Assembly packaging could mean a bright future.
Conferences around the world are buzzing over the growth projections for high brightness light-emitting diodes (HB LED). In Korea, almost 200 people attended the International LED and Green Lighting Seminar in June, while more than 20,000 people visited the colocated LED and OLED Expo. More than 200 people attended special sessions on LEDs during SemiCon West in San Francisco this past July. Companies are looking at the explosive industry growth, and all hope for a piece of the pie. Strategies Unlimited, Credit Suisse, and McKinsey predict that the LED market – driven by three major applications (general lighting, backlighting, and automotive) – will grow at a 40% CAGR through 2015.
According to the Optoelectronics Industry Development Association, three segments – automotive, lighting, and mobile applications – are expected to grow to 89% of all applications in 2021. What is driving the growth? In the case of lighting, much of the growth comes from pending bans on incandescent bulbs and government mandates that lighting be converted to LEDs for energy savings.
The technology to produce LED die is based on semiconductor diode technology. The emitted wavelength of light depends on the specific semiconductor material used. The key to producing high-quality LED die is the growth of the epitaxial layers and an active p-n junction on an optimum substrate. The p-n junction is critical in determining the device’s internal quantum efficiency (IQE), and the substrate is important because it should match the lattice constant of the semiconductor. The matching lattice constant helps reduce dislocation defects. Dislocation defects can adversely affect the performance of the LED.
Key tools for the growth of the epitaxial layers include MOCVD equipment. Wafers are typically two to four inches with a constant pressure to continue moving to larger wafer sizes.
The packaging and assembly process is also critical to the performance, reliability, and lifetime of the LED product. LED packaging tends to be nonstandard, with every package unique to the supplier.
LED packaging options include use of one large LED die in a package versus multiple die. Advantages and disadvantages are still being debated. It is not clear if one method will dominate or if both solutions will be used. In some cases, multiple die are mounted directly on a metal core substrate or on leadframes.
Thermal issues. Thermal issues have been reported to account for as much as 50% of the failures in lighting. Effectively removing this heat is critical to control junction temperatures of the LED chips, to ensure long device lifetimes, and to maintain the spectral and efficiency characteristics of the light source. Controlling the LED junction temperature (Tj) from 50° to 100°C is critical to LED performance. As the junction temperature (Tj) of the LED changes, the wavelength of emitted light shifts, making color control difficult. If Tj is excessively high, the active region of the LED and the electrodes of the device can degrade, leading to a decrease in the device light output. High Tj can also reduce LED life. Failure to effectively remove heat from the LED source can also degrade the phosphors in the LED package. as well as other packaging materials. Some examples of the latter are yellowing of the encapsulation material and higher absorption losses at reflective surfaces. Depending on the temperature changes, the coefficient of thermal expansion and glass transition temperature of these materials can lead to mechanical stresses that cause failure of the package or the LED die. If catastrophic failure does not occur, these effects often just reduce the lumens output of the package and thus the overall efficiency (lumens per watt) of the source. Excess heat can also lead to premature failure of the LED package from delamination and die fracture.
Packaging materials will also significantly impact the optical efficiency of an LED package. Reflectivity, transmissivity and index of refraction are all materials’ properties that could affect the number of lumens out of the package, and also, the stability of packaging materials (encapsulants and lenses) with exposure to elevated temperatures, UV and other wavelength radiation. As mentioned, elevated temperatures can cause materials to turn yellow, thus reducing the number of photons getting through or changing the color of the light.
The adoption of new technologies is often hyped to the point that unrealistic expectations are created. The story with HB LEDs will be different. The difference with LEDs is that there is a technology pull (the desire for energy efficiency) and government mandates. If companies can produce new, less expensive lighting, the future is bright. The packaging and assembly process plays a critical role in making this possible.
E. Jan Vardaman is president of TechSearch International (techsearchinc.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. Her column appears bimonthly.
