Thermal management of high-power LEDs

With the coming of energy-saving era, high power light-emitting diodes (LEDs) are promising to replace other technologies such as incandescent and fluorescent bulbs in signaling, solid state lighting, and vehicle headlight applications due to improved luminescent efficiencies and extended lifetime. Power dissipation ratings ranging from 500mW to as much as 10 watts in a single package have become a standard and are expected to increase in the future. However, current packaging efficiencies clearly indicate that conventional packages are inadequate for the demands of any current and future applications. Heat accompanied by higher power not only causes efficiencies to lower down, but also influences long-term reliability of LED devices. Consequently, thermal management of high power LEDs is extremely crucial for proper operation and extended life. Optimal heat dissipating material and package method should be well designed to fit the growing power needs.

Heat Transfer Procedure

In order to maintain a low junction temperature to keep good performance of LED, every method of releasing heat form LEDs should be considered. Conduction, convection, and radiation are the three means of hear transfer. In LEDs, heat is almost transmitted through conduction. 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. Thus, heat is generated from the PN junction and conducted to outside ambience through a long and expensive 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 straight into atmosphere, starting from filament to the glass and ending with the thermal resistance form glass to 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 (R_JA). Different components in the heat conduction path can be modeled as different thermal resistances. The total power dissipated by the LED (P_LED) 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 (T_J) is the sum of the ambient temperature (T_A) 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: T_J = T_A + (R_JA × P_LED) , and R_JA = R_JC + R_CB + R_TIM + R_H

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, R_JC will range from 2.6^oC/W to 18^oC/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 one thermal resistance Rconvection 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

Here below lists some considerations for passive thermal designs to ensure good thermal management for high power LED operation.

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 – Material selection of heat sinks directly affects the dissipation efficiency through conduction. Consequently, material with higher thermal conductivity is desired. The material normally used for heat sink construction is aluminum, although copper may be used with an advantage for flat-sheet heat sinks.
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 decreases the thermal resistance.
Mounting method- Heat-sink mountings with screws or springs are often better than regular clips. Thermal conductive glue or sticky tape should only be used in situations where mounting with clips or screws is not possible.

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 conductance.
Separation - Separate the LED drive circuitry from the LED board so that the heat generated by the driver will not contribute to the LED junction temp.

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 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 backside of the LED chip, and there is usually a build-in reflective layer between the light emitter and the solder joints to reflect the light emitted downwards up. Commercially, several companies have adopted the flip-chip based approach to package their high-power LED. About 60% reduction in the thermal resistance of the LED is achieved while keeping its thermal reliability.