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Documentation

Thermal Effects in High Power LED Packaging

This validation case, for high power LED packaging with one heat sink, aims of to validate the following parameters:

  • Heat transfer solver
  • Multiple materials and contact heat transfer

The simulation results of SimScale were compared to the theoretical results presented in [Adam]\(^1\).

Geometry

The typical geometry used for the case is as follows:

geometry model validation case thermal high power led packaging
Figure 1. Main dimensions of the quarter model of the heat sink, with shown gap of 10 mm.

It represents a heat sink of dimensions 100 x 100 \(mm\) with 25 LEDs attached through Thermal Interface Material (TIM) patches of dimensions 7 x 7 x 0.1 \(mm\). Only one-quarter of the geometry is modeled to leverage the symmetry. Gaps of 10, 5, and 1 \(mm\) between LEDs were tested, with each model shown below for comparison:

geometry variations validation case thermal high power led packaging
Figure 2. Models with gaps between LEDs of 10, 5, and 1 mm (left to right).

Analysis Type and Mesh

Tool Type: Code Aster

Analysis Type: Heat transfer, linear, steady-state.

Mesh and Element Types:

CaseGap
\([mm]\)
Mesh TypeNumber of
Nodes
Element Type
A101st order tetrahedral828917Standard
B51st order tetrahedral824569Standard
C11st order tetrahedral824369Standard
Table 1. Mesh details.

The meshes were computed using the Tet-dominant algorithm with manual mesh sizing and local refinements for the TIM regions. The goal was to keep 2 elements across the thickness of the thin-walled parts, like the heat sink fins and TIM patches.

tetrahedral mesh validation case thermal high power led packaging
Figure 3. Typical tetrahedral mesh for 5 \(mm\) gap model.
tetrahedral mesh detail validation case thermal high power led packaging
Figure 4. Typical mesh refinement showing local details of the tetrahedral elements on a TIM body.

Simulation Setup for High Power LED Packaging

Material:

  • Aluminum Heat Sink:
    • Density \( \rho = \) 2700 \( kg/m^3 \)
    • Thermal conductivity \( \kappa = \) 202.40 \( W/(mK) \)
    • Specific heat \(C_p = \) 897 \( J/(kgK) \)
  • TIM (Thermal Interface Material):
    • Density \( \rho = \) 7870 \( kg/m^3 \)
    • Thermal conductivity \( \kappa = \) 0.22 \( W/(mK) \)
    • Specific heat \(C_p = \) 480 \( J/(kgK) \)

The thermal conductivity of the TIM was computed to achieve a total resistance of 9 \(K/W\), according to [INFINEON]\(^2\) with the relation:

$$ \kappa = \frac{t}{RA} $$

Where:

  • \(\kappa\) is the conductivity, \( W/(mK) \)
  • \(t\) is the thickness of the body, 0.1e-3 \(m\)
  • \(R\) is the thermal resistance, 9 \(K/W\)
  • \(A\) is the cross-sectional area, 4.9e-5 \(m^2\)

Boundary Conditions:

  • Heat Flux Load:
    • Surface heat fluxes of 1, 3, and 5 \(W\) per LED applied to the top surfaces of TIM bodies.
  • Convective Heat Flux:
    • Convective heat fluxes with heat transfer coefficients of 10, 25, 50, 75, and 100 \(W/(m^2K)\) with reference temperature of 300.15 \(K\) on all heat sink faces except the contact patches with TIM bodies and symmetry planes.

The following table relates the simulation runs for each case and the combinations of applied boundary conditions:

RunLoad
1 \([W]\)
Load
3 \([W]\)
Load
5 \([W]\)
Convective flux
10 \([\frac{W}{m^2K}]\)
Convective flux
25 \([\frac{W}{m^2K}]\)
Convective flux
50 \([\frac{W}{m^2K}]\)
Convective flux
75 \([\frac{W}{m^2K}]\)
Convective flux
100 \([\frac{W}{m^2K}]\)
1XX
2XX
3XX
4XX
5XX
6XX
7XX
8XX
9XX
10XX
11XX
12XX
13XX
14XX
Table 2. Simulation runs and their respective boundary conditions matrix.

Reference Solution

The reference solution is of the analytical type, as presented in [ADAM]\(^1\). It is given in terms of the temperature at the center point as a function of the thermal load and convection coefficients.

Result Comparison: High Power LED Packaging

Comparison of temperature at the mid point is shown for each case:

results comparison validation case thermal high power led packaging
Figure 5. Results comparison for Case A.
results comparison validation case thermal high power led packaging
Figure 6. Results comparison for Case B
results comparison validation case thermal high power led packaging
Figure 7. Results comparison for Case C

The deviation of the results with respect to [ADAM]\(^1\) in the cases of gap 5 mm and 1 mm can be attributed to a non-uniform temperature distribution at the base of the fins:

temperature plot validation case thermal high power led packaging
Figure 8. Temperature contour plot for Case C, Run 12: 1 mm gap, 5 W/LED load and Convection coefficient of 50.

Tutorial: Thermal Analysis of a Differential Casing

References

  • Christensen, Adam, and Samuel Graham. “Thermal effects in packaging high power light emitting diode arrays.” Applied Thermal Engineering 29.2 (2009): 364-371.
  • “Thermal Resistance Theory and Practice – Infineon” http://www.infineon.com/dgdl/smdpack.pdf?fileId=db3a304330f6860601311905ea1d4599

Note

If you still encounter problems validating you simulation, then please post the issue on our forum or contact us.

Last updated: August 5th, 2020

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