Innovative packaging improves LEDs’ light output, lifetime, and reliability
HB-LED (high-brightness-light-emitting-diode) efficacy may grab the headlines, but packaging is where the action is in this market. Getting the light out of the die, through the encapsulant and lens, and onto the application’s surface at a consistent, reliable color for a long lifetime is a challenge, and LED manufacturers accomplish this task through the use of innovative packaging for the devices. HB-LED users may be unaware that Big 4 manufacturers Cree, Nichia, Osram, and Philips Lumileds control almost all HB-LED chips. Although China is home to hundreds of LED-chip manufacturers, those vendors produce devices with only 10 to 70 mA of current, which eventually find use as indicators rather than illumination devices. The remaining HB-LED vendors are for the most part packagers of the tiny HB-LED chips that they purchase from the Big 4 manufacturers.
R&D efforts to coax more lumens from an LED die are enabling the use of LEDs in new markets and applications. Just as important, though, are manufacturers’ packaging methods. These methods enhance efficiency to meet the needs of unique applications, such as solid-state-lighting, automotive, signage, and medical applications. To select the best HB LED for your application, you need to understand its packaging as well as some of the methods its manufacturer used to squeeze out and precisely place as many photons as possible. Packaging affects light extraction, heat extraction, and lumen maintenance—important characteristics of LEDs. Light extraction determines how much emitted light will reach the intended application; thermal extraction affects how much heat the LED chip will experience and, hence, its lifetime and performance; and lumen maintenance indicates the lifetime of the LED. The HB-LED package determines—or at least strongly influences—all three of these characteristics, which in turn affect the LED’s performance, lifetime, and reliability.
HB LEDs currently have no standardized package footprint. Unlike for other electronic components, such as ICs, MOSFETs, and most passive components, every HB-LED manufacturer uses a unique package. This practice can be annoying to LED designers who want to line up second sources for their products, but LED packages comprise intricate combinations of the LED chip, encapsulant, primary optics, and substrate. The package for a 20- to 70-mA indicator-type LED dissipates heat through its leads. The epoxy encapsulant serves as a lens and provides a rigid protective structure for the die and leads. Compared with low-power LEDs, HB LEDs must rely on greater heat-dissipation capability for their packaging, and they typically come in SMT (surface-mount technology) or through-hole packages (Figure 1). SMT tends to find use in HB LEDs of 1W or higher power because it mounts directly onto a heat sink, whereas through-hole packages generally suit midrange HB LEDs of 0.5 to 1W.
An LED chip generates light in an epitaxial structure. Ideally, all of the hole/electron combinations would result in photons. Material characteristics and imperfections, however, cause some of the combinations to produce heat rather than light. The next challenge is getting the photons out of the die because of the difference in index of refraction between the layers of the chip, the encapsulant, and the lens. Air has an index of refraction of approximately one, whereas some die materials have an index of approximately 1.4. A difference in indexes for two materials causes light to reflect back at the boundary when the light exceeds a certain angle. Total internal reflection is the most significant barrier in extracting the light from an LED chip.
To manufacture white HB LEDs, manufacturers cover blue LEDs with a phosphor that emits light in the white region when it irradiates with blue light. Developing the phosphor that most efficiently converts blue light to white is part of the IP (intellectual property) of some LED vendors that purchase LED chips from other vendors and then package them with their own phosphor mixture in a package. Because of the LED chips’ high index of refraction, the package needs a transition layer between the air and the chip/phosphor combination. This layer also serves as a protectant and a lens. Low-power indicator LEDs use an epoxy encapsulant, but epoxy tends to yellow when you expose it to high heat and UV (ultraviolet) radiation, according to Kee Yean Ng, product-marketing manager for solid-state lighting and displays in Avago’s optoelectronics-products division. “Silicone is now almost universal as both the lens and the encapsulant for HB-LED packages,” he says. The silicone helps both to extract the light and to protect the chip from air and moisture.
Some HB LEDs are surface-emitting, meaning that most of the light comes from the top surface of the die. However, many HB LEDs, especially those that HB-LED packagers purchase, are edge-emitting, and the package must redirect the light so that it exits the top of the package. Several proprietary methods exist for redirecting edge-emitted light. One method is surface roughening, which is a secondary manufacturing process that roughens the surface to make it less reflective to the internal light. Lumex uses primary optics—those that are part of the HB-LED package—that redirect the light from the top in a pinpoint pattern (Figure 2).
Most of the electron-hole combinations that don’t result in photons create heat—the biggest drawback to LED life. LEDs generate less light as they get hotter. In addition, all manufacturers specify and test their LEDs at room temperature with a test pulse rather than a continuous current, so the LED never heats up. In practice, however, an LED’s junction temperature is never at room temperature when the LED is on. Thermal performance also affects LED lifetime: The hotter the junction temperature, the shorter the LED lifetime will be. Cooler LEDs lose less light due to thermal inefficiency, have a more efficient light output, and last longer.
You can compare the thermal performance of various LEDs by looking at the thermal-resistance rating. The lower it is, the less temperature difference there will be between the junction and the solder point. The thermal path for any LED system starts at the solder point and includes the case temperature of the LED out to the ambient temperature, the PCB (printed-circuit board) that it’s mounted on, and its thermal-interface material. These thermal-design steps are the same ones engineers use to determine the operating points for power MOSFETs. Even though you can’t measure the junction temperature of the LED chip, you can measure the solder-point temperature to determine the power going through the LED and use it to calculate the junction temperature. The junction temperature is useful for understanding lumen maintenance, or how long you can expect the LED to remain operational at its operating temperature.
Lumen maintenance is the percentage of initial light output an LED can maintain over its operational life. The predominant failure mode of an LED is to get dimmer over time rather than to burn out as an incandescent bulb does. Lumen maintenance is an agreed-upon number that defines end of life for an LED and generally is L70, which means that the LED is emitting 70% of the light it did at its maximum output when it was new. Energy Star requirements for solid-state lighting set the lifetime at L70, as well. “We’ve done a lot of higher-temperature testing and found out that the primary mode of degradation for HB LEDs is the package itself,” says Paul Scheidt, product-marketing manager at Cree. “If you keep the package cool, the chip doesn’t degrade all that much over time—maybe 2 to 3%—not that much of a contributor compared with a total 30% degradation over years.” The predominant failure mode is due to degradation of the materials, including silicone and plastic, in the package. Both deteriorate over time in different amounts at different temperatures and light intensities.
The question of aging, reliability, and lifetime for LEDs is not a simple one: High-power LEDs have not been in existence long enough to have life-testing numbers. A common number for LED life is 50,000 hours, or almost six years, which is longer than the most recently introduced high-power LEDs and their packages have existed. But how useful is it to know that an LED has a 50,000-hour lifetime if a manufacturer measured that lifetime in an environment that differs from the one in which your application must operate? For example, a flashlight may require a lifetime of only 1000 to 2000 hours. Users who don’t need the full 50,000 hours want to know how to calculate lifetime numbers under different operating conditions. “We have what we call the four critical parameters: junction temperature, drive current, solder-point temperature, and ambient temperature,” says Scheidt. “Knowing these parameters, users can get a good estimate of how long the lifetime is going to be, not just of whether they’re going to achieve 50,000 hours.”