Low Cycle Fatigue in RF Microwave Module Housings

Hermetic housings are required for the vast majority of electronics used in RF microwave electronics to protect the gallium arsenide and silicon devices from the environment. A common housing style includes a metal ring frame that is brazed onto a metal or ceramic base. The housing is populated with electronic devices and circuits and then hermetically sealed with a thin metal lid. For high volume manufacturing, lids are often attached by a resistance weld using a seam seal process. The interior hermitic volume is sealed at or near one atmosphere internal pressure. Since the housing may be subjected to a substantial number of pressure cycles that can yield the lid material, a low cycle fatigue evaluation is required to establish the long-term reliability of the hermetic housing. Kovar is a common lid and housing material because of a good coefficient of thermal expansion match with glass, ceramic and other materials used in RF circuitry. Unfortunately only high cycle fatigue data is available for kovar. A method is proposed to generate an estimate of high cycle end of the low cycle fatigue response from the high cycle fatigue data. The proposed method is verified by comparing a predicted fatigue life with experimental results from hermetic housings subjected to pressure cycling.