Improvements In Microwave Laminates For Power Amplifier Reliability And Efficiency

By George Qinghua Kang, Michael T. Smith, John C. Frankosky, Arlon Materials for Electronics

Abstract
Demands for higher data rates and capacity have continued to drive RF and Microwave electronics continue toward higher frequency and higher power requirements. These power and frequency demands have increased the heat burden on power amplifiers and related electronics while competing requirements have pushed to reduce device size and weight while exposing electronics to greater environmental conditions. The resulting combination has been a decrease in overall efficiency resulting in higher operating temperatures and a resulting decline in reliability. Most RF system engineers highlight the "Arrhenius Equation," in which a 10°C increase in operating temperature doubles the failure rate for a typical component. In other words, the ability to get heat away from components, reduce or eliminate hot-spots, reduce overall device operating temperature will increase product life. Innovations focused on increasing thermal conductivity and temperature stability of the dielectric while maintaining low loss are being introduced in the industry. A benefit to these advances can result in greater phase stability, which is critical to impedance network transformers utilized for matching networks of power amplifiers. For power amplifiers, the shift in dielectric properties with temperature increases reflections and directly reduces efficiency. For antenna designs, a significant shift in resonance frequency and bandwidth roll off at specific frequencies, results in lower gain performance. The resulting combination of new materials with better heat transfer and better thermal stability of the dielectric results in devices that operate more efficiently and more reliably over time. Applications and test data have shown the benefit of increased board thermal conductivity on reducing the maximum case temperature of the RF power amplifier FET transistors, as demonstrated by the hot spot thermal images of experimental boards with different thermal conductivity properties. TDR (time-domain reflectometer) tests have also shown that the temperature stability of dielectric constant in RF/Microwave laminates provides greater stability of electrical phase or electrical length in high frequency circuit elements that phase shifts greatly affect the performance, such as the impedance matching networks in power amplifiers.