Vacuum reflow furnace technology greatly improves the reliability of UVLED

In recent years, as a subdivided field of LED industry, UVLED industry has developed in full swing. UV LED (ultra violet light emitting diode) is a kind of ultraviolet light source that can emit different wavelengths of 200nm-400nm. Compared with traditional light sources, it has the advantages of long service life, environmental protection, low energy consumption, no thermal radiation, small volume, fast curing time and so on. At present, UV-A LED (wavelength 320-400nm) is mainly used for curing, drying and other applications, accounting for at least half of the whole UV LED application industry; UV-C LED (wavelength 200-280nm) is mainly used for sterilization, disinfection, purification and other applications. It is expected to become a new driving force of UV LED market in the future.

Different from traditional LED, UV LED needs special electrical, optical and thermal design and special scheme verification. At present, there are still great technical bottlenecks in the development of the industry, and the products have problems such as low yield and poor reliability. In order to improve the reliability of UV LED, packaging enterprises need to upgrade the use of vacuum welding process, ensure the high quality of raw materials and do a good job in the secondary heat dissipation design.

Vacuum reflow soldering: reduce void rate and improve reliability

According to different packaging methods and integration, UV LED is divided into discrete devices and integrated modules. Among them, the integrated module is divided into cob (chip on board) and DOB (device on board). Cob is to directly weld multiple LED chips on a substrate, while DOB is to first package the LED chips in the device, and then weld multiple devices on a substrate. Compared with cob, DOB is more conducive to standardized mass production. In case of poor manufacturing, DOB only loses one device. In case of light source failure during use, DOB only needs to replace the failed devices.

The research shows that the welding quality of DOB interconnection layer (including solid crystal layer and solder paste layer) has a great impact on the luminous output, total thermal resistance and reliability of UV LED. At present, the welding technology of solid crystal layer has been relatively mature, but the welding layer between device and substrate will inevitably produce bubbles and form cavities due to process problems.

According to the research, the influence of holes on the thermal resistance is: multiple randomly distributed small holes (total percentage v%), the influence on the total thermal resistance (RJC) of the device is RJC = 0.007v% + 1.4987, while the influence of multiple relatively large holes on the total thermal resistance of the device is RJC = 1.427e0.015v%.

The smaller the void rate, the better the heat dissipation. The larger the void rate, the worse the heat dissipation. Large void rate and poor heat dissipation capacity lead to low yield and poor reliability of UV LED products, which also reduces the service life of UV LED chips. Some UV LED packaging and application enterprises have suffered great losses. At present, the traditional reflow welding process is mostly used in the industry, and the welding void rate is generally as high as more than 30%

In view of the above problems, some leading enterprises in the UV LED industry have changed to vacuum welding process, which greatly reduces the void rate of UV LED chip welding and effectively improves the reliability. It is understood that Beijing Zhongke technology has developed a special vacuum welding furnace for UVLED, that is, a vacuum reflow furnace specially used for the welding of UV LED chips and modules, which can seamlessly replace the imported vacuum welding furnace. Using the UVLED vacuum welding furnace of Zhongke technology, the void rate of UV LED chip welding can be controlled below 3%.

In addition, it is worth mentioning that the special vacuum reflux furnace for UV LED of jinos electronics has been specially optimized for UV LED products. In the process of using UV LED chip, all its energy is emitted through brightness. Other welding processes will damage the surface and affect the luminescence. The Huaqi Zhengbang vacuum reflux furnace will not damage the lighting brightness of UV LED and ensure the complete release of energy.

Ensure the high quality of raw materials and the rationality of secondary heat dissipation design

In order to improve reliability, in addition to using vacuum welding furnace for packaging welding and reducing the void rate of UV LED, the following two points should also be paid attention to:

1. Select high-quality substrates, chips and solder pastes. Especially in the mass production of UV LED products, we must ensure the quality of the substrate. Ont face = 'Arial' > there are two main substrates for UV LED integrated modules, namely copper substrate and aluminum nitride (AlN) ceramic substrate. Compared with aluminum nitride ceramic substrate, copper substrate has the following advantages: lower price; The texture is solid and not easy to crack or even crack; It is easier to realize the change of shape and size. The performance and reliability of devices will vary with the choice of packaging materials.

2. Ensure the rationality of secondary heat dissipation design during the use of UV LED module. You can refer to the following rules:

a. Total electric power of UV LED module × 50% ÷ 1.3 "less than or equal to heat dissipation power" as the theoretical basis for sufficient heat dissipation. In the case of water-cooled cooling, the heat dissipation power is calculated at (outlet temperature - inlet temperature) x flow rate × The specific heat capacity of water or other heat dissipation medium is calculated by the formula. If it is air-cooled cooling, it is calculated in the same way.

b. After the completion of the experimental sample, the temperature of the heat dissipation substrate close to the UV LED is not higher than 55 ° C. (be able to test the temperature on the intermediate pad, take into account the actual equipment or actual working environment, as well as the surrounding temperature and extreme conditions).