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Identify electrically overstressed LEDs (Part 1)

Posted: 22 May 2014 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? electrical overstress? EOS? electrostatic discharge? ESD?

All LEDs are susceptible to electrical overstress (EOS), just like any semiconductor component. Although EOS is by far the leading cause of LED failures regardless of the manufacturer, to date there has been no data to characterise EOS failures or determine the specific conditions that cause EOS failures. We tested more than a dozen commercially available LEDs from multiple manufacturers to identify the conditions that cause EOS failure in midpower, highpower and chiponboard (COB) LEDs.

What is EOS?
It is the exposure of an LED to current or voltage beyond its maximum specifications. EOS failures result from excessive localized heat generated by the current or voltage transient that accompanies the EOS event. Like all semiconductor devices, LEDs have a limited ability to survive overstress, which we refer to as the maximum withstand power.

EOS differs from electrostatic discharge (ESD), the rapid transfer of static charge between a nonoperating part and an object at a different electrical potential. EOS events have a duration that ranges from milliseconds to seconds, which is longer than ESD events that typically range from picoseconds to nanoseconds.

EOS can be a single event or an ongoing periodic or non-periodic event. Following are some typical causes of EOS:
???A driver that produces a current spike.
???Constantly driving the LED over its maximum rated current.
???A power surge from the main AC power input, such as a lightning strike.
???Hotplugging an LED into an energized power supply.

Recognising EOS damage
LED failures caused by EOS vary from subtle to severe damage depending on the amplitude and duration of the overstress conditions. An LED with severe EOS damage does not emit light. An LED with subtle damage does not emit light at low current but does emit light at high current.

An affected LED may exhibit current leakage, a resistive short or an open circuit. LEDs with subtle damage usually contain isolated damage sites within the epitaxial (epi) structure that can be identified only by measuring leakage current through the junction. Under normal operating conditions, these LEDs appear to function properly. During continued operation, localized heating at the damage sites causes the impacted areas to increase in size, which in turn increases the junction leakage. Continued operation often results in short-circuit electrical characteristics. The amount of time it takes for a damaged LED to progress through these states depends on the EOS conditions, junction temperature and operating conditions.

The LEDs
We investigated mid-power, high-power and COB LEDs from several manufacturers and categorised the LEDs based on criteria including device performance, chip structure and power ratings. We subdivided the general categories to explore EOS susceptibility for specific chip characteristics. This further division led to multiple sub-categories within each category.

Test conditions
We applied squarewave pulses of forward currents to the LEDs to simulate EOS conditions, incrementally increasing the pulse voltage until the LED failed. We applied pulse power levels up to 1700 W to the LEDs in forwardbias mode, with time durations ranging from 0.1 to 70ms. We based the failure criteria on junction leakage current measurements performed after each pulse. If the leakage current exceeded the device specifications, we considered the result a failure and conducted failure analysis to investigate the relationship between overstress conditions and the appearance of damage.

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