Sign & Digital Graphics

The 2015 LED & EMC Report

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L E D & E M C • June 2015 • 13 of electrons, while the other has a defi- cit (called holes). When a potential is applied, the difference between the lay- ers causes the electrons to move from one layer to another, producing energy in the form of light. These layers are grown epitaxially in a clean room, and are less than one micron thick. Each slice of the semiconductor, or wafer, is a single uniform crystal. Later in the process impurities are introduced (doping). These impurities actually cause the semiconductor to function. Different dopants are added to cause different wavelengths or colors of light to be emitted. Multiple layers (like layers of a cake) are grown epitaxially in either a liquid or gas phase in a very specific process. Once the wafer is "grown", metalliza- tion contacts are patterned so that elec- trical contact can be made. This pattern is repeated (sometimes up to 10,000 times) on the wafer. Finally, the wafer is cut into smaller sections called the "die" or "chip". At this point the individual dies or chips are mounted into a lamp. Mounting LED Chips In the early days LED chips were mounted on lead frames, coated with a phosphor and then an epoxy domed lens was cast around the lead frame and chip to create a lamp. Many of us remember the first LED sign modules that contained these 3mm or 5mm lamps or later a four- pin cast epoxy lamp called a piranha package. As packaging technology developed, surface mount device LED lamps were produced and are still one of the most common types of LED lamps used in sign modules today. More recently, compa- nies utilize what is called chip-on-board technology in which the chip is mounted directly on the PCB substrate, coated, and encapsulated. There have been multiple develop- ments over the years that have contrib- uted to LED adoption as a mainstream source of lighting for the sign industry. Developments have occurred at the manufacturing level with the advent of new semiconductor materials and dop- ants, advances in substrates, better lamp packaging techniques, as well as pro- cessing larger wafers resulting in better throughputs and lower manufacturing costs. All in all LEDs have improved their efficiencies from 50 LM/W just 10 years ago to over 150 LM/W in commercially available LEDs and at the same time are being produced at a fraction of the cost. So what is coming and what does it mean for the sign industry? Improved Production First, let's talk about chip production. LEDs, as described, are made in a batch process—like baking a cake. In batch processing, it takes about as much work and the same equipment to make a big cake as a small one. So a lot of focus has been around how to grow larger wafers. For example, if you can grow a 1" wafer in a single step and produce 2,000 LED die, growing a 2" wafer allows you to produce 8,000 die of the same size with the same number of steps. Figure 1 shows the growth of LED wafer size over time (i.e., baking larger cakes). Recently Taiwan Semiconductor has developed a platform for bring a fully automated 8" manufacturing process to LED manufacturing. These processing improvements have been and will con- tinue to be a major driver in reducing LED die costs over the next decade. Now let's consider chip efficiency. Many consider the theoretical efficiency of a phosphor based LED to be some- where between 260 and 300 LM/W. As mentioned commercially available LEDs are already around 150 LM/W, so we may see a doubling of LED efficiency over the next decade, but we are not likely to see the massive efficiency improvements that was achieved over the last 10 years with- out the development of radically new materials. So what is new out there in Packaged LED surface mount lamps. Figure 1: Effect of wafer size on LED die yield. A four-inch semiconductor wafer. AC LED chips taped, die mapped and ready for packaging. (Photo courtesy of Principal LED)

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