Secure Attachment in Pulsed Power Applications

In applying high power attenuators and terminations to RF and Microwave circuits there are a number of requirements to be observed to obtain satisfactory life. These are heat-generating devices so the removal of excess heat is key to maintaining a safe film temperature. This heat removal is often accomplished by mounting the resistive element (chip) to a heat sink, which is then in turn mounted to a cold plate. Overheating of the resistive film is to be avoided as it can cause a resistance shift. This shift will interfere with the carefully designed RF impedance match. For the highest performance in the smallest size we want to load the chip to the near maximum film temperature and use the highest conductivity heat sink to redistribute and remove the heat. Materials properties work against here as shown in table 1. Ideally we would use an aluminum heat sink as it is the high thermal conductivity with low density and thus low weight. Unfortunately the corresponding Coefficient of Thermal Expansion (CTE) for aluminum is quite high.