High-power LED thermal substrate development trend

Due to the advancement of LED technology, LED applications are increasingly diversified. From early power indicator lights, LED lighting products have the advantages of power saving, long life, and high visibility. However, since the input power of high-power LEDs is only 15 to 20% converted to light, the remaining 80 to 85% are converted to heat. If the heat is not discharged to the outside in due course, the temperature of the LED die interface will be too high to affect the light emission. Efficiency and luminous lifetime.

LED development is the key to heat dissipation

With the continuous development of LED materials and packaging technology, LED products continue to increase the brightness, LED applications are more and more widely, and provide a stable and growing market for the LED industry. LED as the backlight of the display is a hot topic recently. From small-size display backlights to medium-size LCD TV backlights, it gradually replaces the CCFL backlight. The main reason is that LEDs have advantages over traditional cold cathode fluorescent lamps (CCFLs) in terms of color, brightness, lifespan, power consumption, and environmental protection appeal, thus attracting active input from the industry.

The power of early single-chip LEDs was not high, the heat generation was limited, and the problem of heat was not significant. Therefore, its packaging method was relatively simple. However, in recent years, with the continuous breakthrough of LED material technology, the LED packaging technology has also changed along with it, from the early one-chip artillery shell type packaging gradually developed into a flat, large-area multi-chip packaging module; its operating current from the early 20mA or so The low-power LED, progressed to the current high-power LED about 1/3 to 1A, single LED input power up to 1W or more, even to 3W, 5W.

Packaging more evolved

Since the heat problem derived from high-brightness, high-power LED systems will be the key to affecting the function of the product, to rapidly dissipate the heat from the LED components to the surrounding environment, it is necessary to start with the thermal management of the package level (L1 & L2); The LED die is attached to a soaking plate with solder or thermal paste, and the thermal impedance of the package module is reduced via the soaking plate. This is the most common LED package module on the market. The main sources are Lumileds, OSRAM, Cree, and Nicha. LED international famous manufacturers.

In practical applications, these LED modules can be assembled in an array of linear light sources, arranged in an array or in a circular array, and then bonded to a heat-dissipating substrate as a surface light source. However, for many terminal applications, such as mini projectors, automotive and lighting sources, the amount of lumens required in a given area will need to be in excess of thousands of lumens or tens of thousands of lumens. To cope with, to multi-chip LED package, and chip directly attached to the substrate is the future development trend.

In practical application of LED, whether it is used for display backlight, indicator light or general lighting, a plurality of LEDs are usually assembled on a circuit substrate as required. On the one hand, the circuit substrate plays a role of carrying the LED module structure. On the other hand, as the output power of the LED becomes higher and higher, the substrate must also play the role of heat dissipation to transfer the heat generated by the LED chip out of the material selection, and therefore must Take into account structural strength and cooling requirements.

Need to worry about material costs

Traditional LED power is not large, the heat dissipation problem is not serious, as long as the general electronic copper foil printed circuit board is sufficient to deal with, but with high-power LED more and more popular, this board is not enough to meet the cooling needs, so we need to The printed circuit board is attached to a metal plate, the so-called Metal Core PCB, to improve its heat transfer path.

In addition, there is also a method of directly forming an insulating layer or a dielectric layer on the surface of an aluminum substrate, and then forming a circuit layer on the surface of the dielectric layer, so that the LED module can directly bond the wires to the circuit layer. At the same time, in order to avoid the increase of thermal resistance due to the poor thermal conductivity of the dielectric layer, perforation is sometimes adopted so that the heat spreader at the bottom of the LED module directly contacts the metal substrate, that is, the so-called direct adhesion of the chip.

In addition to metal substrates, in order to respond to the development of high-power LED packages and direct chip adhesion substrates, in addition to considering heat dissipation, the selection of substrate materials must also consider the issue of matching the thermal expansion coefficient of the chip to avoid thermal distortion caused by thermal stress. Because of reliability problems, ceramic substrates and metal composite substrates are currently being developed at home and abroad. These newly developed substrate materials not only have good heat dissipation properties, but also have a coefficient of thermal expansion (between 4 and 8 ppm/K) that matches the LED chips. The only disadvantage is that the price is higher than that of a common metal substrate.

Focused fanless cooling

In fact, LED's heat-dissipating components are similar to CPU heat-dissipating components. They are mainly composed of air-cooling modules composed of heat sinks, heat pipes, fans, and thermal interface materials. Of course, water cooling is also one of the heat countermeasures.

Taking the current most popular large-size LED TV backlight module, the input power of 40-inch and 46-inch LED backlight sources are 470 W and 550 W, respectively. When 80% of them are converted to heat, the required heat dissipation is about 360 W. And about 440W. How to take these heat away, there is a water-cooled way to cool, but there are doubts such as high unit price and reliability; also use a heat pipe with cooling fins and fans to cool (such as Sony 46 "LED TV), but fan power consumption and noise, etc. Therefore, how to design fanless heat dissipation is the key to decide who will win in the future.