History of LEDs

Light emitting diodes (LEDs) have been around for over 50 years; however, LEDs with efficacies and color quality high enough for general lighting have only been available for a short time. LEDs have been used in various areas, for example: in industrial applications from switch cabinets to measuring instruments; in consumer products such as cell phones and computer monitors; and in transportation-related areas such as in traffic signal installations for road and railway, or in interior and exterior automotive lighting... and new applications are being discovered every day.

LEDs are semiconductor light sources that are bright enough to be used as visible light sources. Technically speaking, an LED semiconductor operates only in one direction (like a typical diode) and emits energy as a photon in the course of operation. At first LEDs were used primarily as red indicator lights on electronics products, but by the mid 1990s, advances in LED technology started engineering experimentation into the use of LEDs for lighting a room. Modern LEDs now come in a variety of shapes, sizes and colors, often relying on phosphor powders to produce very specific colors.

As today's white LEDs reach efficacies surpassing the 150 lumens per watt (lm/W) mark (depending on color temperature and color rendering) they are continuously being used to replace "incumbent" lamp sources in traditional lighting installations. That is, in addition to retrofits, lighting that would have been installed as incandescent or fluorescent lighting is now being designed-in as LEDs from the outset.

New LEDs for very small spaces

One of the issues with retrofitting traditional light fixtures with more efficient LEDs is space constraints. The technologies are very different, so the requirements are different. One problem has been that the traditional light fixtures do not have enough space for the newer LED systems. One major contributor to the LED landscape is OSRAM Opto Semiconductors, one of the world's leading manufacturers of optoelectronic semiconductors for the lighting, sensor and visualization markets. OSRAM introduced the "OSLON Square" LED, part of a series of LEDs ideally suited for applications requiring a large amount of light in a compact footprint. This technology advancement, delivering lot of light with a small footprint, is achieved by a tiny 2mm2 chip featuring a highly reflective internal layer for enhanced photon recycling that is contained in a small 3mm x 3mm package and has a lifetime greater than 50,000 hours. The ceramic package with a silicone resin out-coupling lens and a 120 degree viewing angle ensures both homogeneous (uniform) light distribution and excellent thermal management with a low thermal resistance of 3.8 K/W. Out-coupling lenses are a proven means to increase efficiency. Low thermal resistance means that heat isn't retained within the LED but allows it to be dissipated into the thermal system, allowing for smaller heat sinks and thus smaller overall light engines. The 3mm x 3mm LED package is among the smallest available on the market today.

Figure 1: High efficiency: the reflective package of the OSLON Square redirects light reflected back toward the package into the secondary optic, guaranteeing high lumen efficacy.
Image: OSRAM

The Right White LEDs

Phosphor is a luminescent material that is crucial to a large number of lighting applications, especially LEDs. Since phosphor absorbs light at short wavelengths and re-emits the energy at longer wavelengths, most LEDs for general lighting purposes are actually blue LEDs with a yellow phosphor coating over the chip within the LED package.

LEDs are now available over a wide range of warm, neutral and cool white color tones, ideal for ambient to industrial lighting solutions, and the selection from the OSLON Square LED family can deliver the right white for any purpose.

A new trend in designing lighting products with LEDs is to place a phosphor-coated lens some distance away from, or remote to, the blue LED itself. This practice is referred to as "remote phosphor," and provides the designer with the flexibility of changing the color of the light by simply swapping the lens element. The OSLON Square LED also comes in deep blue, which is ideal for use in remote phosphor applications.

Overcoming Thermal Design Issues

LEDs with high intensities logically operate at higher wattages. As wattage increases, so does heat generated by the LED. Thermal resistance (Rth) is defined as the rate of temperature increase for the dissipated power. It is a measure of the capability of a given material to dissipate heat. With a decrease in LED package size and internal thermal resistance, there is a growing need for higher performing substrate materials to manage the increased amount of heat being dissipated by the LED. At times, in an effort to reduce the LED component count in an LED system, fewer LEDs devices are used and are driven at higher operating currents (>1.5A.) This can lead to greater heat which exacerbates thermal challenges.

Thermal Substrates: Thermal Management by Bergquist

In LED circles it is commonly known that the light output and life expectancy of LEDs, especially that of High Power LEDs, are directly attributed to how well the LED is managed thermally. The Bergquist Company, originator of many proprietary and patented thermal management products since the early 1960s, has a solution. Bergquist is known for its pioneering spirit and innovation. As an LED generates heat, a Bergquist Thermal Clad® Insulated Metal Substrate (MCPCB) rapidly transfers it to the metal base, significantly improving LED performance. Bergquist MCPBs were developed as a thermal management solution for today's high power surface mount applications. Present-day high power LEDs are an excellent example of an application that requires an insulated metal substrate with the benefit of reducing heat build-up, which if not controlled can cause a change in light output, color rendering and lifetime. Using thermal management materials properly can ensure the most reliable luminaire design.

