It is often difficult to place the device adjacent to the fan providing the airflow. The path of the airflow might traverse a length on the board before reaching the device, thus diminishing the actual airflow the device receives. The following figure shows a fan that is placed at the end of the board. The airflow at the FPGA is weaker than the airflow at the fan.
You must also consider blocked airflow. The following figure shows a device blocking the airflow from the FPGA, significantly reducing the airflow seen at the FPGA. The airflow from the fan also has to cool board components and other devices before reaching the FPGA.
If you are using a custom heat sink, you do not need to enter the airflow directly into the EPE spreadsheet but it is required to enter the θSA value for the heat sink with the knowledge of what the airflow is at the device. Most heat sinks have fins located above the heat sink to facilitate airflow. The following figure shows the FPGA with a heat sink.
When placing the heat sink on the FPGA, the direction of the fins must correspond with the direction of the airflow. A top view shows the correct orientation of the fins.
These considerations can influence the airflow at the device. When entering information into the EPE spreadsheet, you have to consider these implications to get an accurate airflow value at the FPGA.
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