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		Finite-element study of a board & heat-sink assembly produced by Milper

Company Business:
Engineering consultant for equipment (computers and peripherals) used in high-tech and defense under rugged conditions.

Analysis Goals::
To calculate the mechanical response of the board & heat-sink assembly under various dynamic loads. To calculate the natural frequencies and mode shapes of the assembly. To calculate displacements and stresses under random vibration (PSD), constant acceleration and shock loads according to Mil-SPEC flight requirements.


Simulation Details: The geometry of the model was generated in ABAQUS/CAE, a commercial FE preprocessor, using data supplied by MILPER. The geometry is shown in Figure 1, which included heat sink, PCB, electronic components and connector. The geometry was modeled with Solid Elements; the finite-element mesh is shown in Figure 2.	Figure 1	Figure 2
The electronic components were modeled so that their inertia would be taken into account and transferred correctly to the rest of the model. The heat sink and PCB were modeled as bonded at their interface, to account for the physical glue layer existing between them. The connector was connected to the board and heat sink by two screws, shown in Figure 3.	Figure 3
The following analyses were performed on the FE model: 1. Modal analysis: to extract natural frequencies and mode shapes. 2. Random vibration analysis: to calculate 1-Sigma displacements and stresses developing under an acceleration power spectral density (done for 3-axes). Figure 4 shows the PSD curve applied. 3. Transient analysis: to calculate the structure's response to shock load (in 6 different directions). 4. Static analysis: to calculate model response to constant acceleration on five axes.	Figure 4
Results: Two resulting mode shapes are depicted in Figures 5-6. (For the sake of clarity, only the heat sink and PCB are shown.) The white dotted curve is the undeformed configuration.	Figure 5	Figure 6
Sigma-1 displacement perpendicular to the PCB (Y axis) is shown in Figure 7 for Y PSD excitation direction, where the largest displacement occurs. The displacement field (in direction Y) for the Saw-shock loading in Y is shown in Figure 8.	Figure 7	Figure 8

Benefits:
Using FEA in the preliminary design stage showed that no natural frequency of the analyzed assembly coincides with the known operating frequencies of the structure connected to it. Another benefit of the numerical study was the ability to determine the force that acts on the interface between the electronic components and the PCB under various dynamic loads. This value was then used to verify that no tearing of electronic components is likely to occur.