Study of heatsink grounding schemes for GHz microprocessors

As the operating frequency of processors approaches 1 GHz and beyond, the heatsink dimensions are no longer small when compared to the wavelength. In fact, the heatsinks that are used at present tend to resonate strongly around the 1 GHz frequency. In order to keep the emissions from the heatsink in check, it is necessary to ground the currents that are excited on the heatsink. Since the electrical dimensions of the heatsinks are no longer negligible, a distributed grounding scheme with multiple grounding points must be used. By approximating the ungrounded heatsink structure as a cavity with magnetic side walls, we can obtain analytic expressions for the resonant frequencies. Simulations are carried out on the Ansoft High Frequency Structure Simulator (HFSS) to model the ungrounded heatsink structure. The simulation results show good agreement with the analytical results, boosting our confidence in the simulation tool. Two different grounding schemes are investigated to study their relative efficiency in suppressing the emissions. The first grounding scheme uses four grounding posts at the four corners of the heatsink. This scheme was observed to be effective in suppressing emissions up to 1.5 GHz. The second grounding scheme utilized eight grounding posts evenly spaced around the periphery of the heatsink. This scheme was observed to be effective in suppressing emissions up to 6 GHz.