Experimental Analysis of the Water Absorption Effects on RF/mm-Wave Active/Passive Circuits Packaged in Multilayer Organic Substrates

The feasibility of using near-hermetic organic materials for microwave and millimeter-wave packaging is investigated. The effects of moisture on both passive and active components on/inside the nonhermetic materials are tested. Microstrip patch antennas on three low moisture absorption substrates (0.1% or less) are subjected to immersion tests until saturated and the corresponding measured frequency shifts are found to be less than 0.25% for two of the three materials tested. The recovery time from saturation to normal operation is found to occur from natural evaporation at room temperature in 10 min or less. One of the three materials, a promising organic laminate material called liquid crystal polymer (LCP), showed no measurable change in weight or in antenna resonance frequency between the ambient (air) state and that after extended submersion in water. In addition, a novel laminated LCP package for a 13-25 GHz monolithic microwave integrated circuit (MMIC) formed from seven thin layers of laser-machined LCP is subjected to immersion testing. The seal for the embedded chip cavity (formed by the LCP conforming around the feeding transmission lines) is found to be robust for protecting the embedded chip. Standard hermetic materials are suggested to be a potentially unnecessary requirement for reliable, long-lifetime, high-performance RF systems.