Prognostics health management model for LED package failure under contaminated environment
Lall, P., Zhang, H., & Davis, L. (2015). Prognostics health management model for LED package failure under contaminated environment. In ASME International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems (InterPACK) InterPACK/ICNMM 2015 - 48724, 1-5 Silicon Photonics and LED section, 6-9 July, 2015, San Francisco, CA,.
Apart from the improvement in die technology, the challenges for the high brightness LED lies in the package technology, especially in the package material such as encapsulation, to ensure the reliability of the whole LED system. The industry use epoxy as the encapsulation material few years ago. Epoxy has very good light transmission that is suitable for LED application. However, epoxy LED encapsulants were found to yellow during use which reduces luminous flux and shifts the color point due to absorption of blue photons. With expectation of higher luminous flux output, higher forward current and better long term reliability, silicone materials has been introduced to be new generation of encapsulation materials for LED. Silicones can be formulated with different hardness properties (aka, durometer), which can be gel, elastic, and hard types. Silicone used in LED encapsulation is generally elastic type to give the flexibility to encapsulated wire bound and die attach. This characteristic provides the ability to absorb thermal stress during the soldering process and during operation at high power as well. The chemical structure of silicone provides several advantages over epoxy encapsulation used in the optoelectronic products. The advantage includes the thermal and UV stability of silicone, which are the main characteristics needed for new generation LED products. Due to these optical advantages, silicone materials have been used in the phosphor binder, encapsulation and lens of LED package. During the fabrication of vertical LED package, laser lift off (LLO) and wafer bonding technology are used to transfer the thin film chip to the second silicon based substrate. Ag based material has been used to bond the thin film chip with the conductive substrate. In this work, commercially available vertical structure LEDs were used to examine the failure mechanism in the harsh and contaminated environment. The encapsulation and lens are both silicone materials but with different durometers. Near the top of the chip, there is a combined layer of phosphor particles and silicone binder that is used to produce white light from a blue LED.