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Posts Tagged ‘Thermal management’

Thermal Management of High-power LEDs

15 Jun

High power light-emitting diodes (LEDs) are likely to replace other technologies such as incandescent and fluorescent bulbs in signaling, solid state lighting, and vehicle headlights because they save energy and extend the light’s lifetime. LEDs that use from 500 milliwatts to as much as 10 watts in a single package have become standard, and researchers expect to use even more power in the future. Some of the electricity in an LED becomes heat rather than light. If that heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED more dangerous and less reliable. Thus, thermal management of high power LEDs is a crucial area of research and development.

Heat transfer procedure

In order to maintain a low junction temperature to keep good performance of an LED, every method of releasing heat from LEDs should be considered. Conduction, convection, and radiation are the three means of heat transfer. Typically, LEDs are encapsulated in a transparent resin, which is a poor thermal conductor. Nearly all heat produced is conducted through the back side of the chip. Heat is generated from the PN junction by electrical energy that was not converted to useful light, and conducted to outside ambience through a long and extensive path, from junction to solder point, solder point to board, and board to the heat sink and then to the atmosphere. The heat path of tungsten light bulbs is almost all straight into the atmosphere, starting from filament to the glass and ending with the thermal resistance from glass to the atmosphere. A typical LED side view and its thermal model are shown in the figures.

The thermal resistance between two points is defined as the ratio of the difference in temperature to the power dissipated; the unit is oC/W. From the LED junction to the thermal contact at the bottom of package, the thermal resistance is governed by the package design. It is referred to as the thermal resistance between junction and ambient (RJA). Different components in the heat conduction path can be modeled as different thermal resistances. The total power dissipated by the LED (PLED) is the product of the forward voltage and the forward current of the LED, which can be modeled as a current source. The ambient temperature is modeled as a voltage source. Therefore, the junction temperature (TJ) is the sum of the ambient temperature (TA) and the product of the thermal resistance from junction to ambient and the power dissipated. By “thermic Ohm’s Law”, we have the equation as follows: TJ = TA + (RJA × PLED) , and RJA = RJC + RCB + RTIM + RH

Intuitively, you can see that 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 dissipation, the total thermal resistance from junction to ambient must be minimized. The values for the thermal resistance vary widely depending on the material or component supplier. For example, RJC will range from 2.6oC/W to 18oC/W, depending on the LED manufacturer. The thermal interface material’s (TIM) thermal resistance will also vary depending on the type of material selected. Common TIMs are epoxy, thermal grease, pressure sensitive adhesive and solder. In the most cases, power LEDs will be mounted on metal-core printed circuit boards (MCPCB), which will be attached to a heat sink. Heat flows from the LED junction through the MCPCB to the heat sink by way of conduction, and the heat sink diffuses heat to the ambient surroundings by convection. So, we can also add convection to the thermal model at the end of the heat transmission path. In the package design, the surface flatness and quality of each component, applied mounting pressure, 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 is commonly used to bond LED and board, and board and heat sinks. Using a thermal conductive adhesive can further optimize the thermal performance.

Heat sink

Heat sinks provide a path for heat from the LED source to outside medium. Heat sinks can dissipate power in three ways: conduction (heat transfer from one solid to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid will be air), or radiation (heat transfer from two bodies of different surface temperatures through electromagnetic waves).

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, although copper 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 2 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 fins or by increasing the size of the heat sink itself.

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 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 thermal resistance between the heat sink and LED source. On the other hand, anodizing or etching 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.

PCB (Printed Circuit Board)

MCPCB – MCPCB (Metal Core PCB) are those boards which incorporate a base metal material as heat spreader as an integral part of the circuit board. The metal core usually consists of aluminum alloy. Furthermore MCPCB can take advantage of incorporating a dielectric polymer layer with high thermal conductivity for lower 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.

