Are your LEDs electrically overstressed? Part I
Like any semiconductor component, all LEDs are susceptible to electrical overstress (EOS). Although EOS is by far the leading cause of LED failures regardless of the manufacturer, to date there has been no data to characterize 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 mid‑power, high‑power and chip‑on‑board (COB) LEDs.
What is EOS?
EOS 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 non‑operating 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.
Hot‑plugging an LED into an energized power supply.
Recognizing 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 categorized 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, summarized in Tables 1-3.
Test Conditions
We applied square‑wave 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 forward‑bias mode, with time durations ranging from 0.1 to 70 milliseconds. 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.
We used an oscilloscope to record the waveforms of the current and voltage of each pulse applied to an LED. We calculated the power applied to, and the energy absorbed by, a device from the recorded current and voltage.
We based the test results on a small sample set (on average five samples) of each type of LED for each test condition to achieve an average threshold value for failure. We plotted mean pulse-power-to-failure versus time-to-failure profiles for all categories.
EOS Susceptibility of Mid-Power LEDs
A mid-power LED is typically a plastic package with either one or multiple chips, with the multiple chips connected in series or parallel. These different structures contribute to the overall EOS robustness of the device. Typically, LEDs with higher light output have a higher EOS robustness than those with lower light output, as shown in Figure 1.1
EOS Susceptibility of High‑Power Single‑Chip LEDs
Because LED structure has a strong effect on EOS susceptibility, we investigated how the structures of the high‑power LEDs correlate to EOS robustness. LEDs with different structures have similar light output and power ratings; however, major differences in chip architecture result in major differences in EOS robustness. Architecture differences include attachment of the die, current spreading techniques and packaging contacts. LED device structure is a major factor determining power dissipation and temperature rise, the leading cause of failure. Figure 2 compares the simulation test results for the three high-power sub-categories and shows how the chip structure relates to EOS susceptibility.
Single Chip High-Power LEDs – Structure 1
Structure 1 single‑chip high-power LEDs are the most susceptible to EOS. These LEDs have metal traces and contacts. Figure 3 is a graphical representation of the structure of these LEDs.
These components use bond wires to connect the top‑side contacts to the chips and metal traces for current spreading, which results in lower withstand power than other high‑power LEDs. Figure 4 highlights EOS robustness of the Structure 1 LEDs.
1The data for a sub-category is this and the following charts are presented as an ellipse that includes the data points for the LEDs in the subcategory.
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