Improved grounding method for heat sinks of high speed processors

The clock speed of high performance microprocessors continues to increase at a rapid rate, recently passing the 1 GHz mark for the first time in volume products. This trend raises a concern for mitigation of the electromagnetic interference (EMI) that could result from radiation of high frequency signals coupled to the processor's heat sink, which can act as an antenna if not properly grounded. However, the grounding methods typically used are implemented using mechanical structures that take longer to change than the time to the next increase in processor speed. This results in a number of problems: products shipped with a grounding scheme suitable for older, slower processors, the need to add processor speed-dependent shielding mechanisms to help the product meet FCC or other radiation limits, increased product cycle time to accommodate needed design changes, etc. This paper will present and discuss an improved grounding method for high speed microprocessors that attempts to decouple the design of the grounding method from the processor clock speed, allowing the product developer to concentrate on the function of the product, not on devising last-minute "fixes" to EMI problems found in the late stages of product development. A comparison of EMI test results between this method and more commonly used techniques will be presented. Additionally, some unexpected findings of these EMI studies will be discussed.