Common EMI sources in many systems are integrated circuits (ICs). ICs are also generators of thermal energy which must be efficiently removed via a heat sink. To enable efficient thermal energy flow a Thermal Interface Material (TIM) is used between the IC and a heat sink. Desirable qualities in a TIM are high thermal conductivity and softness to ensure good physical contact between the IC, heatsink and TIM. However it has been found that the electromagnetic properties of the TIM material can increase the EMI radiation leading to failure in regulatory compliance or deterioration in operating efficiency in the device. This has led many users to demand TIM material with a low dielectric constant (dk). Read this white paper to understand why the TIM dk could have an impact on EMI radiation.
Impact of TIM Dielectric Constant on EMI Radiation
Common EMI sources in many systems are integrated circuits (ICs). ICs are also generators of thermal energy which must be effi-ciently removed via a heat sink. To enable efficient thermal energy flow a Thermal Interface Material (TIM) is used between the IC and a heat sink. Desirable qualities in a TIM are high thermal conductivity and softness to ensure good physical contact between the IC, heatsink and TIM. However it has been found that the electromagnetic properties of the TIM material can increase theEMI radiation leading to failure in regulatory compliance or deterioration in operating efficiency in the device. This has led manyusers to demand TIM material with a low dielectric constant (dk). This paper will investigate why the TIM dk could have an impact on EMI radiation.
Radiation from an IC
Integrated circuits contain multiple current paths that will generate radiated electromagnetic energy. The inner workings of an actual IC are far too complicated to model in terms of EM radiation. Currents are not constant and will differ based on the ICs operating mode. Therefore to model the IC we need to make some simplifying assumptions.
Source types
A term for any object or component that radiates electromagnetic energy is an antenna. Depending on their size, shape, material properties, different antennas will radiate in different ways. Some will radiate very well, some not so well. For our purposes there are only two basic categories of antennas, linear and circular. In a linear antenna, the conductive portion is a straight line similar to the radio antenna on your car. The current is constrained by the ends of the antenna. In a circular antenna, the current isnot con-strained and freely propagates out from the energy source then returns to the energy source. We have found that the best model to represent radiation from an IC is a circular (loop) source mounted vertically on a printed circuit board (PCB). The analysisin this paper regarding the impact of the dielectric constant would be very similar for any type of source. The model shown below represents a vertical loop source realized on a PCB, the current path goes from the source at left up one conductive post then across the horizontal conductive portion then down the second conductive post and returns to the source viathe conductive ground plane. In our model the antenna height is 2mm and the antenna length (horizontal portion) will vary.
