The effect of the angle of attack on the thermal resistance measurement

As the requirements and the demand on electronic packages increase, the thermal performance becomes one of the key considerations for electronic packages, especially for high frequency and high power applications. Thermal resistance, known as theta-ja in the electronic industry, is the measurement used to quantify the thermal performance of a package design. Therefore, the specification of the theta-ja measurement is critical and essential in this concern. In the JEDEC and the SEMI standards, the orientation of the testing board with package under testing is specified to be vertical. The board is subjected to an airflow within a wind tunnel, with the corresponding angle of attack prescribed to be zero degree. However, in practice, the angle of attack may not be zero, as it could vary due to the back-slash of the connection fitting or the misalignment of the installation. Previous numerical studies have shown that the effect of this variation in angle increases the thermal resistance. In the present study, it is shown that while the thermal resistance generally increases with the angle of attack, the minimum resistance does not correspond to the zero angle of attack. That is, the minimum resistance corresponds to an angle value slightly greater than zero. This result was confirmed experimentally. The change of airflow velocity on the surfaces was observed from numerical simulation model. The present results confirm the importance of fixing the angle of attack, which leads to the necessity of establishing a standard to ascertain this angle.