EP&T (Electronic Products and Technology) magazine has a terrific little article on thermal interface material (TIM) and how to choose the right one. They’ve got some excellent slides as well that can help you systematically choose what you need. Well worth reading.
You can find the article and resources here at EP&T Magazine: “Higher frequencies leading to rising power and EMI in traditionally low power electronic devices”
ATS has produced a white paper that makes a good companion to the EP&T Magazine Article. You can get your own copy by clicking to, “When to Use a Thermal Interface Material and What Type“.