As socketable LED modules are being developed the need for thermal characterization of heat-sinks aimed at cooling such modules has also arisen. The natural approach from manufacturers is to apply the component level LED thermal testing standards to characterize complete LED module and heat-sink systems. In order to obtain the real thermal resistance of such a complete assembly needs the consideration of the radiant power of the LEDs when calculating the thermal resistance of the complete assembly. Also, the contribution of the LED module, the thermal interface between the module and the heat-sink and the thermal resistance of the heat-sink need to be separated. For the latter problem thermal transient measurements followed by structure function analysis is a solution. To eliminate the need for the measurement of radiant flux of the LED module we suggest building a thermal dummy of the module in question which mechanically is compatible with the module but the heat source is a conventional silicon device instead of LEDs. The paper provides details how such a thermal dummy was created and used in characterizing different heat-sinks aimed at a particular LED module family.