The human eye has a very high resolution for the color and brightness of light, and is particularly sensitive to color differences and changes. Human eyes have different sensitivities to light of different color wavelengths. For example, for light with a wavelength of 585 nm, the human eye can perceive when the color change is greater than 1 nm; for red light with a wavelength of 650 nm, the human eye can only perceive when the color change is 3 nm.
In the early days, since LEDs were mainly used as indicators or display lamps, 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 getting wider and wider. When LEDs are used as array display and display devices, due to the sensitivity of the human eye to the color wavelength and brightness, the use of unsorted LEDs will produce uneven phenomena, which will affect people's visual effects. Unevenness in wavelength and brightness can create an uncomfortable feeling. This is something that LED display manufacturers are reluctant to see, and it is also unacceptable.
LED is usually tested and sorted according to several key parameters such as main wavelength, luminous intensity, luminous brightness, color temperature, working voltage, reverse breakdown voltage and so on. LED testing and sorting is a necessary process in the LED production process. At present, it is the capacity bottleneck of many LED chip and packaging manufacturers, and also an important part of LED chip production and packaging costs.
1. LED sorting method
There are two methods for LED sorting: one is chip-based test sorting, and the other is test sorting of packaged LEDs.
(1) Test and sorting of chips
LED chip sorting is very difficult, the main reason is that 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, and the sorting process requires an accurate mechanical and image recognition system, which makes the cost of the equipment very high and the test speed is limited. The current LED chip test sorting machine price is about 1 million yuan, and its test speed is about 10,000 per hour. If calculated according to 25 days per month, the capacity of each sorting machine is 5KK per month.
At present, there are two methods for chip test sorting:
One method is that the test sorting is done by the same machine. Its advantage is reliable, but the speed is very slow and the production capacity is low;
Another method is that the testing and sorting are done by two machines. The test equipment records the position and parameters of each chip, and then transfers these data to the sorting equipment for rapid sorting. The advantage of this is that it is fast. But 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, and in this process, the epitaxial wafer may be cracked and partially Fragmentation or partial defects make the actual chip distribution inconsistent with the data stored in the sorter, which makes sorting difficult.
The key to fundamentally solving the bottleneck problem of chip test sorting is to improve the uniformity of the epitaxial wafer. If the wavelength distribution of an epitaxial wafer is within 2nm and the change in brightness is within + 15%, you can classify all the chips on this wafer as a single file (Bin), as long as the unqualified chips are removed by testing, Will greatly increase chip production capacity and reduce chip cost. When the uniformity is not very good, you can also use the test to deal with the "more defective products" chip area by inkjet smearing method, so as to quickly get the desired "qualified" chip, but do so The cost is too high, and many chips that meet the requirements of other rooms will be treated as unqualified waste. The final cost of the chip may be an unacceptable level in the market.
(2) LED test sorting
The packaged LEDs can be tested and sorted according to wavelength, luminous intensity, luminous angle and operating voltage. As a result, the LEDs are divided into many files (Bin) and categories, and then the test sorting machine automatically divides the LEDs into different Bin boxes according to the set test standards. As people's requirements for LEDs are getting higher and higher, the early sorter was 32Bin, which was later increased to 64Bin, and now there are 72Bin commercial sorters. Even so, the LED technical indicators of Bin are still unable to meet the production and market demands.
LED test sorting machine is to test the LED under a specific workbench current (such as 20mA), and generally do a reverse voltage value test. The price of the current LED test sorting machine is about 400,000 to 500,000 yuan per unit, and the test speed is about 18,000 per hour. According to the calculation of working hours of 20 hours per day for 25 days per month, the capacity of each sorting machine is 9KK per month.
Customers with large display screens or other high-end applications have higher requirements for LED quality. Especially in the wavelength and brightness consistency requirements are very strict. If LED packaging factories do not put forward strict requirements when purchasing chips, these packaging factories 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 customer, and most of the rest will change Into the inventory in the warehouse. This situation forced LED packaging factories to put forward strict requirements when purchasing LED chips, especially the wavelength, brightness and table voltage indicators; for example, in the past, the wavelength requirement was + 2nm, but now it is required to be +1 nm, even in In some applications, a requirement of +0.5 nm has been proposed. This puts tremendous pressure on the chip makers and must be strictly sorted before the chips are sold.
As can be seen from the above analysis on the selection of LEDs and LED chips, the more economical approach is to test and sort the LEDs. However, due to the large variety of LEDs, there are different forms, invisible shapes, different sizes, different light angles, different customer requirements, and different application requirements. This right of use is completely selected by LED test points for product classification Selection becomes difficult to operate. Moreover, the current application of LEDs is mainly distributed in the range of several wavelengths and brightness. It is difficult for a packaging factory to prepare all forms and types of LEDs required by all customers. So the key to the problem is back to the epitaxy process of MOCVD, how Growing the LED epitaxial wafer with the required wavelength and brightness is the key point to reduce the cost. If this problem is not solved, the LED capacity and cost will still not be completely solved. However, before the uniformity of the epitaxial wafer is controlled, a more effective method is to solve the problem of fast and low-cost chip sorting.
2. Sorting equipment
At present, LED chip test and sorting equipment is mainly provided by American and Japanese manufacturers, while LED test and sorting equipment is mostly provided by manufacturers in Taiwan and Hong Kong. There is no manufacturer in China that can provide similar equipment. The LED chip sorting machine mainly includes two major hardware parts (robot, microprobe and photoelectric tester) and a set of system software, and these three parts are provided by different manufacturers and then integrated; and the LED test sorting machine includes LED mechanical transmission, storage and photoelectric testing are two parts.
3. Development trend of LED selection technology
(1) Before the uniformity of the epitaxial wafer is controlled, develop a fast and low-cost chip sorting process and equipment.
(2) With the development of W-level 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 life test of LED system, develop the technology of rapid price determination of long-term performance and life of LED system.
(4) The LED for lighting works under high current drive, which puts forward higher reliability requirements. The traditional LED screening method is not suitable for high-power LEDs for lighting. New screening test methods must be developed to eliminate early failures and ensure product reliability.
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