Thermal Analysis of High Power LED Array Packaging with Microchannel Cooler

Liulin Yuan'2, Sheng Liu ,Mingxiang Chen ,Xiaobing Luo2,3

1. Institute of Microsystems, HUST, Wuhan 430074, P. R. China

2. Wuhan National Laboratory for Optoelectronics, HUST, Wuhan 430074, P. R. China

3. The Institute of Energy & Power, HUST, Wuhan 430074, P. R. Chian

Corresponding author's E-mail:

Abstract

The efficiency and reliability of solid-state lighting

devices strongly depend on successful thermal management.

High-brightness light emitting diodes (LEDs), as a strong

candidate for the next generation general illumination

applications, were developed by improving luminous

efficiency and integrating multi-chips within limited areas.

One of key problems is cooling in developing high power

LED for illumination. This paper explores the thermal analysis

of high power LED array packaging with a microchannel

cooler, which is a relatively new cooling technology. The

packaging structure of a high power LED array integratedwith a microchannel cooler is discussed. Detailed thermal

performance is analyzed using the FEA (finite element

analysis) technology. The effects are discussed on the coolingof a multi-chip LED module with different internal fin

geometries of module, flow velocity and its total power.

Simulation results, in the form of average die temperature,

show that the microchannel cooler reduces the average die

temperature, and improves the heat dissipation capability of

LED array. However, the results also demonstrates that,without proper design the junction temperature of the module

is non-uniform across the LED array, and the downstream or

central chips were hotter than the upstream or edge chips. This

may accelerate thermal runaway problems and reduce the

reliability of the LED arrays device. The cooling scheme is

optimized by using staggered fins in our microchannel cooler

to increase the heat transfer coefficient of the multi-chip LED

packaging module. The result shows that the packagingstructure of the microchannel cooler with staggered fins

achieves good thermal performance for high power LED

arrays.

Introduction

Light Emitting Diodes (LEDs) have progressed in recent

years from being signal indicators to emitting enough light for

illumination applications. This has opened a new field for

LED applications, resulting in significant advantages over

conventional light sources and creating some application

challenges unique to LEDs. To be practical for generalillumination, LED systems must reach 1200-1500 lumen

levels at acceptable costs while maintaining reliability [1].

Multi-chip construction is inevitably needed. However,

integrating multi-chips in a limited area also adds more

challenges to the current thermal issue of high power LEDs

devices. Because the lumen output and the efficiency of the

devices are strongly dependent on the heat dissipationcapacity extracted from the package. Currently LEDs turn

about 10-20% of the incoming energy into light and the

remaining 80-90% becomes heat [2]. Without efficient cooling

techniques, the device could be overheated and can degradeoptical performance and decrease lifetime of the chip.

Consequently, improved thermal management techniques are

required for high power LEDs system. Thermal solution

development for high power LED array requires modeling and

optimization on the heat sink and package.A typical high power LED has a 1mm2 surface area with a

total power consumption of 1 watt with a 350mA drivingcurrent, and the average LED light efficiency is about 20%.

This contributes to a high heat flux of 80 W/cm2 within LED.

The heat flux will rise over the 80 W/cm2 with the steady

increasing power consumption in LED [3]. When the drivingcurrent is higher than 350mA and its number can be IA or

more, the heat flux can be mich higher. Since most

conventional cooling methods are inadequate to meet such

requirements, advanced new cooling technologies are being

developed to accommodate these high heat flux demands. In

various innovative cooling schemes proposed to date, the

microchannel technique is among the most promising to beat

such challenges.Tuckerman and Pease first introduced the concept of

1-4244-0620-X/06/$20.00 C 2006 IEEE. 2006 7th International Conference on Electronics Packaging Technology

microchannel heat sink for the cooling of very-large-scale

integrated (VLSI) circuits in 1981 [4]. Phillips discussed the

advantages brought forth by foreshortening the fins so as to

take advantage of high heat transfer rates associated with

developing flow. This could be done by interrupting the fins in

the flow direction, in either an inline or staggered fashion [5].

Kishimoto and Ohsaki in 1986 presented their work in

which they mounted a 5x5 array ofVLSI chips with an area of

8 mm2. By their cooling device for large-scale high-densitystacked multichip packaging, an allowable heat dissipationhigher than 400 W per package was realized [6]. Kishimoto

and Sasaki reported the use of a microchannel with

diamond-shaped interrupted micro-grooved cooling fins as a

means of enhancing performance and reducing temperature

rise across the chip from coolant inlet to coolant outlet [7].Colgan researched silicon microchannel coolers with

staggered fins, which showed to be superior to continuous fin

designs with equivalent geometries, and extendable to power

densities of 400 W/cm2 or more [8].In this work, cooling design and analysis are conducted for

the LED array integrated with a heat sink made of silicon. The

heat sink selection and development are depicted in detail. The

maximum power dissipation from the LED array is examined

based on allowable pressure and flowrates. The possible ways

to reduce the junction temperature are discussed.

Problem Statement

The packaging module to be investigated is shown in

Figure 1. The silicon microchannel is directly etched and

bonded with a Pyrex glass, on which the LED chips are then

mounted. This extracts a high heating load emitted by the

LED chips and transfers it into the water propelled by the

micropump inside the micro-cooling system. Carried by the

coolant, this waste heat is finally dissipated into the ambient.

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Figure 1. Integrated microchannel cooling of a 5 x5 LEDs

array module.

The cross-sectional view is illustrated in Figure 2. A

copper layer is located above (used as electrical traces) and

below (metallic substrate) the dielectric layer (electricalbarrier). LED dies are solder-bonded to the top layer usingsolders. The bottom copper layer is also solder bonded to the

top base of the micro-cooler module. The dimension of

microchannel module is 30mmx30mmx 1.2mm.

