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Silver oxalate: towards a new solder material for highly dissipative electronic assemblies
Keywords: thermal conductivity, packaging, silver oxalate
This study is carried out in the context of recent development of high-power devices that can increase power density by a decade: thermal dissipation at the first-level interfaces, where the flux is the highest, becomes the limiting factor in the design of this kind of assemblies. Currently used materials, such as gold-tin alloy, with thermal conductivity of 58 W/m.K, or conductive adhesives, with thermal conductivity up to a few W/m.K, do not offer sufficient properties. For these reasons, highly conductive metals such as Cu, Au, Ag and some of their alloys, are studied as possible alternatives for soldering applications [1]. Silver, in particular, is the best thermal conductor available after the diamond but cannot be used in its bulk form with low-pressure (below 10 MPa) and moderate temperature processes (<300°C) that are usually required. One of the driving ideas is to take benefit from the strong decrease of the melting temperature when the size of particles becomes lower than about 10 nm [2]. The preparation of silver nanoparticles suspensions or pastes thus attracted a great attention [3][4]. However, this kind of materials has several drawbacks, such as particle aggregation that require using organic coatings and potential health and security issues induced by nanoparticle handling. A new route to obtain a highly conductive silver solder at low temperature and pressure is to use suspensions or pastes made of micronic silver oxalates [5]. Indeed, such Ag2C2O4 salts can be decomposed into porous metallic silver under air, inert or reducing atmosphere, around 200°C [6][7]. During this reaction silver nanoparticles with a high propensity to sinter are created. The decomposition is highly exothermic (∆H close to - 122 kJ.mol-1) [7], bringing locally an additional energy which also favours the sintering process. To take full advantage of such precursors, it is better to prepare particles in an hydro-alcoholic medium enabling the creation of well-shaped and small particles that can be more easily dispersed in a liquid for suspension or paste manufacturing. The different steps of the transformation were investigated by Scanning Electron Microscopy on partially decomposed powder, highlighting the creation of nanometric silver on the surface of silver oxalate particles, starting from temperatures as low as 90°C. Due to their high reactivity, silver nanoparticles tended to quickly sinter, favoring the creation of bigger size nanoparticles, spherical or spike-shaped. Microtomy preparations revealed an internal channel structure that is likely to favour silver migration. Suspensions were prepared by dispersing the silver oxalate powder in pure ethylene glycol and samples were prepared by heating to 300°C. Thermal conductivity measurements performed by micro-Raman infra-red thermography on active assemblies showed a thermal conductivity around 100 W/m.K. The photothermal radiometry measurements made on passive samples gave a thermal conductivity up to 120 W/m.K. A new thermal performance study has been undertaken on optimised samples. A reliability study is currently in progress, to evaluate the solder compatibility with space requirements.
K. Kiryukhina, Ph. D. student
Toulouse Cedex 09, Midi-Pyrénées

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