High power light-emitting diodes (LEDs) can use 350

milliwatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor o ...

s or more in a single LED. Most of the electricity in an LED becomes heat rather than light – about 70% heat and 30% light. If this heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED less

reliable Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * Reliability (computer networking), a category used to des ...

, shortens its lifespan. Thus, thermal management of high power LEDs is a crucial area of the research and development. Limiting both the junction and the

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...

particles temperatures to a low value is required, which will guarantee desired LED lifetime. Thermal management is a universal problem having to do with power density, which occurs both at higher powers or in smaller devices. Many

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...

applications wish to combine a high light flux with an extremely small light emitting substrate, causing concerns with LED power management to be particularly acute. Chip-On-Board COB LED Module

Heat transfer procedure

In order to maintain a low junction temperature to keep good performance of an

LED A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...

, every method of removing heat from LEDs should be considered.

Conduction Conductor or conduction may refer to: Biology and medicine * Bone conduction, the conduction of sound to the inner ear * Conduction aphasia, a language disorder Mathematics * Conductor (ring theory) * Conductor of an abelian variety * Condu ...

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

, and

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

are the three means of heat transfer. Typically, LEDs are encapsulated in a transparent

polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

-based

resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...

, which is a poor thermal conductor. Nearly all heat produced is conducted through the back side of the chip. Heat is generated from the p–n junction by electrical energy that was not converted to useful light, and conducted to outside ambience through a long path, from junction to

solder Solder (; North American English, NA: ) is a fusible alloy, fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces aft ...

point, solder point to board, and board to the

heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is thermal management (electronics), ...

and then to the atmosphere. A typical LED side view and its thermal model are shown in the figures. The junction temperature will be lower if the thermal impedance is smaller and likewise, with a lower ambient temperature. To maximize the useful ambient temperature range for a given

power Power may refer to: Common meanings * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power, a type of energy * Power (social and political), the ability to influence people or events Math ...

dissipation, the total

thermal resistance In heat transfer, thermal engineering, and thermodynamics, thermal conductance and thermal resistance are fundamental concepts that describe the ability of materials or systems to conduct heat and the opposition they offer to the heat current. ...

from junction to ambient must be minimized. The values for the thermal resistance vary widely depending on the material or component supplier. For example, R_JC will range from 2.6 °C/W to 18 °C/W, depending on the

manufacturer. The thermal interface material’s (TIM)

will also vary depending on the type of material selected. Common TIMs are

epoxy Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...

, thermal grease, pressure-sensitive adhesive and solder. Power LEDs are often mounted on metal-core printed circuit boards (MCPCB), which will be attached to a heat sink. Heat conducted through the MCPCB and heat sink is dissipated by convection and radiation. In the package design, the surface flatness and quality of each component, applied mounting

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

, contact area, the type of interface material and its thickness are all important parameters to thermal resistance design.

Passive thermal designs

Some considerations for passive thermal designs to ensure good thermal management for high power LED operation include:

Adhesive

Adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advantage ...

is a thermal conductive interface layer, which is commonly used to bond LED and board, and board and heat sinks and further optimizes the thermal performance. Current commercial adhesive is limited by relatively low thermal conductivity ~1 W/(mK).

Heat sink

Heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is thermal management (electronics), ...

s provide a path for the heat from the LED source to outside medium. Heat sinks can dissipate power in three ways: *

conduction Conductor or conduction may refer to: Biology and medicine * Bone conduction, the conduction of sound to the inner ear * Conduction aphasia, a language disorder Mathematics * Conductor (ring theory) * Conductor of an abelian variety * Condu ...

- heat transfer from one solid to another *

- heat transfer from a solid to a moving fluid, which for most LED applications will be air *

- heat transfer from two bodies of different surface temperatures through

Thermal radiation Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...

. Also, heatsink: * Material – The thermal conductivity of the material that the heat sink is made from directly affects the dissipation efficiency through conduction. Normally this is

aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

, although

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

may be used with an advantage for flat-sheet heat sinks. New materials include thermoplastics that are used when heat dissipation requirements are lower than normal or complex shape would be advantaged by injection molding, and natural graphite solutions which offer better thermal transfer than copper with a lower weight than aluminum plus the ability to be formed into complex two-dimensional shapes. Graphite is considered an exotic cooling solution and does come at a higher production cost. Heat pipes may also be added to aluminum or copper heat sinks to reduce spreading resistance. * Shape – Thermal transfer takes place at the surface of the heat sink. Therefore, heat sinks should be designed to have a large surface area. This goal can be reached by using a large number of fine

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...

s or by increasing the size of the heat sink itself. Although a bigger surface area leads to better cooling performance, there must be sufficient space between the fins to generate a considerable temperature difference between the fin and the surrounding air. When the fins stand too close together, the air in between can become almost the same temperature as the fins, so that thermal transmission will not occur. Therefore, more fins do not necessarily lead to better cooling performance. * Surface Finish – Thermal radiation of heat sinks is a function of surface finish, especially at higher temperatures. A painted surface will have a greater

emissivity The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...

than a bright, unpainted one. The effect is most remarkable with flat-plate heat sinks, where about one-third of the heat is dissipated by radiation. Moreover, a perfectly flat contact area allows the use of a thinner layer of thermal compound, which will reduce the

between the heat sink and LED source. On the other hand,

anodizing Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electr ...

etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...

will also decrease the thermal resistance. * Mounting method – Heat-sink mountings with screws or springs are often better than regular clips, thermal conductive glue or sticky tape. For heat transfer between LED sources over 15 Watt and LED coolers, it is recommended to use a high thermal conductive interface material (TIM) which will create a thermal resistance over the interface lower than 0.2 K/W. Currently, the most common solution is to use a phase-change material, which is applied in the form of a solid pad at room temperature, but then changes to a thick, gelatinous fluid once it rises above 45 °C.

Heat pipes and vapor chambers

Heat pipe A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor ...

s and vapor chambers are passive, and have effective thermal conductivities ranging from 10,000 to 100,000 W/m K. They can provide the following benefits in LED thermal management: * Transport heat to a remote heat sink with minimum temperature drop * Isothermalize a

natural convection Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

heat sink, increasing its efficiency and reducing its size. In one case, adding five heat pipes reduced the heat sink mass by 34%, from 4.4 kg to 2.9 kg. * Efficiently transform the high heat flux directly under an LED to a lower heat flux that can be removed more easily.

PCB - printed circuit board

* MCPCB – Metal Core PCB are the boards, which incorporate a metal material base as heat spreader as an integral part of the circuit board. The metal core usually consists of aluminum or copper alloy. Furthermore MCPCB can take advantage of incorporating a

dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

layer with high

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

to reduce thermal resistance. * Separation – Separating the LED drive circuitry from the LED board prevents the heat generated by the driver from raising the LED junction temperature.

Thick-film materials system

* Additive Process – Thick film is a selective additive deposition process which uses material only where it is needed. A more direct connection to the Al heat sink is provided; therefore thermal interface material is not needed for circuit building. Reduces the heat spreading layers and thermal footprint. Processing steps are reduced, along with the number of materials and amount of materials consumed. * Insulated Aluminum Materials System – Increases thermal connectivity and provides high dielectric breakdown strength. Materials can be fired at less than 600 °C. Circuits are built directly onto aluminum substrates, eliminating the need for thermal interface materials. Through improved thermal connectivity, the junction temperature of the LED can be decreased by up to 10 °C. This allows the designer to either decrease the number of LEDs needed on a board, by increasing the power to each LED; or decrease the size of the substrate, to manage dimensional restrictions. It is also proven that decreasing the junction temperature of the LED dramatically improves the LED’s lifetime.

Package type

* Flip chip – concept is similar to flip-chip in package configuration widely used in the

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

industry. Briefly speaking, the LED die is assembled face down on the sub-mount, which is usually silicon or

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

, acting as the heat spreader and supporting substrate. The flip-chip joint can be eutectic, high-

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

, lead-free

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

stub Stub or Stubb may refer to: Shortened objects and entities * Stub, a tree cut and allowed to regrow from the trunk; see pollarding * Pay stub, a receipt or record that the employer has paid an employee * Stub period, period of time over which i ...

. The primary source of light comes from the back side of the LED chip, and there is usually a built-in reflective layer between the light emitter and the solder joints to reflect up the light which is emitted downward. Several companies have adopted flip-chip packages for their high-power LED, achieving about 60% reduction in the thermal resistance of the LED while keeping its thermal reliability.

LED filament

The LED filament style of lamp combines many relatively low-power LEDs on a transparent glass substrate, coated with

, and then encapsulated in the silicone. The lamp bulb is filled with

inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. Though inert gases have a variety of applications, they are generally used to prevent u ...

, which convects heat away from the extended array of LEDs to the envelope of the bulb. This design avoids the requirement for a large heat sink.

Active thermal designs

Some works about using active thermal designs to realize good thermal management for high power LED operation include:

Thermoelectric (TE) device

Thermoelectric devices are a promising candidate for thermal management of high power LED owing to the small size and fast response. A TE device made by two ceramic plates can be integrated into a high power LED and adjust the temperature of LED by heat-conducting and electrical current insulation. Since ceramic TE devices tend to have a coefficient of thermal expansion mismatch with the silicon substrate of LED, silicon-based TE devices have been invented to substitute traditional ceramic TE devices. Silicon owning higher thermal conductivity (149 W/(m·K)) compared with aluminum oxide(30 W/(m·K)) also makes the cooling performance of silicon-based TE devices better than traditional ceramic TE devices. The cooling effect of thermoelectric materials depends on the Peltier effect. When an external current is applied to a circuit composed of n-type and p-type thermoelectric units, the current will drive carriers in the thermoelectric units to move from one side to the other. When carriers move, heat also flows along with the carriers from one side to the other. Since the direction of heat transfer relies on the applied current, thermoelectric materials can function as a cooler with currents that drive carriers from the heated side to the other side. A typical silicon-based TE device has a sandwich structure. Thermoelectric materials are sandwiched between two substrates made by high thermal conductivity materials. N-type and p-type thermoelectric units are connected sequentially in series as the middle layer. When a high power LED generates heat, the heat will first transfer through the top substrate to the thermoelectric units. With an applied external current, the heat will then be forced to flow to the bottom substrate through the thermoelectric units so that the temperature of the high power LED can be stable.

Liquid cooling system

Cooling systems using liquids such as liquid metals, water, and stream also actively manage high power LED's temperature. Liquid cooling systems are made up of a driving pump, a cold plate, and a fan-cooled radiator.{{Cite journal, last1=Deng, first1=Yueguang, last2=Liu, first2=Jing, date=2010-08-01, title=A liquid metal cooling system for the thermal management of high power LEDs, journal=International Communications in Heat and Mass Transfer, volume=37, issue=7, pages=788–791, doi=10.1016/j.icheatmasstransfer.2010.04.011, bibcode=2010ICHMT..37..788D , issn=0735-1933 The heat generated by a high power LED will first transfer to liquids through a cold plate. Then liquids driven by a pump will circulate in the system to absorb the heat. Lastly, a fan-cooled radiator will cool the heated fluids for the next circulation. The circulation of liquids manages the temperature of the high power LED.

References

External links

Thermal Management of Cree® XLamp® LEDs

Thermal management of Osram Soleriq COB LED modules
Light-emitting diodes Optical diodes Semiconductor technology