Package type

Flip chip – The concept is similar to flip-chip in package configuration widely used in the silicon integrated circuit industry. Briefly speaking, the LED die is assembled face down on the sub-mount, which is usually silicon or ceramic, acting as the heat spreader and supporting substrate. The flip-chip joint can be eutectic, high-lead, lead-free solder or gold stub. 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 the light emitted downwards up. Several companies have adopted flip-chip packages for their high-power LED, achieving bout 60% reduction in the thermal resistance of the LED while keeping its thermal reliability.

 

Controlling Junction Temperature of LEDs with Thermal Management Materials

06 Apr

The junction temperature in an LED (the p-n junction temperature) is most critical to consider for LED cooling. If this temperature rises above the prescribed level recommended by the LED manufacturer, the lifetime of the LED as well as its intensity and color may be affected.

As with most electronic systems, the LED assembly location where the highest temperatures are reached is the junction temperature. Many thermal management materials may be used to control this temperature, such as heat pipes or metal core boards, but each of these carry their own thermal resistance. An optimal cooling design is one which includes the lowest sum of thermal resistances for the system.

Ideally, no one thermal management material will be a bottle neck for thermal dissipation, however the materials closest to the heat source are most critical. High performance thermal management materials should be considered here. If the highest resistance measured is at the interface junction, the junction temperature will be raised more than if the bottleneck in resistance were at any other location.

There are various types of LED assemblies, but a typical high power LED is depicted here. In this type of assembly, implementation of high performance thermal management materials would be most critical in the die attach material, heat sink slug, and solder as these are closest to the heat source and will have the greatest impact on dissipating the heat away from the p-n junction.

 

New “Diamond LED” Resists High Temperature and Large Current

12 Nov

A deep UV LED using diamond semiconductor has been developed by National Institute of Advanced Industrial Science and Technology of Japan (AIST), with the details of the development planned to be disclosed at the 56th Spring Meeting of the Japan Society of Applied Physics taking place from March 30.

The new deep UV LED uses a p-i-n-structured diamond semiconductor stacked on the 2mm-square diamond substrate, emitting deep ultraviolet light of 235nm wavelength, with output of 30μW under a current of 320mA.

There are two important characteristics of this new LED according to AIST. Firstly, its luminous efficiency continues to increase even under a large current, secondly, it can well resist high temperatures. Specifically speaking, with a current of a density exceeding 2,000A/cm2 through a 120μm-diametered electrode, the diamond LED still can have its luminous efficiency increasing without being saturated, according to AIST. As to the excellent heat resistance, even when temperature rises to 420°C, the light emission intensity of the LED does not degrade but continues to increase, AIST said.

The diamond LED emits light because of the generation of “excitons.” For general LEDs, the excitons is vulnerable to heat and often deteriorates quickly, while the excitons generated in the diamond LED are very stable and will not break until 600°C, which is the major reason of its excellent temperature resistance.

At present, the commercialization of diamond LEDs is limited by the expensive price of the diamond substrate, with the available diamond substrates usually to be several square millimeters in size.

To solve this problem, AIST said they have been developing a technique to stack a polycrystalline diamond semiconductor film on a Si wafer, with the prototype formed with the new technique having efficiency only an order of magnitude lower than the currently developed diamond LED. “Because it only requires quite common materials such as silicon and methane, when the technique becomes practical, diamond LEDs can be produced at a very low cost.”

 

Improve Thermal Control to Enhance The Lifetime of White LEDs

28 Sep

LED industry has ever tried to develop large-size LED chips in the past in order to achieve the desired goal of getting full white LED light beam, but actually when the imposed power of white LEDs exceeds 1w, the light beam will be weakened, and the luminous efficiency will reduce by 20 ~ 30%, in other words the white LED is several times bright as conventional LED, if we want the power consumption characteristic of white LED to surpass fluorescent lights, then we must first overcome the following four topics including inhibition of temperature increase, ensuring lifetime, improving light efficiency, and equalization of luminescence properties etc.

The detailed solution to isuues related to temperature increase is to lower the package thermal impedance; the detailed methods to maintain the lifetime of LEDs is to improve chip shape, utilize small size chips; the detailed methods to improve the luminous efficiency of LEDs is to improve the chip structure, utilize small size chips; as to the equalization of luminescence, we can achieve by improving LED package method, and all these methods have already been developed in succession.

Solving the thermal control issue of package is the fundamental method

Since increasing power will result that the thermal impedance of package rapidly decrease to 10K/W, the foreign LED industry have developed high temperature resistant white LEDs to improve the above mentioned problems, however, the heat generation of high-power LEDs is actually tens of times higher than low power LEDs, and temperature increase can also sharply decrease the luminous efficiency, even if the package technology allows high heat generation, but the junction temperature of LED chip is likely to exceed the allowable value, the industry has finally realized that the fundamental method is to solve the thermal control issue of package.

As for the lifetime of LEDs, for instance, switching current package materials to silicon and ceramic package materials can improve the lifetime of LEDs by ten times, especially the white LED’s light spectrum includes short wavelength light whose wavelength is below 450nm, traditional epoxy resin package material is easy to be destroyed by short wavelength light, large quantity light output of high-power white LEDs light is easier to accelerate the deterioration of package materials, industry tests showed that if the light has been lit continuously for less than ten thousand hours,  brightness of high-power white LED has been degraded by more than 50%, which totally can not meet the basic requirements of long life for illumination light source.

As for LED luminous efficiency, improving both LED chip structure and package structure, can achieve the same level as low power white LED, which is mainly because that when the current density increases by more than 2 times, it will not be easy to extract light from the large size LED chips , whereas it will cause the luminous efficiency of high power white LEDs is even lower than low-power white LEDs, if we improve the electrode structure of the LED chip, theoretically the light extraction issue can be solved.

Try to reduce thermal resistance and improve thermal control

As for the uniformity of luminescence properties, it is generally believed that as long as we improve the intensity and uniformity of fluorescent material of white LEDs and the manufacture technology of fluorescent material, the above mentioned difficulties can be solved. As noted above, while increasing exerting power, we must try to reduce thermal resistance, improve heat dissipation issue, the specific content is: reducing thermal resistance from LED chip to package, inhibiting thermal resistance from package to the printed circuit board, improving cooling smoothness of LED chips.

In order to reduce the thermal resistance, many foreign LED manufacturers locate LEDs in the surface of the heat sink which is made from copper and ceramic material, and then use metal wire to connect the thermal control system of printed circuit board to the heat sink which uses cooling fan to achieve forced air cooling through welding, according to the German OSRAM Opto Semiconductors Gmb experiment results, the thermal resistance from LEDs with the above mentioned structure to the soldering point can reduce 9K/W, which is only 1/6 as the traditional LED’s, when imposing 2W power to the packaged LEDs, LED chip junction temperature is 18K higher than the temperature of the soldering point, even if the printed circuit board temperature rises to 500 degree Celsius, junction temperature is at most 700 degree Celsius; in contrast, in the past once the thermal resistance reduces, LED chip junction temperature will be affected by the printed circuit board temperature, therefore we must try to reduce the temperature of LED chips, in other words reducing the thermal resistance of LED chip to the the same level as sodering point can effectively reduce the burden of LED chip cooling operation. Conversely, even if the white LED has the structure which can inhibit the thermal resistance, once the heat can not be conducted from the package to the printed circuit board, temperature rise of LEDs will cause the rapid decrease of luminous efficiency, so Panasonic Electric developed the integration technology of printed circuit board and package, the company packaged 1mm square blue LED chip on a ceramic substrate by the way of flip chip, then stuck the ceramic substrate onto the surface of the copper printed circuit board, the thermal resistance of the entire module including printed circuit board is around 15K/W.

