What is the conductivity of common metals used in RF circuits and how does it affect loss?
Metal Conductivity and Its Impact on RF Circuit Loss
Conductor loss in microstrip and stripline circuits scales with the square root of the conductor's resistivity (or inversely with the square root of its conductivity). At microwave frequencies, the skin effect confines the current to a surface layer only a few skin depths thick, making the surface metallurgy the primary factor controlling conductor loss.
Frequently Asked Questions
Should I use ENIG or immersion gold surface finish for RF traces?
ENIG (Electroless Nickel Immersion Gold) adds a nickel layer between copper and gold. Nickel is ferromagnetic with much lower conductivity than gold or copper, increasing loss significantly at microwave frequencies. For RF traces, use immersion gold directly on copper, or hard gold plating without an intermediate nickel layer.
Does aluminum work well as an RF conductor?
Aluminum is about 37% less conductive than copper and forms a native oxide layer. In CMOS RFICs, aluminum or copper metallization is used because semiconductor processing is optimized for these metals. For discrete RF circuits, aluminum is rarely used because copper and gold offer better conductivity and more mature fabrication processes.
How does temperature affect metal conductivity at RF?
Metal resistivity increases approximately linearly with temperature (0.4%/°C for copper). This means a circuit operating at 125°C has about 40% higher conductor loss than at 25°C. This temperature effect is often overlooked but can be significant in high-power amplifiers where conductor temperatures are substantially elevated.