Florian Johannes Schattenmann

Micron and Submicron-Scale Characterization of Interfaces in Thermal Interface Material Systems

One of the key challenges in the thermal management of electronic packages are interfaces, such as those between the chip and heat spreader and the interface between a heat spreader and heat sink or cold plate. Typically, thermal interfaces are filled with mate-rials such as thermal adhesives and greases. Interface materials reduce the contact resistance between the mating heat generating and heat sinking units by filling voids and grooves created by the nonsmooth surface topography of the mating surfaces, thus improving surface contact and the conduction of heat across the interface. However, micron and submicron voids and delaminations still exist at the interface between the interface material and the surfaces of the heat spreader and semiconductor device. In addition, a thermal interface material (TIM) may form a filler-depleted and resin-rich region at the interfaces. These defects, though at a small length scale, can significantly deteriorate the heat dissipation ability of a system consisting of a TIM between a heat generating surface and a heat dissipating surface. The characterization of a freestanding sample of TIM does not provide a complete understanding of its heat transfer, mechanical, and interfacial behavior. However, system-level characterization of a TIM system, which includes its freestanding behavior and its interfacial behavior, provides a more accurate understanding. While, measurement of system-level thermal resistance provides an accurate representation of the system performance of a TIM, it does not provide information regarding the physical behavior of the TIM at the interfaces. This knowledge is valuable in engineering interface materials and in developing assembly process parameters for enhanced system-level thermal performance. Characterization of an interface material between a silicon device and a metal heat spreader can be accomplished via several techniques. In this research, high-magnification radiography with computed tomography, acoustic microscopy, and scanning electron microscopy were used to characterize various TIM systems. The results of these characterization studies are presented in this paper. System-level thermal performance results are compared to physical characterization results.

Comparison of the adhesion strength of epoxy and silicone based thermal interface materials

In electronic packages, strong adhesion between the backside of the silicon die and the heat spreader is crucial in maintaining a robust interface that facilitates heat transfer. The heat spreader is normally attached to the backside of the die with a thermal interface material. This study monitors the adhesion performance of epoxy and siliconebased thermal interface materials, used to attach a heat spreader to the Si die. The adhesion strength was measured by means of a die shear test. Test vehicles were subjected to thermal shock, temperatureihumidity, autoclave testing and reflow sensitivity (EDEC MSL-1) testing, through the course of which the adhesion strength was monitored. Initially, the silicone-based materials had much lower adhesion strengths (400 600 psi) compared to the epoxy-based materials (> 1500 psi). However, while the adhesion of the siliconebased thermal interface materials remained relatively constant throughout the various tests, the adhesion of the epoxy-based materials dropped drastically after thermal shock, temperatureihumidity, and autoclave testing. Consequently, after 1000 cycles of thermal shock ( 5 W to +12S°C) and 1000 hours of temperatureihumidity testing (85OC/85%RH), the adhesion strength of the experimental silicone-based materials was comparable to the adhesion strength of the epoxy-based materials.