Sorting and testing of LED chips and devices

There are two methods for sorting LEDs : one is chip-based test sorting, and the other is test sorting of packaged LEDs.

　　(1) Chip testing and sorting

　　LED chip sorting is very difficult, mainly because the size of LED chips is generally very small, ranging from 9mil to 14mil (0.22-0.35nm). Such small chips require microprobes to complete the test, and the sorting process requires precise machinery and image recognition systems, which makes the equipment very expensive and limits the test speed. The current LED chip test sorting machine costs about 1 million yuan per unit, and its test speed is about 10,000 per hour. If calculated based on 25 days per month, the production capacity of each sorting machine is 5KK per month.

　　At present, there are two methods for testing and sorting chips: one method is that the testing and sorting are completed by the same machine, which has the advantage of reliability, but is very slow and has low production capacity; the other method is that the testing and sorting are completed by two machines, and the testing equipment records the position and parameters of each chip, and then transmits these data to the sorting equipment for rapid sorting. The advantage of this method is speed, but the disadvantage is that the reliability is relatively low and it is prone to errors, 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 broken or partially damaged, making the actual chip distribution inconsistent with the data stored in the sorting machine, causing sorting difficulties.

　　The key to fundamentally solving the bottleneck problem of chip testing and sorting is to improve the uniformity of epitaxial wafers. If the wavelength distribution of an epitaxial wafer is within 2nm and the brightness change is within +15%, all chips on this wafer can be classified into one bin. As long as the unqualified chips are removed through testing, the chip production capacity will be greatly increased and the chip cost will be reduced. In the case of poor uniformity, the chip area with "more unqualified products" can also be treated by testing and inkjet smearing to quickly get the desired "qualified" chip, but the cost of doing so is too high, and many chips that meet the requirements of other guest rooms will be treated as unqualified waste. The final calculated chip cost may be at a level that the market cannot accept.

　　(2) LED testing and sorting

　　After packaging, LEDs can be tested and sorted according to wavelength, luminous intensity, luminous angle, and operating voltage. The result is that LEDs are divided into many bins and categories, and then the test sorting machine automatically packs the LEDs into different bins according to the set test standards. As people's requirements for LEDs are getting higher and higher, the early sorting machines were 32 bins, which were later increased to 64 bins, and now there are commercial sorting machines with 72 bins. Even so, the technical indicators of binned LEDs still cannot meet the needs of production and the market.

　　LED test sorting machine tests LEDs at a specific workbench current (such as 20mA), and generally also performs a reverse voltage test. The current price of LED test sorting machine is about RMB 400,000 to 500,000 per unit, and its test speed is about 18,000 per hour. If calculated based on 25 days per month and 20 hours per day, the production capacity of each sorting machine is 9KK per month.

　　Customers of large display screens or other high-end applications have high quality requirements for LEDs. In particular, they are very strict about the consistency of wavelength and brightness. If LED packaging factories do not put forward strict requirements when purchasing chips, they will find that only a small number of packaged LEDs can meet the requirements of a certain customer after a large number of packaging, and most of the rest will become inventory in the warehouse. This situation forces LED packaging factories to put forward strict requirements when purchasing LED chips, especially the indicators of wavelength, brightness and working table voltage; for example, in the past, the requirements for wavelength

　　It used to be +2nm, but now it is required to be +1nm, and even in some applications, it has been required to be +0.5nm. This has created tremendous pressure on chip factories, and they must conduct strict sorting before selling chips.

　　From the above analysis of LED and LED chip sorting, it can be seen that the more economical approach is to test and sort LEDs. However, due to the wide variety of LEDs, different forms, different shapes, different sizes, different luminous angles, different customer requirements, and different application requirements, it becomes difficult to sort products completely through LED testing and sorting. Moreover, the current application of LEDs is mainly distributed in several wavelength bands and brightness bands. It is difficult for a packaging factory to prepare all forms and types of LEDs required by all customers. Therefore, the key to the problem returns to the MOCVD epitaxial process. How to grow LED epitaxial wafers with the required wavelength and brightness is the key to reducing costs. If this problem is not solved, the production capacity and cost of LEDs will not be completely solved. But before the uniformity of the epitaxial wafer is controlled, the more effective method is to solve the problem of fast and low-cost chip sorting.