Insulated metal substrate (IMS) materials are becoming accepted in automotive power electronics assemblies. Power electronics surface mount technology assemblies use IMS materials to provide electrical isolation and thermal management. There are differences of opinion regarding the test method used to evaluate the thermal conductivity of the electrical isolation used in IMS's. Techniques have been developed to extend usefulness of test methods included in ASTM D 5470-95 to electrical isolation materials that are thin, have high glass transition temperature and high modulus.. The data generated meet the needs of finite element thermal analysis modeling.