Steve,
I guess in all the years I have never seen a semiconductor mounted via a conductive pad when connection to the thermal surface is required or desired. Usually the device is directly mounted to the cooling surface especially if a low resistance path is required. There have been some applications I have dealt with where the entire heat conducting surface (heat sink) is insulated above the chassis when the attached semiconductor is above ground potential and mounted without any insulating pad. This is usually done when other devices are attached to the same heat sink and need to ¡°talk¡± to each other electrically.
Greg
