The value is influenced by several factors. Humidity and elevated temperature will reduce the voltage breakdown because absorbed water will degrade the electrical properties of the material. The size of the test electrode will affect the observed breakdown voltage. A larger test electrode will typically yield a lower breakdown voltage. The presence of partial discharge, as well as mechanical stresses imposed on the interface material, also reduce voltage breakdown.
Volume resistivity is a measure of the bulk electrical resistance of a unit cube of a material.
When determined per ASTM D257, volume resistivity can give an indication of how well an interface material can limit leakage current between an active component and its grounded metal heat sink. As with voltage breakdown, volume resistivity can be significantly lowered by humidity and elevated temperature. Interface materials exhibit properties typical of highly filled elastomers, namely compression deflection, compression set and stress relaxation. Unlike foams, a solid elastomer can not be compressed under normal compressive loads. As a compressive load is applied, the material is deformed but the volume of the material remains constant.