as long as the failed chip is removed by testing. Greatly increase the capacity of the chip and reduce the cost of the chip. In the case where the uniformity is not very good, it is also possible to test and discharge the area of ​​the chip with “more defective products†by ink-jet smearing to quickly obtain the desired “qualified†chip, but doing so The cost is too high, and many chips that meet the requirements of other rooms will be treated as scraps of unqualified certificates. Finally, the calculated chip costs may be unacceptable to the market.
(2) LED test sorting
The packaged LEDs can be tested and sorted according to wavelength, luminous intensity, light emitting angle, and operating voltage. As a result, the LED is divided into a plurality of bins and categories, and then the test sorting opportunity automatically divides the LED into different bins according to the set test standards. Due to the increasing demand for LEDs, the early sorters were 32Bin and later increased to 64Bin. Now there are 72Bin commercial sorters. Even so, the LED technology index of Bin can't still meet the demand of production and market.
The LED test sorter tests the LEDs at a specific table current (eg, 20 mA), and generally performs a reverse voltage value test. The current price of LED testing and sorting machines is about 40 to 500,000 yuan/set, and the test speed is about 18,000 per hour. If calculated according to the working time of 20 hours a day for 25 days a month, the capacity of each sorter is 9 KK per month.
Large-scale display screens or other high-end application customers have high quality requirements for LEDs. In particular, the requirements for wavelength and brightness consistency are very strict. If LED packaging factories do not make stringent requirements when purchasing chips, these packaging companies will find that after a large number of packages, only a small number of products in the packaged LED can meet the requirements of a certain customer, and most of the others will change. Inventories in the warehouse. This situation forced the LED packaging plant to impose stringent requirements when purchasing LED chips, especially for wavelength, brightness, and bench voltage; for example, in the past, the wavelength requirement was +2 nm, but now the requirement is +1 nm, even in some applications. Above, a requirement of +0.5 nm has been proposed. This has created tremendous pressure on the chip factory, which must be rigorously sorted before it can be sold.
From the above analysis of LED and LED chips, it can be seen that the more economic approach is to test and sort LEDs. However, due to the variety of types of LED, there are different forms, different shapes, different sizes, different lighting angles, different customer requirements, and different application requirements. This right to use can be determined by the LED test. Selection becomes difficult to operate. At present, the application of LED is mainly distributed in the range of several wavelength segments and luminance segments. It is difficult for a packaging plant to prepare all kinds and types of LEDs required by all customers. Therefore, the key issue is back to the MOCVD epitaxial process. The growth of LED epitaxial wafers with the required wavelength and brightness is a key point to reduce costs. This problem is not solved and the LED production capacity and cost will still not be completely resolved. However, before the uniformity of the epitaxial wafer is controlled, an effective method is to solve the problem of fast and low-cost chip sorting.
2, sorting equipment
At present, LED chip testing and sorting equipment is mainly provided by the United States and Japan, and LED testing and sorting equipment is mostly provided by the Taiwan region, Hong Kong manufacturers, China has not yet provided similar equipment manufacturers. The LED chip sorter mainly includes two hardware components (manipulator, microprobe, and optoelectronic tester) and a set of system software. These three components are separately provided by different manufacturers and integrated; and the LED test component selector includes LED mechanical transmission, storage and optoelectronic testing in two parts.
3, LED selection technology development trend
(1) The rapid and low-cost chip sorting process and equipment are developed before the uniformity of the epitaxial wafer is controlled.
(2) With the development of W-class power LED technology, traditional LED product parameter detection standards and test methods can no longer meet the needs of lighting applications, and new test standards and methods must be developed to include more lighting-related optical content.
(3) In the LED system life test, the long-term performance and life-span rapid measurement and evaluation technology of the LED system is developed.
(4) The LED for lighting is driven by a large current, which imposes higher reliability requirements on it. Traditional LED screening methods are not suitable for lighting high-power LEDs. New screening testing methods must be developed to eliminate early failure products and ensure product reliability.
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