LED aii-avi X ~~~dielectficcooper soldei, A =!

Silicon niWci octiaiuiel glass

Figure 2. Cross-sectional view ofthe module.

The LED chip has a 4mm2 surface area with a power

consumption of 5 W. The total power consumption of the 5x5

array is added up to be 125W. The 5x5 die array is perfectlycentered on the module and dies are spaced about 5.5mm

center-to-center distance across the length of the module [9,10].

Analysis and Simulation

In the modeling, only dies are modeled, without phosphorand resin on the dies. Before the simulation, it is assumed that

the coolant is water with an inlet temperature of 25 C, and the

constant flow rate of tm/s flowing through the channels. To

explore the cooling effects on the LED array packaging with

microchannel cooling module, such as channel dimension,liquid inlet flow velocity, allowable module power dissipationand staggered fins structure [11].

It is first assumed that the channel is straight with the

width of channel and fin both of 0.15mm. Figure 3 shows the

temperature distribution across the LED dies with water-flow

going from top to bottom, which represents the maximum

temperature on each die. Because of the relatively highconductivity of the materials in the package, there are not

large temperature gradients within the LED dies. However,

there is about 3.9°C difference between dies. In the stream

direction, the temperature of the chip in upstream is higherthan that in downstream. Temperature for chips located alongthe upstream and downstream edges is lower than that of

central chips. In the transverse direction of stream, the

temperature of chips in central area is higher than that in the

edge. These results indicate that with straight channel the

central chips has a bad thermal runaway, and the whole chipshave a non-uniform temperature distribution.

3 m m mTemperature (dge C)

.............. ................................

flowdi(ectliu | 3 4 25

Figure 3. Die temperature distribution ofthe LED array with

straight fins in microchannel Cooler

Figure 4 shows the relationship between average

temperature, flow resistance and hydraulic diameter. The

smaller the hydraulic diameter is, the larger the dissipationareas are, and the higher the dissipation efficiency is. With the

decreasing of hydraulic diameter, the flow resistance is

increasing. And the flow velocity will be decreased, with a

bad heat dissipation capacity. These results indicate that we

should make a balance between average temperature and flow

resistance to choose a proper hydraulic diameter.

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Figure 4. Average temperature and flow resistance vs

hydraulic diameter.

Figure 5 shows the relationship between average

temperature and flow velocity. With the increasing of inlet

flow velocity, the average die temperature decreases rapidly

and then decreases mildly. At the same time, the flow

resistance is increasing linearly, which will degrade the

reliability of the system. Consequently, we should design a

proper flow velocity.

Figure 6 shows the results for the average die temperature

of the LED array module with various total heat dissipation

powers at the flow velocity tm/s. For high power

consumption more than 500W, the microchannel cooler

module achieves the goal of average LED die junction

temperature below 80°C. The average die temperature is

increasing linearly with total heat dissipation powers.

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Figure 6. Average temperature vs. various total heat

dissipation power.

Based on above analysis, we then expect the introduction

of staggered internal fins the cooling module could increase

the heat dissipating and enhance the convective heat transfer.

In order to reduce temperature variation between upstream

and downstream dies, we use staggered fins by increasing the

hydraulic diameter of channels located in both the upstream

and downstream, and decreasing the hydraulic diameter of

channels in the center, as illustrated in Figure 7. With this

improved design, the total convective area in the central fins is

almost twice compared to the upstream fins. The resultingtemperature is more uniform , as shown in Figure 8. The

average die temperature decreases to 37. 0 °C and the

maximum variation between dies is only 1.8°C.

resistance compared to the straight fins. The hydraulic

diameter in the upstream is greater than that in the center, and

then the hydraulic diameter increased. With this special design,

the flow resistance changes gradually, which decreases the

whole flow resistance of the system compared with the

straight fins in microchannel cooler.

Figure 7. Staggered fins in microchannel cooler.

Temperature (deg C)

34 611M H Wi I ~~~~~~~~~~31 A08.......2 ........

Figure 8. Die temperature distribution of the LED array

with staggered fins in microchannel cooler

Compared to the straight fins in the front, the current

design of staggered fins can lower the average die temperature,

and also decrease the difference in temperature of individual

dies. For further analysis, we compute the variance of the

temperature of all dies, as illustrated in Figure 9, in which real

line show the temperature while broken line show variance.

The staggered fins in microchannel cooler can decrease the

variance of temperature of all dies while decreasing the

average die temperature. With the increasing of inlet flow

velocity, the variance is decreasing. When the velocity

approaches to 2m/s, the change of variance is mild.

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Figure 9. Average temperature and flow resistance with

staggered and straight fins in microchannel cooler.

Figure 10 shows the relationship between flow resistance

and flow velocity. With the same inlet flow velocity, the

staggered fins in microchannel cooler can decrease flow

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microchannel cooler.

Conclusions

This paper describes a process of applying a FEA technique

to simulate and analyses a light emitting diode (LED) array

integrated in microchannel cooler module. The cooling

module with different internal configurations, heat source

density, and heat dissipation capacity corresponding with

different flow velocity are investigated. From the analyses, the

special design of internal staggered fins in microchannel

cooler could reduce the average die temperature, the

difference in temperature and the flow resistance compared to

straight fins in microchannel cooler. With the flow velocity

increasing, the average die temperature decreases, but being

kept below target temperature approximately 80°C with a

high power consumption of 500W.

Acknowledgement

The author would like to thank Mr. Zetao Ma for his

technical discussion throughout the project.

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