Semiconductor-based light-emitting diode (LED) devices first appeared in the 1960s. Today, with the advances in materials, design, and manufacture of LED devices, we are seeing a wide spectrum of LEDs that are cheaper, more colorful, more efficient, more intense, and more reliable.
Lighting typically has the biggest consumption of on-site electricity. In this case, the LED provides unique advantages in solid-state lighting for its power efficiency and reliability. As an example, the Pharox 500 from Lemnis Lighting — equivalent to a 60-watt conventional bulb — consumes 7 watts and lasts for about 25 years. The cost is currently projected to be under $40 and can go down to $10 in five years driven by demand and improvements in the electronics.
The exciting solid-state lighting market developments will pose interesting reliability challenges. For example, solid-state lighting depends on high-power LEDs, which generates much heat. Additionally, they are often used in uncontrolled environment such like the outdoors. LED reliability will be determined by all the its constitutive parts and their reactions upon environmental or operational stresses. The primary drivers from the perspective of the LED die and package will be discussed.
From the semiconductor die perspective, the two dominant stresses affecting LED reliability are the LED forward drive current and operation temperature.
Atomic defects such as dislocations contribute to the reduced probability of photon generation, thus the degradation of the LED light output. These defects serve as non-radiative recombination centers and generate heat instead of light. In many materials systems such as GaAs/AlGaAs, high forward drive current density causes electro-migration, which is responsible for the nucleation and growth of dislocations and other defects in the region where the radiative recombination occurs. The speed at which these lattice defects increase depends on the magnitude of the forward current density.
The atomic defect generation and growth can be accelerated at higher temperatures. The acceleration behavior is often modeled by the empirical Black’s equation. There can be interactions between the drive current density and temperature. These two factors can not only accelerate the atomic defect growth, they can also lead to thermal runaway — especially for high-power LEDs. In addition to reliability concerns, critical performance parameters such as emitted light intensity as well as wavelength may shift as the junction temperature rises.
The LED package serves in protecting the semiconductor die from direct environmental exposures, and in facilitating the interconnection of LED to the system, it can also affect LED reliability.
Optical: Package optical properties can be affected by epoxy and phosphor degradations when exposed to temperature, moisture, and UV radiation. Optical performance is also affected when air gap forms, e.g., due to delamination, or when materials’ optical property/ geometry changes. Such changes can modify the optical path, reflectivity, and index of refraction matching across the device layers before photons can be emitted efficiently.
Thermal: Thermally, low-power LEDs dissipate heat primarily through the lead whereas highpower LEDs depend more on the packages. Most high-power LEDs are surface-mount technology (SMT) type that can be directly mounted onto a heat sink. In some cases, package degradation can contribute more to the light output degradation than the LED die itself, as silicone and epoxy may deteriorate faster over time. Packages for state-of-the-art high-power LEDs are much more sophisticated than early LEDs, e.g., they may be mounted on metal-core printed circuit boards (PCBs) to provide efficient heat transfer.
Heat can also degrade the different phosphors used in white LEDs. The degradation rates for these phosphors may vary, causing changes in the eventual output light color. For example, purple and pink LEDs with an organic phosphor formulation may degrade after just a few hours of operation resulting in output color shifts.
In addition to high temperatures and high humidity operation degradations, the LED package can be susceptible to failures during temperature cycling and when the package is exposed to low temperatures. Thermal cycling can cause thermal fatigue failures related to wire bonding, die attach, and die-package delamination. Presence of manufacture defects such as poor intermetallic formation at the ball bond or inadequate die attach can be particularly susceptible in these scenarios. At low temperatures, the LED package may contract and exert mechanical stress on the LED die, to the extent of causing die cracks.
Mechanical: Mechanical integrity of the LED is another critical aspect that demands attention throughout the package design, assembly, and system integration. The compound semiconductor used in LEDs possesses different mechanical strength when compared to, e.g., Si. The wire bonding process parameters need to be carefully designed and controlled in order to form a quality bond. In LED assembly to printed circuit board assemblies (PCBAs), package stress can be introduced due to lead bending or soldering actions, which can introduce package and die defects leading to early failures.
Electrical: As a semiconductor device, LEDs will be susceptible to electrostatic discharge (ESD) and electrical overstress (EOS). ESD may cause immediate failure at the diode junction, or a shift in its parameters, or latent defect causing delayed functional failures. As EOS examples, power-line coupled transients and surges can degrade LEDs. The reverse-breakdown mode for some LED types can occur at very low voltages where any excess reverse bias may cause immediate degradation.
In integrating LED devices into a system, the first order of business is to carefully evaluate the application environmental and operational stress conditions in order to select the LEDs with the proper strengths. For example, thermal and moisture environment exposures and controls, and voltage/current fluctuations including the likelihood of ESD/EOS incidents, UV light intensities, etc. should be examined. Don’t forget details such as soldering temperature exposure, and mechanical stress on the package due to specific mounting methods can all contribute to the reliable operation of the LED. Additionally, one can expect reliability variations across vendor bases, technology platforms, production processes, and more importantly, specific system design and deployment. Frequently, a quality LED that is properly integrated into a system can experience much longer life than the rest of the system and may be the last concern when evaluating the overall system reliability.