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

Abstract

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 materials 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.

Filler-depletion layer adjacent to interface impacts performance of thermal interface material

“…Therefore, it is likely that the thermally conductive fillers are not functioning in the region due to the presence of the filler depletion layer. Such filler-depletion layer (FDL) has been observed for a curing TIM 14,15 but the current result suggests that it can also occur for non-curing TIM even without phase transition. Figure 3(a)-3(c) also shows that the TDTR signal obtained for pure Silicone oil agrees well with the heat conduction model with the bulk thermal conductivity of Silicone (0.15 Wm…”

“…subtracting the separately measured thermal resistance of suspended TIM), they have found that ITR can be significant and the values range from 2 mm 2 KW −1 to 20 mm 2 KW −1 . Furthermore, Gowda et al 15 observed the interface between the cured TIM and Al by computed tomography, acoustic microscopy, and scanning electron microscopy, and showed the existence of delamination and filler-depleted/resin-rich region at the interface.…”

Capacitive sensing of local bond layer thickness and coverage in thermal interface materials

“…Many techniques, such as C-Mode Scanning Acoustic Microscopy (CSAM) [5,6], radiography [7], infrared microscopy [7], scanning electron microscopy [8], and transient thermal tomography [9], have been employed in an effort to characterize the physical integrity of TIM bond layers while under stress, or after being stressed.…”

Void Detection in Dielectric Films Using a Floating Network of Substrate-Embedded Electrodes

“…Various nonintrusive means have been used to characterize the quality of TIM layers, such as acoustic microscopy [2,3] and Xray methods [3]. An infrared microscopy technique to characterize a TIM layer through a silicon wafer was demonstrated by Hu et al [4].…”

“…Thermography techniques [5,6] may be used to characterize the transient thermal response of an interface, and thus infer the existence and location of voids. High-resolution methods, such as scanning electron microscopy [3], have also been applied to TIM layers. Although the above techniques provide microscale or nanoscale resolution, the necessary equipment, sample preparation, and expertise required for these methods may be prohibitive in many industrial applications.…”