Printed Circuit Board Design and Layout Questions

3. Can EMI be attenuated by using lossy thermal gaskets between the IC and the heatsink?

The short answer to this question is "No." The space between the IC and the heatsink is not a good place to add loss. If the material has too much resistance, the interface between the IC and the heatsink is capacitive and provides no attenuation at resonances. If the material has too little resistance, it looks basically like an extension of the heatsink. The interface is still capacitive and there is no attenuation at resonance (although the resonant frequency may be shifted slightly). A careful analysis [1] reveals that there is no value of resistance that can be designed into a thin layer of gasket material that will result in a significant level of attenuation. This also applies to materials designed to lie on top of an IC without a heatsink. Grounding the heatsink to the structure on the opposite side of the IC (e.g. a circuit board ground plane) is generally a better strategy.

References

[1] Y. Huang, J. Butler, M. de Sorgo, R. DuBroff, T. Hubing, J. Drewniak and T. Van Doren, “EMI considerations in selecting heat-sink-thermal-gasket materials,” IEEE Transactions on Electromagnetic Compatibility, vol. 43, no. 3, Aug. 2001, pp. 254-260.