The engineer is going to define a Thermal Dielectric Pad or Thermal Interface Material (TIM). Thermal Dielectric Pad Materials are typically thin and hard, yet flexible. Thermally conductive pads are specifically designed to be electrically isolating. These types of thermal pads are typically used on Power Transistors that have an electrically energized base that will short out if directly in contact with a metal heat sink. The differences in Dielectric Pads comes primarily in the form of thermal performance, isolation performance, price points and whether or not the material is available in a high volume format, such as kiss cut on a roll.