A Little Drop of Silicone - Wacker Chemie AG

A Little Drop of Silicone

… but a big step for efficient mass production of LEDs. High-performance LUMISIL silicones from WACKER now allow LED manufacturers to create optical lenses by placing a drop of silicone directly onto the chip—and eliminating complex injection-molding steps in the process.

Cross-section through an LED with silicone lens

The LED Success Story

In the three decades following their invention in 1962, light-emitting diodes were only a niche market. Emitting only colored light (primarily red, yellow and green), LEDs were used in applications such as automobile dashboard displays or standby indicators in electronic equipment. The infrared version was used in IR transmitters for remote controls.

It was not until blue and white models came onto the market in the 1990s that applications for LEDs began to expand steadily. According to the market research agency DisplaySearch, every fourth LCD television sold throughout the world in 2010 was made with LED backlighting, and LED running lights have made the transition from the luxury class to mid-sized and compact cars.

The market for general lighting (street, storefront and ambient lighting), long neglected by LED manufacturers, has been experiencing rapid growth for roughly two years now. OSRAM, one of the world’s leading lighting manufacturers, expects the market volume for LEDs to nearly triple by 2012. Growth in the general lighting market can also be attributed to bans on classic incandescent bulbs, which, due to their poor energy efficiency, are being phased out in most industrialized nations.

Longer Life, but Complicated Production Process

LEDs not only represent an improvement over incandescent bulbs, however – in many respects they are also superior to what is currently the most common alternative, i.e. the compact fluorescent tubes, popularly known as energy-saving bulbs. With an average life of 50,000 hours, LEDs last considerably longer than energy-saving bulbs (roughly 10,000 hours, depending on the model), they have a higher light output (which will eventually reach some 200 lumens per watt in top models), they do not require toxic mercury and their light appears warmer and more natural in color than the colder light of fluorescent tubes. Fluorescents, however, are less expensive and distribute light more evenly throughout a room.

The high purchase price is currently all that stands in the way of the widespread adoption of the LED. In addition to the diode itself, however, there is another factor that drives up manufacturing costs: the silicone lens, which radiates the light and, at the same time, protects the semiconductor component from the environment. Until now, LED manufacturers made these lenses in a complex injection molding process requiring not only expensive injection molding equipment, but also high-precision molds as well as sorting, dispensing and assembly equipment – a total of five steps, which, to make matters worse, also had to be performed under cleanroom conditions.

Cost-Efficient Production

Rheometry samples in the lab

WACKER, however, now sells a new silicone elastomer (under the brand name LUMISIL®), which gives manufacturers the ability to make optical LED lenses in a single step: a drop of the silicone is dispensed directly onto the chip, where it is activated with UV light to cure it and shape it into the lens geometry.

“LUMISIL® allows us to manufacture optical lenses for light-emitting diodes in a way that is both economical and extremely fast – at an appropriately high throughput and with no complex molding step,” explains Dr. Phillip Müller, who bore the responsibility for developing this high-performance optical silicone. LUMISIL® has very smart flow and cross-linking properties, Müller points out, adding that the encapsulation process is sufficient to produce an optical lens with the ability to radiate light in a way that is ideally suited to general lighting purposes.

High-Quality Material

The benefits of LUMISIL® apply to more than the manufacturing process, however: its use is also recommended by the quality of the resulting materials. The silicone is light-resistant, does not turn yellow or brittle, and protects the semiconductor chip from heat, cold and other sources of stress, such as humidity. Lighting manufacturers use lead-free soldering techniques when processing an LED, which generates temperatures of 270 °C or more. Unlike other materials (such as the epoxy resins that used to be used), however, the silicone lens can easily withstand even this level of heat. “The use of WACKER’s UV-activated silicone plays a major part in the 15 percent increase in brightness that our LEDs produce,” says Dr. Michael Kruppa, senior manager for back-end technology materials development at OSRAM Opto Semiconductors, the lighting manufacturer’s LED subsidiary in Regensburg, Germany.

Select Applications

OSRAM Opto Semiconductors uses LUMISIL® for LEDs in its Golden Dragon Plus series, one of the manufacturer’s most widespread product lines for general lighting applications.

The following photo gallery showcases five applications – from South African World Cup stadiums to street lamps in Siberia to lighting for Danish greenhouses – featuring OSRAM’s Golden Dragon Plus series, which uses LEDs encapsulated with LUMISIL® from WACKER.