Measurement of thermal resistance of TIMs, heat sinks and interfaces in thermal management systems for IC packages

With the trend towards ever-powerful chips and evermore-intense heat fluxes, the thermal resistance of interfaces is becoming an important consideration. Thermal interface materials (TIMs) are playing a critical role in the thermal management of advanced chip packages. While the thermal conductivity of TIMs may be relatively easily measured in the laboratory, it is often not easy to establish the value of the thermal resistance of the assembled TIM in an actual package. This is because the thermal resistance of the TIM in an actual package depends on several parameters including bond line thickness, presence of voids, uniformity of bond line and quality of adhesion between TIM and contact surfaces. Hence, it is imperative to establish an easy-to-use and accurate method for evaluating the thermal resistance of TIMs, heat sinks and interfaces in thermal management systems for IC packages. This paper describes the use of a thermal transient method for the characterization of the TIMs of two different functional chip packages, which incorporate integrated heat spreaders. The overall junction to ambient thermal resistance of the packages was also determined. Three different tests were conducted on each package - natural convection test, natural convection with a passive heat sink and forced convection with an active heat sink. In the tests, a step change in heat input to the chip was applied and the resulting thermal transient response of the package was measured every microsecond until equilibrium was reached. The thermal transient profile was then analysed to obtain the structured functions, thermal resistance of the TIM and the junction-to-ambient thermal resistance. The paper also describes the thermal resistance, and hence heat transfer coefficients of surfaces, which are cooled by spray cooling. It is concluded that a quick transient testing after the completion of packaging can serve as a reliability tool to ensure if all the parts of a package are assembled properly and will also provide values of parameters such as thermal resistance of TIMs and junction-ambient thermal resistance.