Customizing solutions for thermal build-up in any application

A typical MCPCB substrate is a three layer laminate construction as shown in Figure 2. This substrate is comprised of three different materials; a copper circuit layer, the dielectric layer, and the base layer. The thermally conductive dielectric layer is an electrical insulator that has the ability transfer heat efficiently.

Figure 2: Substrate Layers Image: Bergquist

The copper circuit layer can serve two purposes; first and primarily as an electrical conductor, but it also can be used as a heat "spreader." Optimized circuit pad geometry and copper thickness can improve heat spreading. The second layer is the dielectric layer. Bergquist offers several types of dielectrics depending on thermal performance and electrical isolation requirements. Bergquist dielectrics range in thermal conductivity from 1.3 to 3.0 W/mK, with dielectric thickness ranges from 38 μm (0.0015") to 225 μm (0.009") depending on the isolation requirements. For comparison, an average human hair is only 100 μm (100 microns, or 0.004") wide. The base layer thicknesses can range from 0.5mm (0.020") to 5mm (0.197") and can be copper or aluminum or even a more exotic material depending on the thermal and mechanical requirements. Bergquist is able to combine the three and vary the combination such that mechanical / electrical / thermal performance can be optimized for an enormous range of applications.

Thermal properties of LEDs for general lighting

LEDs have increased in power density and reduced in thermal resistance to a point where good thermal management is absolutely necessary to ensure consistent light output, color rendering, and life time. Bergquist Thermal Clad (MCPCBs) substrates minimize thermal resistance and conduct heat more effectively and efficiently than older solutions, such as standard FR-4 printed wiring boards (PWB's). Bergquist's Thermal Clad is a cost-effective solution, which makes it possible for denser packaging designs and a potentially reduced component count.

The most advanced LEDs, like OSRAM's OSLON Square LED, have power densities approaching 2W/mm2. Bergquist High Power Lighting(HPL) is a dielectric specifically formulated for high power lighting LED applications with demanding thermal performance requirements. This thin dielectric, at 38 μm (0.0015"), has the ability to withstand high temperatures with a glass transition temperature (Tg) of 185°C and phenomenal thermal performance of 0.30°C/W (RD 2018) that resists thermal aging.

What is glass transition?

Tg, or glass transition, is the point at when a material begins to transition from a solid to a plastic state. Under heated conditions, high Tg materials have better mechanical strength, dimensional stability, adhesiveness, moisture absorption, thermal decomposition and thermal expansion than lower Tg materials. In addition to high Tg, Bergquist also chooses a polymer for its electrical isolation properties, its ability to resist thermal aging and its high bond strengths. Ceramic filler enhances thermal conductivity and maintains high dielectric strength. The result is a dielectric layer that can maintain these properties even at a thickness of 38μ m (0.0015") and one can design a light engine, a combination of an LED module and its associated control devices, that does not overheat. Bergquist Thermal Clad HPL circuits are available in a variety of form factors including single piece parts, v-scored arrays, and punched parts.

Figure 3: Bergquist Thermal Clad HPL circuits are available in a variety of form factors including single piece parts, v-scored arrays, and punched parts.
Image: Bergquist

OSRAM and Bergquist take the guesswork and worry out of thermal management for LED systems leading to maximum system performance. OSRAM's OSLON Square LEDs are an ideal fit and offer the latest in power chip technology. Designing with Power LEDs is easier with Bergquist IMS Substrates in a configuration that fits the OSLON Square LED. Thermal Clad® IMS substrates are available in custom configurations or as a standard star or square format from Bergquist, available in single, strip, or array configurations with more than 10 different LED footprints.

About OSRAM Opto Semiconductors

OSRAM AG (Munich, Germany) is a wholly-owned subsidiary of Siemens AG and one of the two leading light manufacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications. OSRAM Opto Semiconductors has production sites in Regensburg (Germany) and Penang (Malaysia). Its headquarters for North America is in Sunnyvale, California (USA), and for Asia in Hong Kong. OSRAM Opto Semiconductors also has sales offices throughout the world. For more information go to www.osram-os.com.

About the Bergquist Company

The Bergquist Company designs and manufactures high performance thermal management materials used to dissipate heat and keep electronic components cool. Headquartered in Chanhassen, MN, Bergquist supplies the world with Thermal Clad® insulated metal substrates along with some of the best-known brands in the business: Sil-Pad® thermally conductive interface materials, Gap Pad® electrically insulating Gap Fillers, Hi-Flow® phase change grease replacement materials, and Bond-Ply® thermally conductive adhesive tapes. www.bergquistcompany.com