The ATLAS upgrade detector foreseen at the phase 2 upgrade of LHC requires a complete new inner detector using silicon pixel and strip detectors. For both technologies, a specific mechanical and thermal design is required. Such a design may use soft thermal interfaces such as grease between the various parts. One foreseeable use would be between the cooling pipe and the thermal block allowing the strip modules to be decoupled from the mechanical and cooling structure. This note describes the technique used and the results obtained when characterizing a few grease samples. The results have been compared with thermal FEA simulations. A thermal conductivity measurement for each sample could be extracted from the measurements, with a systematic uncertainty of less than 6%. Some samples were irradiated to the expected fluence at sLHC and their resulting thermal conductivity compared with the non-irradiated samples
