The Computational Fluid Dynamics (CFD) modeling of light emitting diode (LED) components has become increasingly more important as it is now being applied to the design process. This paper compares the results of Avago Technologies’ high power LED package (ASMT-MX00 ) on a metal core printed circuit board (MCPCB) and double layer FR4 substrate with heat sink to experimental data. After the comparison discussion, a thermal modeling technique for LED packages with heat sinks is noted. The results are quite impressive, indicating that this technique can be used for LED system levels.
Nomenclature
RJA
Junction to ambient thermal resistance (°C/W)
RJB
Junction to solder point thermal resistance (°C/W)
RBA
Solder point to ambient thermal resistance (°C/W)
TJ
Junction temperature (°C)
TB
Solder point temperature (°C)
MCPCB
Metal core printed circuit board
CFD
Computational Fluid Dynamics
I. Introduction
The prediction of the thermal performance of LEDs is becoming a necessity in reducing the time it takes to bring products to market. However, with increasing heat flux and package density, heat dissipation of the LED package module is becoming a challenge; therefore, the thermal analysis and design of the module is becoming even more critical. CFD simulation is a widely used method for thermal analysis of electronic products in the early design stage. CFD is concerned with the numerical simulation of fluid flow, heat transfer, and other related processes such as radiation.
This paper presents work done to create a high power LED package on an MCPCB with heat sink and a double layer FR4 with heat sink. First, a detailed model of an LED package-on-substrate is created. Then a heat sink is created on the bottom of the LED package. Finally, this simulation data is compared to experimental data.
II. Thermal modeling technique
The LED package, an MCPCB, double layer FR4, and heat sink were modeled using Flotherm, a CFD tool from Flomeric.
A. Model description
Two models of heat sinks have been developed: the detailed model and the compact model. The intent is to compare the error percentage between these two models. The detailed geometry parameters of the LED package and thermal conductivity of the package materials are shown in Table 1.
A schematic of the front view and layout of the LED package is shown in Figure 1a and 1b. The solder paste is filled in, between the package and the substrate. When the package reaches the maximum power of 1.3 W, the standard natural and forced convection cooling of air cannot maintain the junction temperature within the acceptable range. The additional heat sink helps meet the requirements. In order to mount the heat sink onto the LED, adhesion thermal tape is attached to the backside of the heat sink, and the heat sink is placed on the bottom of the LED substrate.
Figure 1a: Front and side views of the Avago Technologies’ Power LED Package (ASMT-MX00).
Figure 1b: LED package on substrate with heat sink.
No.
Component
Material
Thermal Conductivity (W/m.K)
Dimension
1
Lead frame
Cu
364.25
Refer to above
2
Reflector
PA9T
0.2
8.5 mm x 8.5 mm x 3.3 mm
3
Chip
Sapphire
23.1
Junction about 0.11mm from bottom
4
Encapsulant
Silicone
0.2
—
5
PCB base
Aluminum-Double Layer FR4
2000.3
37 mm x 26 mm x 1.6 mm in thickness
6
Metallization
Cu
385
35 μm in thickness
7
Dielectric layer
Alox
8
75 μm in thickness
8
Solder paste
SnPb37
50.9
25 μm in thickness
9
Thermal tape
—
2
Thickness of 0.125 mm
10
Heat sink
Aluminum
200
110 fins, base 23 mm x 23 mm x 1.5 mm Fin height 8 mm, thickness 0.8 mm, Fin pitch size 1 mm
Table 1: Construction details of the LED package with heat sink and thermal conductivity of the package materials.
B. Grid and boundary conditions
For CFD analysis, the following properties are assumed:
- Three dimensional
- Steady state
- Airflow velocity of 0.2 m/s
- Air properties are constant
- Ambient temperature is 25°C
- Computational domain is 305 mm x 305 mm x 305 mm
- Heat is dissipated through natural convection and conduction
- Radiation effect is neglected because radiation effects are roughly 2 to 3%
The total grid cells for the LED package-on-substrate, with both the detailed heat sink model and the compact heat sink model, are near 600,000 and 150,000, respectively. For the grid cells setup, at least 3 cells are recommended between the fins of the heat sink. (This is the default setting of Flotherm.)
III. Results
A. Sample package configurations
The LED package is mounted on an MCPCB and double layer FR4. It has dimensions of 32 mm x 27 mm x 1.6 mm. The heat sink is a typical finned-type with 110 fins, and a base made of extruded aluminum is attached to the back of the MCPCB and double layer FR4 with thermal tape. The package is driven at 1.2W; the temperature of the solder point (TP) is measured at the heat slug of the package. Based on this data, the thermal resistance from the solder point to the ambient, RBA, can be calculated.
B. Numerical vs. experimental
Measurement data comparisons of the detailed model heat sink and the compact model heat sink are shown in Table 2. The visualization simulation results are shown in Figures 2a and 2b. As the approximation gets coarser, the agreement with real data becomes poorer. However, the percentage of error is acceptable for industrial applications. The fact that the simulated temperature is higher than the measured temperature indicates that the numerical model fails to account for some cooling phenomena. One source of cooling, which was ignored, was radiation. This difference could be due to measurement accuracy
Measured RBA
(°C/W)
Simulated RBA
(°C/W)
Percentage Difference (%)
LED package on MCPCB with detailed model heat sink
25
23
8
LED package on MCPCB with compact model heat sink
—
27
8
LED package on FR4 with detailed model heat sink
37
35
8
LED package on FR4 with compact model heat sink
—
32
13.5
Table 2: Simulated results vs. measured results.
Figure 2a: Visualization result of LED package on MCPCB with detailed heat sink model.
Figure 2b: Visualization result of LED package on FR4 with compact heat sink model.
IV. Thermal design considerations
If the LED package has a design constraint of improving the package thermal performance, then the following approaches can help to reduce the temperature on the substrate and the junction temperature of the LED.
- Furnishing the backside with aluminum plates or heat sink
- Using separate PCBs for the driver circuitry and LEDs
- Using higher thermal conductive material for the dielectric layer
- Using fans to remove the heated air and to augment convection cooling
V. Conclusion
This study showed that the CFD modeling technique can be used for simulating the LED package-on-substrate with heat sink. The results clearly show that the detailed and compact heat sink models provide results that closely match the actual measurements; however, the detailed heat sink model can be more time consuming. The compact heat sink model is good for performing a fast analysis. The percentage of error is acceptable for industrial applications, while saving time. The CFD is a good tool to assist in the design of the power LED for the real application.
Acknowledgment
The authors would like to thank Pang Siew It for providing support and information, and Hwang Yi Feng for providing the measurement data.
References
- Aizar Abdul Karim, “Thermal Modeling of Light Emitting Diode—Simulation and Verification,” SID R&D, Avago Technologies.
- Sridhar Narasimhan, Rajesh Nair and Avram Bar- Cohen, “Thermal Compact Modeling of Parallel Plate Heat Sinks,” IEEE Trans. Comp, Vol.26, No.1, March 2003.
- Ronald L. Linton and Dereje Agonafer, “Coarse and Detailed CFD Modeling of a Finned Heat Sink,” IEEE Trans. on Comp, Packaging and Manufacturing Tech— Part A, Vol.18, No.3, September 1995.
- Yingjun Cheng, Gaowei Xu, Dapeng Zhu, Wenjie Zhu, and Le Luo, “Thermal Analysis for Indirect Liquid Cooled MultiChip Module Using Computational Fluid Dynamic Simulation and Response Surface Methodology,” IEEE Trans. on Comp. and Packaging Tech, Vol.29, No 1, March 2006.
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