LED manufacturers demonstrate their thermal design capability

Since the adhesion between heat sink and printed circuit board directly impacts the heat conduction effects, the printed circuit board design becomes very complicated, under such circumstance, lighting device and LED package manufacturers such as Lumileds in the United States and CITIZEN in Japan, have developed simple cooling technology for high-power LEDs, CITIZEN’s white LED package which started sample shipments in 2004 can emit the heat of heat sink with a thickness of 2-3mm out requiring no special bonding technique, although the thermal resistance of 30K/W from LED chip junction to the heat sink is higher than the OSRAM’s 9K/W, and under normal environment the room temperature will increase the thermal resistance by about 1W, however even under the condition that traditional printed circuit board has no cooling fan to achieve forced air cooling, the white LED module can be lit continuously.

The high power LED chips that Lumileds began sample shipments in 2005 allow a junction temperature as high as +1850 degree celsius, which is 600 degree Celsius higher than the same level products of other companies, when using traditional RF4 printed circuit board to package, ambient temperature range within 400 degree Celsius we can input the current equivalent to 1.5W power. So Lumileds and CITIZEN utilized the method of improving allowable junction temperature, while OSRAM located LED chips in the heat sink surface, and reached 9K/W ultra-low thermal impedance record, the record is 40% less than the thermal resistance of same level products developed in the past, it is worth mentioning that when packaging the LED module, they use the method of flip chip which is the same as traditional method, however when bonding LED module and heat sink, they choose the surface which is nearest to light-emitting layer of LED chips as the joint face, which can conduct and emit the heat of light-emitting layer in the shortest distance.

In 2003, Toshiba Lighting ever built low thermal resistance white LEDs with a luminous efficacy of 60lm/W on the surface of 400mm square aluminum alloy, there were no special cooling components such as cooling fan, they tried to make LED module with a light beam of 300lm, due to rich trial manufacturing experience of Toshiba Lighting, the company indicated that as a result of advances in simulation analysis technology, since 2006 the white LEDs with luminous efficacy of more than 60lm/W, can easily use light fittings, framework to improve thermal conductivity, or use cooling fan to achieve forced air and design the thermal control of lighting devices, the module structure without special heat dissipation technology can also use white LEDs.

Change package materials to inhibit material deterioration and the decreasing speed of light transmittance

As for the longevity of LEDs, LED manufacturers now utilize the method of changing package materials, meanwhile distribute fluorescent material within the package materials, especially silicon package material can more effectively inhibit material deterioration and decrease speed of light transmittance than the traditional blue and near-UV LED chip package material-epoxy resin material. Since the absorption percentage of epoxy resin to light with wavelength of 400 ~ 450nm light is as high as 45%, which of silicon package material is as low as 1%, the time for epoxy resin to degrade LED brightness by 50% is less than ten thousand hours, while silicon material can extend the time to around forty thousand hours, which is almost the same as the designed lifetime of lighting devices, which means there is need to replace white LEDs during the using period of lighting devices. However, silicon resin material is a kind of high elastic soft materials, when processing we must use the production technology which will not scratch the surface of the silicon resin, in addition during processing silicon resin is easy to attach dust, so in the future technology of improving the surface property must be developed.

Although silicon package material can ensure forty thousand hours lifetime of LEDs, the lighting device manufacturers in the industry have different views, the main argument is that the lifetime of traditional incandescent and fluorescent lamps is defined as “the brightness degrading by less than 30%”, the time for LEDs to degrade brightness by 50% is around forty thousand hours, if it is converted into the time to degrade brightness by 30%, it will be only about twenty thousand hours. There are two measures currently to extend the service life of components, inhibiting the overall temperature rise of white LEDs and stopping using the resin package method.

