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.
Non-conductive paste (NCP) formulation for chip-on-board (CoB) flip chip packages should contain a large amount of inorganic filler to achieve the low CTE (coefficient of thermal expansion) of cured joint and resultantly the reliability of overall packages. As the interconnection pitch decreases, the filler particle size should also be decreased because the particles trapped between bump and pad at the joint may deteriorate the joint reliability. In this study, effects of submicron-sized silica filler additions on the uncured and cured properties of NCP formulation and the void formation behavior at the flip chip joint were investigated. Two kinds of spherical fused silica fillers having particle size of 1 ㎛ and 100 nm were used. The abrupt increase point of viscosity of NCP formulation was shorten in the finer silica addition due to the increased probability of agglomeration and collision of particles. It resulted in the maximum filler content of 60 wt% and 30 wt% for 1 um and 100 nm silica, respectively. The addition of 100 nm silica showed lower effect on the CTE reduction and increased void formation at the flip chip joint compared to 1 ㎛ silica. It was considered that the defects at the interface between epoxy resin and silica filler originated from the surface contaminants, such as moisture and adsorbed gases, of silica particles was mainly responsible for the bad effects of the finer silica particles on CTE reduction and void formation behaviors.
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