From the tiny electronics in smart watches to the massive server racks in data centers, today’s state-of-the-art technologies would fail if they couldn’t manage one thing: heat. Solid-state lighting is no exception.
“Heat affects every aspect of an LED,” says Chris Reed, director of product for San Antonio, Texas–based Lucifer Lighting, “and each element is affected differently.” High heat loads can reduce the efficiency of an LED’s phosphors (which convert blue and violet light to white light), degrade the color of optical-grade silicone, and shorten the life span of the diodes. “If you don’t manage heat,” Reed says, “not only do you get lumen depreciation over lifetime, but you also get color change.”
The first generation of LEDs used for lighting, which emerged around the early 2000s, did not manage their heat and essentially “flopped,” says Justin Wang, founder and CEO of AXP Lighting in San Francisco. “Some of them worked just three days and [then] went dead.”
Over the next decade, original equipment manufacturers (OEMs) quickly learned to introduce heat sinks into their products. As a thermal management technique, these heat sinks worked—the LEDs performed better immediately—but they were heavy, accounting at times for more than 50 percent of a fixture’s overall weight. This resulted in increased material consumption, higher shipping costs, and bulkier designs that interfered with lumen output, says Tim Rider, a senior global portfolio manager at Philips.
Today, the options for thermal management are far more numerous and sophisticated. The result is a wave of lighter and sleeker fixtures, and, in some cases, completely new form factors.
Material Review
In 2008, the U.S. Department of Energy announced the L
Prize, a technology competition that included a $10 million cash prize for the
company that could develop a 60W replacement A-lamp with a life span of 25,000
hours. Philips won with a then-new type of aluminum heat sink that had fins to
increase surface area and heat dissipation, and set a precedent for new lamp
form-factors and heat-sink architecture.
Many heat sinks continue to utilize fins, often in a radial pattern or in a grid layout, the latter of which is known as a pin fin design. Aluminum also remains popular for its conductivity, light weight, and low cost relative to other metals, such as copper. More recently, manufacturers have employed high-quality alloys, such as forged aluminum, to create heat sinks with more complex shapes.
Companies are also experimenting with ceramics, thermally conductive plastics, and graphite. Ceramics—inorganic, nonmetallic, and noncorrosive materials—are already used in many LEDs because they conduct heat but not electricity. In a typical diode stack, a minimum of three layers separate the chip and the heat sink: a printed circuit board (PCB), a conductor, and an adhesive layer. “[C]eramics eliminate all these layers so there is a significantly reduced potential for delamination,” says Robert Christensen, the North American business development manager for the German company CeramTec, which manufactures heat sinks.
At the opposite end of the heat-sink performance spectrum are thermally conductive plastics, or thermoplastics, which are loaded with a conductive material such as aluminum nitride. Unlike aluminum, they are naturally noncorrosive, which makes them ideal for outdoor applications, where lumen demand is low and weight is a concern. According to Taiwan-based manufacturer Nytex Composite, thermoplastics are also 20 to 30 percent less expensive than aluminum, in part because they are simpler to make.
Other companies are using longstanding materials in new ways. Copper, one of the best conductors of heat, is typically too expensive for LED applications, but Reggiani Lighting, based outside of Milan, has combined copper and extruded aluminum to reap the best of both metals. Marketing communication manager Filippo Devoti says the company redesigned its heat sink geometry to have a greater number of thinner fins and thus more dissipating surface.
The Overlooked TIMs
One oft-forgotten aspect of an LED’s thermal management
system is the thermal interface material (TIM), a hyperthin substance applied
between the conductive layer and the heat sink to facilitate heat transfer.
Depending on its makeup, this sole layer can cause substantial variance in the
heat sink’s effectiveness, Lucifer’s Reed says.
TIMs can come in the form of adhesives, greases, gels, pads, or solder alloys. Greases, which are often silicone based, can degrade over time—as can many of the aforementioned substances. One alternative is Thermal Clad, an insulated metal substrate by German company Henkel. The material is reportedly more resilient and adheres to a heat sink with a pre-applied conductive adhesive tape.
Another option is Hitherm, a super-lightweight flexible material made from graphite. Graphite is the next best thing to diamond, Reed says, because carbon is an excellent conductor of heat. Made by GrafTech, headquartered in Independence, Ohio, Hitherm comes in sheets that are die-cut and then applied to the LED “almost like a sticker,” Reed says.
Because of graphite’s superior conductivity, Reggiani Lighting has started to use the material for the heat sink of some of its fixtures. Devoti says these heat sinks are traditional in shape but perform better than aluminum, which make them worth their higher cost.
Forgo the Heat Sink?
