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|ELECTRODEPOSITED COPPER-GRAPHITE AND COPPER-GRAPHENE COMPOSITES FOR LOW-CTE INTEGRATED THERMAL STRUCTURES|
|Keywords: Low-CTE Copper composites, Electrodeposition, High-power packages|
|High-temperature and high-power packages range from lead frames to direct-bonded copper or insulated metal substrates, with increasing trend towards integration of gate drivers, logic and passive components. These invariably require thick conducting structures for thermal vias and heat-spreaders. Addressing challenging issues such as substrate warpage, interfacial and interconnection stresses are becoming increasingly important in these packages. The crux of reliability issues arise from coefficient of thermal expansion (CTE) mismatch between different components of the system. Copper has a very high CTE compared to the other components, and induces stresses between interfaces of adjoining materials. This paper addresses this crucial problem by using innovative low cost fabrication procedures to reduce the CTE of copper without degrading its thermal conductivity. Low-CTE thermal and electrical conductors are available as metal matrix composites with material systems such as Al-SiC, Cu-graphite and Al-graphite. [1,2,3] These structures are formed through metal infiltration into porous preforms as thick substrates, and cannot be easily integrated into thin packages. To address this challenge, this paper demonstrates microfabricated low-CTE Cu-graphite and Cu-graphene composites using pattern-plating approaches. Two electrodeposition approaches were developed for the fabrication of these composite coatings. The first approach involves co-electrodeposition of copper and graphite or graphene. The second approach is based on a sequential process of electrophoretic deposition of graphite to form a porous preform, into which copper is electroplated as a second step to form smooth uniform coatings of copper-carbon composite. A surfactant cetyltrimethylammonium (CTAB) was used in both processes to improve the positive surface charge of the carbon particles and improve their electrophoresis. The key parameters such as current density and particle concentration affecting both techniques are studied. Co electrodeposition was carried out using copper sulphate solution, an ionic electrolyte with additives to improve the planarity of the plated surface, loaded with graphite particles and CTAB of different concentrations. Plating rates were optimized to improve the mechanical integrity of the deposit. The thickness of the coatings was controlled based on time and current density. Surfaces were characterized using Scanning electron microscopy (SEM) to visualize coating quality and uniformity along with energy dispersive spectroscopy (EDX) to determine volume percent of carbon in the coating. Initial results show upto 30 vol.% incorporation of carbon in the composites without inducting porosity. Higher volume loadings was achieved, however, with the introduction of porosity that can be eliminated by a subsequent sintering step. Analytical models predict that the composites retain most of the thermal conductivity, while attaining reduced CTE. The final paper will report the measured thermomechanical properties of the composite film, and reduction in warpage and stresses during thermal cycling using shadow-moire measurements and micro Raman spectroscopy.|
Packaging Research Center, Georgia Institute of Technology