In the LED production process are many failure sources to be considered. Consequently, this leads to accelerated ageing of the LED. Since the interface area of the chip to substrate bond essentially influences heat dissipation, disruptions of the interface layer cause an increase of thermal resistance and chip temperature. Delamination in the junctions between the chip and bond wire respective the chip and the substrate have also been detected. Interruptions in the current path respective the increase of electrical resistance have turned out to be frequent failure causes leading to reduced light emission, temporally instable function or even the entire failure. Furthermore, penetration of humidity or other contaminants, latent ESD (Electro Static Discharge) damage as well as an instable power supply can result in an accelerated degradation of epitaxy layers.Ī catastrophic defect like a sudden failure can be caused by ESD or EOS (Electrical Over Stress) due to electrical overload resulting in a serious damage of the epitaxy layer. LED efficiency loss within a period lower than expected life, is caused by adverse factors like low quality of epitaxy layers as well as, often, an excess junction temperature due to insufficient heat dissipation.
Therefore a sufficient control of these parameters is imperative for reaching expected lifetime.Īccelerated ageing, i.e. Usually a 30% or 50% decrease of optical performance is defined as defect while expected operation life is between 20,000 h and 100,000 h.Īgeing due to extension of defects is considerably dependent on junction temperature Tj and current. Performance decrease is caused by growing defects in the epitaxy layers or on their boundaries, resulting in an increase of not radiating recombination and a decrease of optical efficiency. In turn, this means a limitation in lifetime. Due to the very different assembly technologies and types of constructions, as well as varying applications, an extended range of failure mechanisms can be observed.ĭuring normal operation optical performance of LEDs gradually decreases during lifetime. The defects occurring on LEDs can be related to different categories which are: the chip, as the central element and the internal and the external packaging. For white LEDs phosphor is added to convert blue LED in a broadband, white appearing spectrum. LEDs are encapsulated in general with transparent material like silicone or epoxy. Essential and important criteria are a stable current path through bonding- solder- and glued connection, a sufficient cooling of the chip by a good thermal contact to the environment or appropriate heat sink, as well as a high extraction of light from the LED by optical elements and areas of reflection. LEDs are available on the market in different technologies. The primary parameters for the function are current and temperature Tj within the active layer, influencing both the power consumption and the color, which significantly determine the lifetime as well. The material and the quality of the epitaxial layers essentially determine the efficiency factor of the generation of light.
For different colors differential material-combinations are used: InAlGaP - red, InGaN - blue, GaAlAs - IR, AlGaN - UV.
The active zone is a complex structure of epitaxial layers. The LED-chip as the central element of the LED is a semiconductor that generates light in a PN-junction by electron p-hole recombination. LEDs – Principal Function and Technologies
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