Thermal Conductivity Measurement Method for the Thin Epoxy Adhesive Joint Layer

Epoxy adhesives, particularly for non-conductive pastes, are used in 3D chip-stack flip-chip packages to reinforce the mechanical strength of joints. Although the thickness of the adhesive layer is relatively small, its thermal conductivity is known to have a major effect on the heat dissipation behavior of chipstack packages. Because conventional thermal conductivity measurement methods such as the laser flash method are based on the bulk specimens having thicknesses greater than several mm, they are limited in their ability to measure the thermal conductivity of thin adhesive layers between silicon dies. In this study, a modified guarded hot-plate method is proposed using standard joint layer samples of known thermal conductivity, and the measurement results are compared with those of the laser flash method. Results showed that, based on a constant heat flux from heat source to heat sink, the temperature difference at both sides of the joint layers was proportional to the thermal resistivity of the joint layer materials. The thermal conductivity of the under-test joint layer could therefore be determined from the thermal conductivity spectrum of the known samples using a graphical method. Although the measured values by the modified guarded hot-plate method were slightly higher than those derived from the laser flash method due to the thickness effect, it was concluded that the modified guarded hot-plate method could be a practical method in measuring the thermal conductivity of thin adhesive joint layers.

Numerical Simulation of Thermal Diffusivity Measurements with the Laser-Flash-Method to Evaluate the Effective Property of Composite Materials

Laser Flash Method (LFM) is commonly used to measure the thermal diffusivity of homogeneous and isotropic materials, but it can be also applied to macroscopically inhomogeneous materials, such as composites. When composites present thermal anisotropy, as fiber-reinforced, LFM can be used to measure the effective thermal diffusivity (aeff) in the direction of heat flux. In the present work, the thermal behavior of composites during thermal diffusivity measurements with the LFM was simulated with a Finite Element Model (FEM) using commercial software. Three composite structures were considered: sandwich layered (layers arranged in series or parallel); fiber-reinforced composites; particle composite (spheres). Numerical data were processed through a non-linear least-square fitting (NL-LSF) to obtain the effective thermal diffusivity of the composite. This value has the meaning of "dynamic effective thermal diffusivity". Afterward, the effective thermal conductivity (?eff) is calculated from the dynamic effective thermal diffusivity, equivalent heat capacity and density of the composite. The results of this methodology are compared with the analytically calculated values of the same quantity, which assume the meaning of "static effective thermophysical property". The comparison of the dynamic and static property values is so related to the inhomogeneity of the samples, a deviation of the temperature vs time trend from the solution for the perfectly homogeneous sample gives information about the sample's lack of uniformity.