Two LED sorting methods based on chip and package

The human eye has a very high resolution for the color and brightness of light, and is particularly sensitive to differences and changes in color. The human eye has different sensitivities to light of different color wavelengths. For example, for light with a wavelength of 585nm, the human eye can sense when the color changes by more than 1nm; while for red light with a wavelength of 650nm, the human eye can only perceive when the color changes by 3nm.

In the early days, since LEDs were mainly used as indicators or display lights, and generally appeared as single devices, the requirements for wavelength sorting and brightness control were not high. However, with the continuous improvement of LED efficiency and brightness, its application range is getting wider and wider. When LEDs are used as array display and display screen devices, due to the sensitivity of the human eye to color wavelength and brightness, the use of unsorted LEDs will produce unevenness, which will affect people's visual effects. The unevenness of wavelength and brightness will make people feel uncomfortable. This is something that all LED display manufacturers are unwilling to see, and it is also unacceptable to people.

LEDs are usually tested and sorted according to several key parameters such as dominant wavelength, luminous intensity , luminous flux, color temperature , operating voltage, reverse breakdown voltage, etc. LED testing and sorting is a necessary process in the LED production process. At present, it is the production capacity bottleneck of many LED chip and packaging manufacturers, and is also an important part of the production and packaging costs of LED chips.

1. LED sorting method

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 being reliable, but very slow and low in production capacity;

Another method is that testing and sorting are completed by two machines. The testing equipment records the location and parameters of each chip, and then transmits this 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. This is because there is usually a process of substrate thinning and chip separation between the two steps of testing and sorting. 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 cost of the chip 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, and later increased to 64 bins. 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, these packaging factories 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 workbench voltage; for example, the wavelength requirement was +2nm in the past, but now it is required to be +1nm, and even in some applications, a requirement of +0.5nm has been put forward. This creates huge pressure on chip factories, and strict sorting must be carried out before the chips are sold.

From the above analysis of LED and LED chip sorting, it can be seen that the more economical approach is to test and sort LED. However, due to the wide variety of LEDs, there are different forms, shapes, sizes, luminous angles, customer requirements, and application requirements. It is difficult to sort products 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 plant to prepare all forms and types of LEDs required by all customers. So the key to the problem goes back 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.

2. Sorting equipment

At present, LED chip testing and sorting equipment is mainly provided by American and Japanese manufacturers, while LED testing and sorting equipment is mostly provided by manufacturers in Taiwan and Hong Kong. There is no manufacturer in mainland China that can provide similar equipment. LED chip sorting machine mainly includes two hardware parts (robot, microprobe and optoelectronic tester) and a set of system software, and these three parts are provided by different manufacturers and then integrated; while LED test sorting machine includes two parts: mechanical transmission, storage and optoelectronic testing of LED.

3. Development trend of LED sorting technology

(1) Before the uniformity of epitaxial wafers is controlled, develop fast and low-cost chip sorting processes and equipment.

(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. It is necessary to develop new test standards and methods that include more lighting-related optical content.

(3) In the LED system life test, develop technology for rapid measurement and evaluation of the long-term performance and life of the LED system.

(4) LEDs for lighting are driven by high current, which places higher reliability requirements on them. The traditional LED screening method is not suitable for high-power LEDs for lighting . New screening test methods must be developed to eliminate early failure products and ensure product reliability.