EMI considerations in selecting heat-sink-thermal-gasket materials

Specific design criteria are proposed to mitigate radiated emissions from a resonant enclosure excited by a heat sink acting as a microstrip patch ant enna source. In this particular application, the EMI mechanism is assumed t o be due to coupling from the dominant TM010z mode to one or more resonant modes associated with the enclosure dimensions. The enclosure is then presu med to radiate, at the enclosure resonance frequencies, through one or more apertures, slots, or seams. The EMI-reduction strategy consists of shiftin g the resonant frequency of the dominant-patch antenna mode by dielectrical ly loading the patch antenna with thermal-gasket material having a specifie d electric permittivity. Specific formulas and graphs will be presented sho wing how to select the electric permittivity of the thermal-gasket material in order to obtain a given frequency shift. A comparison of experimental m easurements with the predictions of the design criteria indicates that freq uency shifts of up to approximately three times the bandwidth of the patch resonance can be predicted with reasonable accuracy. In at least two differ ent commercial products that we are aware of, changing the electrically ins ulating heat sink gasket materials has solved specific radiated EMI problem s.