Carbon based Thermal Interface Material for high performance cooling applications

Currently, a variety of Thermal Interface Materials (TIMs) are required to meet specific requirements for various products with no single TIM meeting the needs for all CPU market segments. Surface treatments, reflow temperatures, mechanical load, performance targets all play a role in choosing a specific TIM. Based on the above multitude of conditions, it is desirable to find a universal TIM that would enable all the products across all markets. To meet these requirements and based on legacy data, efforts should focus on TIMs that (1) have high bulk conductivity (i.e. > 20 W/mK) and (2) do not have hard or brittle contact surfaces and 3) are reliable over the years of normal usage. The focus of this paper is on a novel material called Vertical Carbon TIM (VCTIM). This composite TIM consists of a soft polymer matrix and carbon flakes aligned in the z-direction (i.e. the direction of the heat flow in the semiconductor package). Previous efforts of graphite/graphene and different matrix material were tried by academia and industry [1]-[5], but this TIM, is one of the first non-metal solid TIMs to achieve high z-bulk thermal conductivity (i.e. > 30 W/mK) and low contact resistance in the TIM tester [6]. These fundamental thermal properties are showing promise in meeting the requirements listed above. The first part of the paper will focus on presenting the fundamental thermal/mechanical material properties which have a significant effect on overall package thermal performance. Through separating the bulk thermal resistance from the contact resistance, we are better able to explain the degradation of VCTIM thermal performance in a package. The empirical data gained through this characterization is then compared to a published physics based model of thermal contact resistance [6], [7]. The model predicts a contact resistance that scales with contact area which in turn is a function of surface height variation (or roughness), hardness, and pressure. The interfacia- thermal contact resistance of VCTIM is found to fit well with this model. Additionally, it is found that changes in thermal performance of VCTIM upon exposure to reliability conditions can be explained within the context of this model. The second part will show thermal data which include the effect of different reliability stresses, including bake, temperature and humidity exposure, and temperature cycling. The fundamental explanation of this behavior, which relates the material properties to the mechanical boundary conditions, will be included with the focus on delamination at the interface and increased hardness during bake or temperature and humidity exposure. Contrary to polymeric TIMs, [9] it will be shown that VCTIM has small thermal degradation through thermal degradation in bake at 125°C. We will also show the effect of matrix material composition changes on significantly eliminating degradation through temperature and humidity exposure (highly accelerated stress testing (HAST) 110°C/85% relative humidity). Other factors examined in the paper include VCTIM thickness, die thickness, and IHS thickness. Also it will be shown that increasing TIM thickness improves reliability and compressibility performance. The material challenges like limited compressibility and the plastic behavior will also be discussed. The final paper section will provide TIM2 package build evaluation data.