“The thing to do is to supply light and not heat.”  

—Woodrow Wilson

 

Sometimes heat is nice. In the grip of winter, for instance, the last thing most people are thinking about is finding ways to get rid of heat.

 

But for LEDs, it’s a different story. For LEDs of any kind to operate in an effective way (read: stay bright long enough to keep the customers happy), whether winter or summer, the heat has to go. LEDs are pretty efficient at converting input power to light, but still on average, somewhere north of 70 percent of LED input power is converted to heat.

 

The LEDs inside channel letters in locations like those found at the Aria in Las Vegas must operate effectively in what are sometimes extremely hot ambient conditions.

SHORT PRIMER REVIEW

The reason heat is such an issue with LEDs compared to say, the tungsten filament inside an incandescent bulb, is that relatively low temperatures wreak havoc on the delicate arrangement of materials that make up the LED semiconductor chip.

 

• Heat causes chemical changes, which cause shifts in color and lumen depreciation.

 

• Heat causes physical changes, which cause layers of semiconductor material to delaminate, soldered connections to loosen, and protective encapsulations to open.

 

• In extreme situations, the result of too much heat is catastrophic failure of the LED.

 

In an excellent two-part article that was published in the March 2005 and April 2005 issues of Sign Business, Dan Watts explained the basics of heat issues that relate to LEDs, with an emphasis on how those issues affect channel letters and other types of signage. Thermal management is an evolving technology that has advanced since that article was written, but the principles have remained constant: heat always moves from a hotter place to a colder place; and the mechanisms for moving heat are defined as:

 

• Radiation—Heat radiates as infrared waves from anything hot to anything else that is cooler

 

• Convection—Heat that moves in a fluid medium such as water or air

 

• Conduction—Occurs when two objects come in direct physical contact

 

All of these mechanisms for moving heat are used in various ways and configurations by LED manufacturers. There is no one-size-fits-all thermal management solution for LEDs because different applications have different requirements in terms of space, budget and aesthetics.

 

The development of efficient thermal management for LEDs is driven in part by a phase-out of incandescent light bulbs. (OSRAM press picture)

WHAT’S HOT

In the beginning, when LEDs were mostly used for small indicator lights that weren’t real bright and didn’t get very hot, the temperature/heat issue was not so important and a small heat sink would be sufficient. But with the introduction of high-brightness LEDs that operate at higher temperatures, effective thermal management has become a critical aspect of LED performance.

 

Controlling P-N junction temperature is one of several factors that together influence overall LED product reliability. But without temperature control, the LED product cannot be reliable. 

 

Without getting overly technical, the electronic action in an LED takes place at the P-N junction. There’s a wealth of information about P-N junctions and other semiconductor technology on the Web and from other sources, which is mostly beyond the scope of this article. For now, suffice it to say that when forward voltage is applied, the P-N junction is the place where photons are emitted and also the place where a lot of the heat is generated in a LED. It’s also where the layers of semiconductor material are most susceptible to the harmful effects of heat. In a nutshell, better light output efficiency of an LED device is achieved when the P-N junction temperature is maintained as low as possible.

 

Basic components of an LED Module. (Courtesy SloanLED)

Two additional sources of heat within an LED system also cause problems. The first is simply the air in the immediate vicinity of the LED, commonly called ambient air. The second is the driver, or power supply. Each of these three heat sources must be considered in order to optimize the removal of heat in an LED system, especially in situations that utilize multiple arrays and clusters of LEDs in enclosed spaces.

 

WHAT’S COOL

The pathways to get the heat from the P-N junction to the ambient surroundings where it can dissipate become the thermal management system for that LED system or device. Factors influencing heat removal from the P-N junction include circuit board materials, component configurations and spacing, and choice of thermal interface materials used at critical spots where different components come in contact.

 

While other light sources simply shed excess heat directly away from the source—mostly through radiation and some convection—all the components of an LED system contribute to the overall flow of heat away from the P-N junction. For most devices, the journey begins with the printed circuit board, which is attached to the heat sink with some kind of thermal interface material.

 

SloanLED’s new V180, a bright light in an very small package. (Courtesy SloanLED)

Most heat sinks are passive, leveraging relatively large surface areas to allow heat to escape into the surrounding air. When possible, system designs that keep heat sources apart—such as remotely locating power supplies away from the LEDs—will be more efficient. And when the ambient air is also kept cool, further efficiencies can be realized. In some large signs in Las Vegas for example, active systems using fans and even air conditioners are used to cool the ambient air.

 

WHAT’S EVEN COOLER

Although the screw-type Edison socket is not the most efficient design for encouraging the removal of heat from the LED package, they are found in billions of light fixtures around the world. And there is a full-tilt push to come up with an energy efficient replacement for the 60-watt incandescent Edison light bulb that easily screws into those fixtures. To meet the challenge, some highly developed thermal management solutions have emerged that address the special needs of removing heat from an LED designed for an Edison socket fixture, where all the heat-generating sources are squeezed into a very tight space.

 

These include both active and passive thermal management systems. One example of an active system is made by Austin, Texas-based Nuventix. It consists of controlled jets of air directed precisely at heat sources within the light fixture, an arrangement that enables more compact heat sinks and thus, smaller and more compact fixtures. On the passive side, thermally conductive plastics can be injection molded into shapes that meet specific heat sink design criteria for LED heat management. Narragansett, R.I.-based Thermal Solutions Resources LLC is an example of a company involved with design, prototyping and manufacturing of thermally conductive plastics for LED heat sink applications. Another example is a new space-age material called Pyrolytic Graphite, a sheet product from Panasonic Electronic Components, which is said to have twice the thermal conductivity of copper. An added bonus is that this material can be easily fabricated into custom shapes.

 

SloanLED’s head and humidity testing chambers. Modules are tested at 70°C for 1,000 hours, subjected to repeated thermal cycling from -30°C to 70°C and junction temperatures at 25°C, 50°C and 70°C. (Courtesy SloanLED)

TRICKLE DOWN TECH?

While technological breakthroughs in thermal management for the general illumination LED market continue to attract media attention and the public’s curiosity, sign making and general illumination are two very different applications.

 

“With a light bulb, you have one point source from which you expect to light up a room,” says SloanLED’s Drew Ferrie. “That means it has to be an array in one central location, which requires a mass of heat sinking. In the case of channel letters, modules are spread throughout the letter, which provides a much larger area for spreading out the heat.”

 

Essentially, that means that heat build-up inside a channel letter is not nearly as intense as can be found in an Edison socket fixture.

 

Still, Ferrie says manufacturers can’t rely on channel letter materials to act as heat sinks and expect to get long-term performance. “We’ve found you can’t always rely on that,” says Ferrie. “There are a lot of plastics and other surfaces people mount to, so we try to make our modules stand on their own as far as heat sinking goes rather than to rely on some secondary surface to provide it.”

 

He says new module designs are developed working closely with the manufacturers of the LEDs. “They have defined specs for how much heat the LED creates and how much is allowable. When we design a module, we make sure we stay within those parameters. The way we do that typically, using a normal FR-4 circuit board, is to maximize the metal on that board. For other higher power LED modules, we’d probably use a metallic circuit board to get the heat out. But for the most part, you just want to maximize the amount of copper. You can do that by having thicker copper, or having a larger physical area of the circuit board or with multiple layers of copper.”