It is believed that if the above two life extension measures can be fully implemented, the time for LEDs to degrade brightness by 30% can achieve forty thousand hours. We can utilize the method of cooling LED package printed circuit board to inhibit white LED temperature rise, which is mainly because that package resin under high temperature and strong light will quickly deteriorate, when the temperature decreases by 100 degree Celsius, the lifetime of LEDs will extend to 2 times as original life. Stopping using resin package  can completely eliminate deterioration factor, because the light generated by LEDs will be reflected within the packaging resin, if we use resin material reflector which can change the light travelling direction of LED chip side, the reflector will absorb light, so the quantity of light extraction will rapidly decrease, which is also the reason that LED manufacturers are more willing to utilize ceramic and metal package materials.

 

How to Effectively Improve Thermal Management of High-power LEDs

17 Sep

For a long time, LED display application has been a main field which demands large quantity of LEDs, when the heat dissipation requirements for LEDs is not very strict, LEDs are usually packaged by using traditional resin substrate.

After the year 2000, as technology developed, LEDs became higher bright and more efficient, coupled with substantial improvement in the light efficacy of blue LEDs and the continuous decreasement of LED manufacturing, the application fileds of LEDs, and the industries which are willing to utilize LEDs has been continuously expanding, the industries including LCD, home appliances, automobile and so on also started to actively consider the possibility of LED application, For instance, expectations of consumer products industry to high-power LEDs are that they can achieve energy saving, high brightness, long lifetime, high color rendering, which means achieving high heat dissipation capability is indispensable condition of high power LED package substrate.

In addition, LCD panel industry is facing the EU RoHS specifications, they have to confront the environmental protection pressure of making all cold cathode fluorescent lamps mercury-free, which causes a more urgent market demand for high-power LEDs.

In addition to protecting the internal LED chip, LED package can also perform the functions of connecting LED chip with the outside electricity and heat dissipation.

Epoxy resin can no longer meet the technical requirements of high-power LEDs

One LED can reach the light output of hundreds of lumens, which is basically not a big problem, the main problem is how to improve thermal management. After producing such a high lumen, how to maintain the stability and continuity of brightness is another important issue, if thermal management is not treated well, LED brightness and lifetime will drop rapidly, for LEDs, how to achieveeffective reliability and thermal conduction is very important.

LEDs are packaged by using resin with low heat conductivity in the past, but this is regarded as one of the reasons affecting thermal properties, in addition, heat resistance of epoxy resin is very poor, and the situation which may happen is that before LED chip itself runs out its whole lifetime, the color of epoxy resin will have already changed, therefore, improving heat dissipation is the key factor.

In addition, not only the heat can cause the change of epoxy resin, but also short wavelength will cause problems to epoxy resin, which is because that the white LED light spectrum also includes short wavelength light, while epoxy resin is quite easy to be broken by short wavelength light in the white light LED, even low-power white LEDs has already been able to aggravate the phenomenon of breaking epoxy resin, let alone that high-power white LED emits more short-wavelength light, the deterioration is certainly more rapid than the low-power type, even some products only have a lifetime of 5,000 hours after lighting continuously, or even less! Therefore, instead of overcoming the color changing problems caused by old package material “epoxy resin”, we should pay our efforts to search for new generation of package material.

Metal substrate became a new focus

Therefore, the old package material is gradually replaced by high thermal conductivity ceramic, or metal resin package structure in recent years, which is the effort to solve heat dissipation, and strengthen the original features. The common approaches to improve power of LED chips are enlarging the chip size, improving the luminous efficiency, utilizing package with high light extraction efficiency, and enlarging the current. Although the current and light output amount of such practices rises in proportion, the heat generation will also increase.

For High-power LED package technology, since cooling problems cause difficulties to a certain extent, under the background of which, metal substrate technology with high cost-effectiveness became a new concerned focus after improving the efficiency of LEDs.

In the past, since LED output power is low, we usually use conventional FR4 glass epoxy resin substrate to package LEDs, which will not cause serious heat dissipation problem, but for the high-power LEDs used for illumination, they have a luminous efficiency of about 20% to 30%, although chip area is very small, the overall electricity consumption is not high, but the heat generation per unit area is great.

