Just the other day, one of the LED light bulbs in our kitchen short circuited. It made a huge popping sounds and released a lot of smoke as it popped out of the ceiling socket. The bulb burnt out and generated so much heat that it caused a hole at the base of the LED bulb.
Images of our kitchen ceiling light socket where the LED bulb popped out from, the burnt epoxy resin in the discoloured LED bulb, the hole in the LED packaging from the overheating/short circuiting of the LED bulb.
Theoretically, LED light bulbs can last for as long as one hundred thousand hours (~11 years) under perfect conditions (Zhaohui et al. 2011). But just like other light bulbs (e.g. incandescent and CFL), material degradation and damage to the packaging from electrical, thermal, chemical or mechanical stress can lead to its early failure. LED failure usually means the light dims over time or burns out completely. Electric or structural damage can cause the suddenly failure of the LED, like ours which burned out.
The main component of LED light bulbs is the LED-chip, which is a semiconductor that generates light. The chip consists of P-type and N-type semiconductors with a transition layer in between these two types of semiconductors.
But without putting our burned out LED in an electron microscope, it’s hard to say exactly what caused it to fail. A couple of research studies (Lu et al. 2009, Shailesh et al. 2012) looking at ways LEDs can fail, report that thermal or electrical stress are two causes of failure. Thermal stress failure is caused by cyclical heat in the LED where the internal temperature is over its maximum rating.
Two common causes of failure are electrical overstress (EOS) damage and Electrostatic Discharge Breakdown (ESD) (Lu 2009). EOS occurs when a voltage or current level exceeds the rating for periods exceeding 100 to 1000 nanoseconds while the LED is in use (Lu 2009). EOS can be caused by voltage or lightning strikes or temporary and unexpected connection to power or ground according to the study’s authors. EOS can damage the wires inside the LED package or the LED chip itself (Lu 2009). This is a key issue during manufacturing and handling according to Lu (2009).
The transition layer between the P-type and N-type semiconductors of the LED chip is known as a PN junction. In ESD, the junction can breakdown when excessive current flow increases the local temperature (Lu 2009). The heat can destroy the junction, resulting leakage or short-circuit (Lu 2009).
LED packaging serves as the electrical connection between the LED chip and external circuit. The packaging protects the LED chip from damage such as ESD. A GaN-based chip is typically packaged with other materials including an epoxy lens, silicone glue, phosphor coating.
As these materials degrade they can lead to failure of the LED package. For example, interface delamination between layers that make up the LED chip can cause open circuit or heat dispersion problems (Fan et al. 2011).
Researchers are continuously studying to understand the failure mechanisms of LED light bulbs to come up with ways of improving their reliability. One area they are focusing on is improving the heat dissipation of the LED packaging structure. Good heat dissipation will prevent heat from accumulating and raising the temperature of the LED chip.
Additionally, ESD protection circuits have been used to protect against ESD failures by getting the current caused by an electrostatic discharge to bypass the LED and flow through the protection circuit instead. Even though LED bulbs will eventual fail they are currently still reliable alternatives to incandescent bulbs, and the reliability of LED bulbs will likely improve over time.
Chen, Z., Zhang, Q., Wang, K., Luo, X., & Liu, S. (2011). Reliability test and failure analysis of high power LED packages Journal of Semiconductors, 32 (1) DOI: 10.1088/1674-4926/32/1/014007
Fan, J., Yung, K., & Pecht, M. (2011). Physics-of-Failure-Based Prognostics and Health Management for High-Power White Light-Emitting Diode Lighting IEEE Transactions on Device and Materials Reliability, 11 (3), 407-416 DOI: 10.1109/TDMR.2011.2157695
Lu, G., Yang, S., Huang, Y. (2009). Analysis on Failure Modes and Mechanisms of LED IEEE DOI: 10.1109/ICRMS.2009.5270043
Shailesh, K.R., Kurian, C.P., Kini, S. G. (2012). Solid State Lighting Reliability from Failure Mechanisms Perspective: A Review of Related Literature. International Electrical Engineering Journal, 3 (2), 764-768