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Electrical and Thermal Conductive Properties of High Packing Density Conductive Filler
Keywords: high packing density powder, erecirical conductivity, thermal conductivity
Due to the size reduction of mobile phones, the diameters of via holes for multilayer interconnection have became smaller such as 0.1 mm or less. This has created new demands on highly electrically conductive pastes. Also, in the IC chip die-bonding field there is requirements for higher thermally conductive pastes. One of the solutions for these requirements is to introduce high packing density (HPD) filler technology. HPD fillers are made by combination of mono-dispersed 5 to 10m silver powder/flake or silver plated copper and dispersed fine silver powder. Relative tap density(RTD) of HPD fillers are 70 to 79%. Electrical resistivity of solvent type thermoset pastes made with a HPD filler of 78% RTD, which was composed of fine silver powder and silver flake, was around 6ʃ-cm. When a thermoplastic binder was used and dried at 110 for 45 minutes the volume resistivity was 18 ʃ-cm. When the drying temperature is increased to 180 , the volume resistivity decreased to 9ʃ-cm. If a finer silver powder is used, resistivity is further improved by 40%. When shape -transformed silver-plated copper and silver powder/flake was used to make HPD filler it was surprisingly found to have anti-silver migration properties. Pastes made with the HPD filler composed of shape transformed silver plated copper had such good anti-silver migration properties that it is now being used for PC boards incorporated in automobiles. The silver to copper ratio of this filler was around 50:50. However, if the silver plated copper powder has not been shape-transformed, the level of anti-silver migration property is not as good as pure silver HPD fillers. (The shape-transform process is proprietary technology.) Thermal conductivity of HPD fillers is remarkable. Measured by laser flash method, 68 to 108 W/m-K was obtained with solvent type thermoset silver pastes.
Hideji Kuwajima, Chief Engineer
NIkka Techno Service Co., Ltd.
Hitachinaka, Ibaraki 312-0003,
Japan


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