In the industry, heatsinks have commonly been oriented on IC packages so that their plan outlines are edge-wise parallel to those of the package. However there are situations where a rotated orientation is preferable, wherein the plan outline of the package is not ‘aligned’ with that of the heatsink assembly — in other words a situation where the heatsink location/orientation remain unchanged while the package itself is rotated in-plane. Mechanical design considerations may drive the need for such a non-traditional orientation, since the rotated package is anticipated to have lower mechanical stress levels in the silicon than the non-rotated one under the same heatsink-induced clamping load. In this study we examine the impact of such package rotation(s) on both the junction temperature performance of CPU packages and the package-level clamp-load induced mechanical stresses. Results show that the stress reduction in the rotated package is in the range of 15% to 60%. The thermal analysis also demonstrates that the effects on the hot spot temperature with 45 degrees rotation is an increase of almost 2°C compared with the non-rotated die case. This increase in junction temperature is expected to be even higher with lower airflow as seen in typical computer systems. Thus it may be inferred that it is important to consider the effects of die rotation on package performances.

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