Thermal Effects in High Power LED Packaging

Overview

The aim of this test case is to validate the following functions:

Heat transfer solvers

The simulation results of SimScale were compared to the analytical results presented in [Adam]. The mesh used was created using first order tetrahedral with local refinements meshing algorithm on the SimScale platform.

Import validation project into workspace

Geometry

The pitch is the Light Emitting Diodes (LEDs) separation distance between each other. Total of 25 LEDs with dimension and thickness of 7 mm x 7 mm and 100 μm respectively mounted on a heat sink of 10 cm x 10 cm were simulated. The pitch of 1 cm, 5 mm and 1 mm were tested. Due to symmetry, only quarter of the model was taken for the analysis. The geometries used for the analysis (with different LED pitches) are shown in the figure below:

Geometries used in the analysis with 1 cm, 5 mm and 1 mm of separation distance between LEDs

Analysis type and Domain

Tool Type : CalculiX, Code_Aster

Analysis Type : Heat transfer

Mesh and Element types :

Information of different tested cases¶
Case	Pitch	Mesh algorithm	No. of nodes	No. of 3D elements	Solver
A	1 cm	Tetrahedral with local refinements	766903	2826272	CalculiX
B	1 cm	Tetrahedral with local refinements	766904	2826273	Code_Aster
C	5 mm	Tetrahedral with local refinements	764085	2814528	CalculiX
D	5 mm	Tetrahedral with local refinements	764086	2814529	Code_Aster
E	1 mm	Tetrahedral with local refinements	760497	2799922	CalculiX
F	1 mm	Tetrahedral with local refinements	760498	2799923	Code_Aster

Mesh used for all the cases are shown below. In order to get better results, at least 2 elements were placed over the thickness of TIM (Thermal Interface Material) layer.

Simulation Setup

Material:

Aluminum: isotropic: $ρ$
TIM (Thermal Interface Material): isotropic: $ρ$

Initial Condition:

Temperature = 300.15 K

Heat Flux Loads:

Each case contains total of 14 simulation runs with different configuration setups. The applied surface heat flux and convective heat flux boundary condition for all the simulation runs are given below:

Surface heat flux:

Surface heat flux of 1 W, 3 W and 5 W per LED was applied on all the red highlighted surface of different LED packages as shown in figure above under ‘Geometry’ section. The table below shows the run cases with applied surface heat flux:

Applied surface heat flux for all the cases¶
Case	Surface heat flux [W/m²]
1 – 5	20408.2 (1 W/LED)
6 – 10	61224.5 (3 W/LED)
11 – 14	102040.8 (5 W/LED)

Convective heat flux:

Convective heat flux of 10 W/(m²K), 25 W/(m²K), 50 W/(m²K), 75 W/(m²K) and 100 W/(m²K) was applied to all the heat sink surfaces accept the surfaces along symmetry plane and surfaces in contact with TIM. The table below shows the run cases with applied convective heat flux:

Applied convective heat flux for all the cases¶
Case	Convective heat flux [W/(m²K)]	Reference temperature [K]
1 & 6	10	300.15
2, 7 & 11	25	300.15
3, 8 & 12	50	300.15
4, 9 & 13	75	300.15
5, 10 & 14	100	300.15

Reference Solution

Since the overall resistance of 9 K/W was used for TIM (Thermal Interface Material) in [Adam], the resistance was converted to conductance in order to apply this value under material definition. The formulation used to convert this value is shown below: