UV LEDs offer numerous advantages over traditional UV light sources, such as energy efficiency, longer lifespan, lower power consumption, and precise wavelength selection. Depending on the emitted wavelength, UV LEDs can be categorized into three types: UVA (315–400 nm), UVB (280–315 nm), and UVC (200–280 nm). Typically, wavelengths above 300 nm are classified as near-ultraviolet (NUV), while those below 300 nm are considered deep ultraviolet (DUV). In terms of packaging and integration, UV LEDs can be divided into discrete devices and integrated modules.
Integrated modules include COB (Chip On Board) and DOB (Device On Board) configurations. In COB, multiple LED chips are directly soldered onto a substrate, whereas in DOB, the chips are first encapsulated in a device before being mounted on the substrate. For near-ultraviolet UV LEDs, which typically have a vertical structure, silver paste is commonly used for die attach due to its high electrical conductivity and thermal performance. Solder paste is often used for device placement during the assembly process.
The interconnect layer plays a critical role in determining the reliability and performance of UV LEDs. Research has shown that the quality of the interconnect layer significantly affects light extraction efficiency, total thermal resistance, and overall reliability. For example, studies have demonstrated that voids in the interconnect layer increase thermal resistance, thereby reducing the device's reliability. Amy S. Fleischer et al. found that small randomly distributed voids affect thermal resistance in a linear manner, while larger voids have an exponential impact.
High void ratios in the interconnect layer can lead to increased operating temperatures, which in turn reduce the lifespan of the device. It is well-documented that every 10°C rise in temperature can double the failure rate of electronic components. Therefore, controlling the void ratio in the interconnect layer is essential for ensuring the performance and reliability of UV LEDs.
Currently, different UV LED manufacturers use varying packaging technologies, leading to differences in interconnect quality. For instance, some companies report void ratios in their interconnect layers exceeding 20%, as seen in X-ray images of UV LED chip die-attach and device placement layers. This issue becomes even more challenging when dealing with large chips (larger than 40 mils) where achieving low void ratios is particularly difficult.
To address these challenges, this paper explores the process optimization of both the solid crystal layer and the solder paste layer for UV LED discrete devices and integrated modules. Through extensive testing and analysis, optimal process parameters were identified. For example, using a multi-head dispensing system, the void ratio in the solid crystal layer was reduced to less than 3%. Similarly, for the solder paste layer in DOB modules, the void ratio was controlled to around 5% or lower, with over 80% of samples meeting this standard.
These improvements significantly enhance the thermal performance and reliability of UV LEDs. By optimizing the dispensing process, stencil design, and reflow oven parameters, it is possible to achieve consistent and high-quality interconnect layers. This not only improves the photothermal characteristics of the devices but also extends their operational life, making them more suitable for demanding applications.
In summary, the interconnect layer is a key factor in the performance and reliability of UV LEDs. Through process optimization and advanced manufacturing techniques, the industry can overcome the challenges associated with void formation, leading to more efficient and durable UV LED solutions.
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