LEDs, being semiconductor components, are susceptible to failure if overstressed electrically. This is true regardless of the manufacturer and electrical overstress or EOS is the leading cause of failure of LEDs. In fact, LED components are subject to transient conditions that can cause EOS and subsequently result in a catastrophic failure.
Like all semiconductor components, LEDs too have their maximum specifications of voltage, current and power. An exposure beyond the maximum current or voltage levels can lead to EOS. Typically, a current or voltage transient, accompanying the EOS event, may cause generation of localized heat – leading to EOS failure. As with any semiconductor device, an LED also has only a limited ability to survive overstress, and this is its maximum withstanding power.
EOS must not be confused with electrostatic discharge or ESD. Electrostatic discharge is the result of a rapid transfer of static electric charge between a non-operating part and an object at a different electrical potential. ESD events typically range from pico- to nano-seconds, whereas EOS events are much slower, ranging from milli-seconds to seconds. Moreover, EOS can be only a single event, an ongoing periodic event or even a non-periodic event. Common causes of EOS are:
• A driver producing current spikes
• A driver constantly driving an LED over its maximum rated current
• A lightning strike or similar power surge from the AC mains power input
• A user hot-plugging an LED into an energized circuit
Depending on the duration and amplitude of the overstress conditions, LED failures due to EOS can vary from subtle to severe damage. For example, an LED with subtle damage may not emit light at low currents, but does so at higher current levels. On the other hand, a severely damaged LED may not emit light at all. Both may exhibit current leakage, an open circuit or a resistive short. The amount of time that it takes for an LED to be damaged by EOS, depends on the conditions of the EOS, operating conditions and the LED junction temperature.
LEDs may be classified into three types – mid-power, high-power and COB. Test laboratories typically use square-wave pulses of forward current for simulating EOS conditions in LEDs. This allows variation of all test parameters such as voltage, current, power and time. For example, pulse power levels of up to 1700W may be applied to LEDs in forward-bias mode, while the time duration may range from 0.1 to 70 milliseconds.
Most mid-power LEDs are typically enclosed in a plastic package and contain either one or multiple chips. The multiple chips may be internally connected in parallel or in series. The EOS robustness of the device depends on the internal structure. As a thumb rule, LEDs with higher light output tend to be more robust to EOS.
The EOS robustness of high-power single-chip LEDs depends on their architecture. LED device structure, such as the packaging contacts, current spreading techniques and attachment of the die, are major contributors to determining temperature rise and power dissipation and hence EOS robustness.
COB or chip-on-board LEDs are similar to high-power single-chip LEDs, with one major difference. There are bond wires connecting the top-side contacts to the chips and metal traces for current spreading, resulting in lower withstanding power as compared to other high-power LEDs.
