This paper describes the characterization of a thermally conductive, electrically insulating aromatic silicone film adhesive used in a flexible heat sink assembly that is part of an optical telescope aboard the planned New Horizons mission to Pluto and the Kuiper Belt. This application requires high thermal conductivity, high electrical resistance, low-temperature flexibility, resistance to creep, and very low outgassing. Post-cure conditioning in thermal vacuum was shown to drastically reduce the total mass loss (TML) and collected volatile condensable material (CVCM) in standard ASTM outgassing tests. Thermal vacuum treatment also affected activation energy and diffusion rate of the adhesive as determined by multi-rate and isothermal thermogravimetric analysis. Dynamic mechanical testing was performed to compare stiffness of the adhesive under representative loads at cryogenic temperatures to its stiffness at ambient temperature. The material was shown to remain relatively flexible at the minimum operating temperature for this mission. Test data also indicate the adhesive is resistant to creep at sub-ambient temperatures.