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Lossy Heatsink Termination for Reduction of Radiated Emissions

IP.com Disclosure Number: IPCOM000126973D
Original Publication Date: 2005-Aug-16
Included in the Prior Art Database: 2005-Aug-16
Document File: 3 page(s) / 56K

Publishing Venue

IBM

Abstract

Disclosed is a method of reducing radiated RF (radio frequency) emissions from heatsinks by using lossy terminations. The heatsink is connected to the reference plane of a circuit card through resistive elements to reduce heatsink resonances and improve the EMC (electromagnetic compatibility) performance.

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Lossy Heatsink Termination for Reduction of Radiated Emissions

    Background: With higher operating frequencies of computing devices now well into the gigahertz (GHz) range, controlling heatsink radiation is a major obstacle in maintaining compliance with domestic and worldwide electromagnetic compatibility (EMC) standards. These standards place legal limits on the levels of radio frequency (RF) noise that are emitted from digital devices.

    A common source of electromagnetic noise is the silicon die which is often comparatively small compared to the wavelength of the operating frequencies, and is thus an inefficient radiator; however, once a heatsink is placed on top of a chip, there is efficient capacitive coupling between the two due to the close proximity, and the heatsink becomes a very efficient radiator. Presently, there are only a few well-known solutions to control this noise. One typical solution is to implement a multipoint grounding scheme that connects the heatsink directly to the ground plane at several locations around the perimeter of the heatsink (anywhere from 2 to 16 point grounding is typical, with more grounding points required at higher frequencies). Implementation of this solution is often costly in order to obtain a sufficient number of grounding points to be effective. Other solutions typically involve adding additional shielding to the enclosure itself to contain the noise within the enclosure, which is often more costly than reducing the noise closer to the source.

    Summary: This invention describes a new method of reducing heatsink emissions. Heatsinks are connected through lossy (resistive) terminations to reduce resonances and control electromagnetic interference (EMI). This termination solution permits the use of fewer heatsink clips than direct grounding, or alternatively allows for even lower radiated emissions levels with the same number of clips used in direct grounding methods. A basic diagram of this concept is shown in figure 1. The number of termination points may vary, and additional EMI improvement is achieved by adding more resistive terminations around the perimeter of the heatsink.

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Figure 1. Basic Diagram of Lossy Termination for Heatsinks

    Description: Metal clips are added to make electrical contact between the outer edges of the he...