Dr. Sabine Nieland Profile

Dr. Sabine Nieland

at CiSMT

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Publications (3)

Proceedings Article | 1 March 2019 Paper

Enhanced heat dissipation for high-power UV LED devices using sintering

S. Nieland, D. Mitrenga, O. Brodersen, M. Schaedel, T. Ortlepp

Proceedings Volume 10940, 109401V (2019) https://doi.org/10.1117/12.2509438

KEYWORDS: Light emitting diodes, Silver, Ultraviolet light emitting diodes, Gold, Ultraviolet radiation, Resistance, Scanning electron microscopy, Thermography, Reliability, Polymers

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Higher optical power and higher UV doses come along with a higher operation temperature of UV LED based light sources. Silver sintered pastes offer a robust lead-free alternative to solder pastes increasing the lifetime of the device and enabling higher heat dissipation. Due to the design of UV LEDs, they have to be connected to a heat spreading submount by flip chip joining. Well established processes are flip chip soldering and thermal compression bonding. However, both methods do not achieve optimal heat dissipation in practice. Solder joining material offers a thermal conductivity in the range of 50-60 W/mK which can be further reduced by voids or uncovered areas. Gold contacts for thermal compression bonding offer excellent thermal conductivity of 320 W/mK, but show a maximum coverage of 50- 70%. Silver sinter paste adapted to the UV LED contact system, is a promising alternative flip chip joining material. In order to evaluate different joining methods, UV LED devices were assembled by thermal compression bonding, diffusion bonding, soldering and sintering and compared according to, thermal resistivity, optical-electrical and mechanical behavior and reliability issues. In addition, different silver sinter pastes were tested and their thermal resistivity was adjusted via processing parameters (pressurization, sintering temperature and time). For pressureless approaches the thermal conductivity and layer thickness are in the range of solder material or below. Using pressure for sintering, several advantages will be introduced. The interconnection thickness can be adjusted to be as thin as possible (below 5 micron), which enhanced the heat dissipation. A thin sintered layer of a few microns shows a lower shrinkage and a better adhesion to the joining partners. The thermal conductivity can be enhanced as well. After sintering, the silver interconnection layer is thermally stable up to 800 °C. These facts speak for sintering pastes as a real alternative for UV LED assembly.

Proceedings Article | 1 March 2019 Presentation + Paper

Discussion on reliability issues for UVB and UVC LEDs

S. Nieland, M. Weizman, D. Mitrenga, P. Rotsch, M. Schaedel, O. Brodersen, T. Ortlepp

Proceedings Volume 10940, 1094009 (2019) https://doi.org/10.1117/12.2509418

KEYWORDS: Light emitting diodes, Ultraviolet light emitting diodes, Ultraviolet radiation, Reliability, Silver, Strontium, Skin, Manufacturing, Gold, UV optics

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The usage of UV LEDs is getting attractive for application such as phototherapy, plant growth, and disinfection due to the wavelength selective, narrow-band emission and a high potential for miniaturization of LED devices. Besides these benefits, the demands on optical power and long-term stability for these applications can often be well satisfied. For UVB LEDs most promising applications are in the field of medical skin therapy and novel concepts of horticulture and plant growth (irradiation of plants for the generation of phytamines or to reduce hormone-like mixtures). UVC applications focus on disinfection of air, surfaces and water at 265 nm or 280 nm. Each application field requires an individual UV dose, which is connected with the optical power output of the LED, and thus the number of LEDs and their long term stability. Typical doses for skin irradiation is 20 mJ/cm² at 310 nm and for water disinfection 20-60 mJ/cm² at 280 nm depending which target reduction factor log reduction of germicides is required. In this work a discussion on different factors influencing the reliability of LED modules, summarizing several years of research in this field will be given. Degradation effects are shown depending on LED design itself as well as the device assembly architecture including different mounting techniques. The most promising assembly technique was tested by a sample series of twice 400 single LED packages with a total yield of 87.7 % after mounting of UVB LEDs in single LED cases, cascading to an array on a main board by secondary soldering and burn-in of 48 hour at 50mA. In total 4% of the yield loss results by soldering issues of the LED on submount as well as another 8 % yield loss was measured after cascading of single LED packages on main board. Due to the burn-in process additional twenty UVB LEDs were lost. These reliability issues will be discussed using selected “state-of-the-art” LED device structures and examples of testing these LED devices in UV lighting lamp systems built at OSA opto Light will be given.

Proceedings Article | 16 February 2017 Presentation + Paper

Influence of interconnection on the long-term reliability of UV LED packages

S. Nieland, D. Mitrenga, D. Karolewski, O. Brodersen, T. Ortlepp

Proceedings Volume 10124, 101240S (2017) https://doi.org/10.1117/12.2250289

KEYWORDS: Light emitting diodes, Reliability, Lamps, Ultraviolet light emitting diodes, Ultraviolet radiation, Gold, Aluminum nitride, Ceramics, Thermography, Power supplies

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