Generally speaking, the resin substrate can only support the heat dissiapation of LEDs woth a power less than 0.5w, LEDs with a power more than 0.5w, usually use metal or ceramic high heat conductivity substrate to package, which is mainly because that thermal property of substrate will directly affect LED’s lifetime and performance, package substrate became the key point of designing high brightness LED products.

LED package substrate thermal design can be divided into two parts currently, one is heat conduction from LED chip to the package body, the other is the thermal conduction from package body to outside world. When we use high thermal conductivity materials, the temperature difference inside the package will become smaller, and the heat will not centralize to some certain parts, the overall heat generated by LED chip will radially flow into every corner inside the package, therefore utilization of high thermal conductivity material can improve the internal thermal dissipation.

Improvement on the heat conduction is entirely dependent on upgrading material to solve the problem. Most people agree that, as LED chips become large in size, use higher current and higher power, the progress of metal package replacing traditional resin package will be accerlerated.

For current metal high conductivity substrate material, there are mainly two types, one is rigid substrate and the other is flexible substrate, for structure, rigid substrates are traditional metal material, metal LED package substrate use materials such as aluminum and copper, for insulation parts, they are almost filled with high heat conduction inorganic material with high thermal conductivity, machinability, electromagnetic shielding, heat impact resistance and other metal properties, the thickness is usually greater than 1mm, most of the materials are widely used in the LED lamp module and illumination module and so forth, technically they have the same high heat conductivity as aluminum substrates, which enables them to be qualified as package material for high-power LEDs under high heat dissipation requirements.

The package industry is actively developing flexible substrates

Flexible substrates are supposed to be developed to make LCD backlight module of car navigation system thinner, and high-power LEDs can achieve three-dimensional package requirements, basically flexible substrate mainly use aluminum as the material, it takes advantage of aluminum’s high thermal conductivity and light weight characteristics to make high-density package substrate, when the aluminum substrate becomes thinner, it can be flexible, and also has high thermal conductivity property.

Generally speaking, thermal conductivity of metal package substrate is around 2W/(mK), but the thermal effect of high-efficiency LEDs is higher, so in order to meet thermal conductivity of up to 4 ~ 6W/(mK), there are already metal package substrates with thermal conductivity of more than 8W/(mK). As the main purpose of rigid metal package substrate is to meet the high power LED package, the package substrate industry is actively developing the technology which can improve the thermal conductivity. Although using aluminum reinforced substrate can improve heat dissipation property, there are restrictions on the cost and assembly, which can not solve the problem at all.

However, the disadvantage of metal package substrate is that metal has a high thermal expansion coefficient, and when it is welded with low thermal expansion coefficient ceramic chips, it is easy to be impacted by thermal cycle, so when using aluminum nitride to package, disharmony phenomenon of metal package substrate may occur, it is very important to overcome the thermal stress difference caused by different materials with different thermal expansion coefficient inside LEDs and improve the reliability of the package substrate.

High thermal conductivity flexible substrate is sticking metal foil on the insulating layer, although the basic structure of the substrate is identical with the traditional substrate, the insulating layer is made with soft epoxy resin filled with high thermal conductivity inorganic material, it has a high thermal conductivity of 8W/(mK), and it is also soft and can be flexible with high thermal conductivity and high reliability, in addition, flexible substrate can also be designed as single-side double-layer or double-side double-layer structure substrate as per customer’s demand. Experimental results show that when using high thermal conductivity flexible substrate, LED temperature decreases by about 100 degrees Celsius, which means the LED lifetime decrease caused by temperature will be highly improved due to the variety in substrate design.

In fact, in addition to high power LEDs, high thermal conductivity substrate can also be used in other high-power semiconductor components, it is suitable for for limited space or high-density package environment. However, only depending on package substrate can not meet the actual demand, so the coordinate of external materials around substrate also becomes more important, for instance, when matching with 3W/(mK) thermal conduction membrane, the heat dissipation property can be further effectively improved.

 
 
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