More recently, some LEDs have been marketed as having no
heat sink at all. In reality, the heat sink exists but is integrated into the
design of the luminaire. For example, the die-cast aluminum body of Lucifer’s
CY3-AD downlight serves as its heat sink, while Philips’ 60W replacement
SlimStyle A19 uses the LEDs’ circuit board to draw the heat away from the
chips. The lamp features 26 LEDs—13 on each side of the circuit board—with
generous spacing, which drives its flat form.
Manufacturers have also turned to convection. Vents cut in
the top and bottom of Cree’s 4Flow A19 lamp create cross-flow ventilation that
whisks heat away from LEDs. Cree uses the same philosophy for its OSQ Series
LED outdoor area light. “With outdoor lighting, a heat sink becomes a
sculptural element because it is the housing,” says Cree vice president of
product strategy Gary Trott, who is based in Atlanta. “So we [develop]
something that is both functional and attractive, and something that integrates
with various architectural sites.”
Hong Kong–based Cledos uses both strategic LED placement and
convection to control heat in its AirLED lamps, which “are comprised of
multiple low-powered chips dispersed and wired in a proprietary process, using
air circulation for thermal management,” says COO Eric Steinmeyer. “The
resulting physical design uses less raw material and is sleeker, lighter, and
more streamlined.”
AXP Lighting’s Filament LED takes an aesthetic approach to
thermal management. The 60W replacement lamp features Edison-style lamp
filaments made up of 28 individual LED chips mounted to a transparent sapphire
substrate. AXP’s sealed lamp is filled with helium, which has a higher
conductivity than oxygen and dissipates heat more efficiently. Importantly, the
lamp only generates a small amount of heat in the first place because AXP
underdrives its chips by design, running them at about 20 percent of capacity. This
means fewer lumens per LED, but AXP makes up for this with a greater number of
diodes. “Sometimes people use only one chip and overdrive it at 120 or even 200
percent,” AXP CEO Wang says. “If a chip can produce 80 lumens per watt, we only
demand about 15 lumens per watt.”
Active Cooling
For some lamps, passive cooling—through conduction with a
heat sink or convection—still may not suffice. When a manufacturer needs to
pack a lot of lumens into a small form factor, it can make up the difference
with an active cooling strategy, such as using a fan. Some in the industry are
skeptical, however, about the reliability of fans and the potential noise they
create. Reed says that a fan can reduce the size of the heat sink by up to 90
percent, but if it stops working, “your heat sink is grossly undersized.”
However, Reed continues, these are the failings of individual brands, not active cooling as a whole. “A lot of manufacturers went out and didn’t do their homework,” he says. Lucifer uses German manufacturer Ebm-Papst’s high-performance fans, which run at 7 decibels—“about the same noise level of someone sleeping in an ultraquiet room,” he says.
U.K.-based LED Eco Lights recently released a replacement for a high-pressure sodium lamp with a fan that generates less than 0.5 decibels—reportedly the quietest on the market. Using magnetic levitation technology, the fan uses no bearings, which are often the point of failure.
Looking Forward
The progress made in thermal management technologies,
including the introduction of new materials and form factors, are happening
alongside developments in nearly every other aspect of an LED’s performance.
“These improvements will lead to things that have never before been possible,”
Philips’ Rider says.
As thermal management devices become smaller, more efficient, and better integrated into a fixture design, the burden OEMs previously faced of designing around a heat sink may soon become a thing of the past. The possibility, Rider says, of “[creating] the same light effect and the same experience, but with a much smaller form factor or with a completely new design is exciting.”
Resources
A list of references on LEDs, heat, and heat-sink architecture.
“Understanding Heat Transfer Mechanisms in Recessed LED
Luminaires,” by Tianming Dong and Nadarajah Narendran, Ninth International
Conference on Solid State Lighting, Proceedings of the SPIE, 2009. Available at: bit.ly/1LgVwzz.
“Thermal Management Solutions Utilizing High Thermal Conductivity Graphite Foams,” by James Klett and Bret Conway, Society for the Advancement of Material and Process Engineering Journal, 2000. Available at: 1.usa.gov/1OC86K3.
“Voids in Thermal Interface Material Layers and Their Effect on Thermal Performance,” by Arun Gowda, Proceedings of the Sixth Electronics Packaging Technology Conference, 2004. Available at: bit.ly/1Vm80oO.
“Is the Thermal Resistance Coefficient of High-Power LEDs Constant?” by Lalith Jayasinghe, Tianming Dong, and Nadarajah Narendran, Seventh International Conference on Solid State Lighting, Proceedings of the SPIE, 2007. Available at: bit.ly/1jqlKTO.
