When a diode is encapsulated in a potting material, the generated heat in the diode must be dissipated through the material to an outside surface or heat sink. The thermal conductivity of the potting material used can be very critical. The thermal conductivities of silicon potting materials are typically lower than those of rigid epoxies, although there are other factors that may make the use of a silicon potting material more desirable.
Other material, such as glass or alumina, can be added to the potting material to increase its thermal conductivity. Thermal conductivities for various materials are given in Table 1.
Along with the thermal conductivity of the potting material used, the mechanical configuration or layout of the diode in the package will have an effect on thermal impedance. The formula used for thermal impedance is:
Stated another way, the thermal impedance from diode to outside surface = length of the thermal path divided by the thermal area.
In many practical cases, the area or length of a thermal path may be difficult to determine exactly. Also, in some cases there are several thermal paths that must be considered in parallel. The above formula should be used to arrive at a close approximation of the thermal impedance of the package, followed by an actual test of the junction temperature of the diodes in the package.
As the above formula indicates, thermal impedance is inversely proportional to the thermal area, so that thermal impedance and junction temperature can be reduced by increasing the thermal area. The thermal area can be increased by adding metal heat dissipators to the leads of the diodes. Also, the shorter the distance between the diode and the heat sink or outside surface of the package, the lower the thermal impedance. In high voltage applications, the minimum distance between the diode and outside surfaces will depend on the voltage stress of the package and on the dielectric strength of the potting material used.
Surface Mount Diode