Signal Attenuation
Power dissipation occurring as an electromagnetic wave propagates through a dielectric medium and along conducting traces governs the signal strength in high-speed circuits. This transmission loss consists of both dielectric loss from the substrate resin and conductor loss from the copper traces. As signal frequencies increase, the attenuation rises, reducing the voltage margins at the receiver.
Measuring and minimizing this loss is a primary task for designers of high-speed digital boards. Without careful control of this parameter, signals traveling across backplanes will degrade below the detection threshold of the receiver IC.
Conductor Roughness
Trace geometry and copper surface treatments determine the resistive portion of signal attenuation. At gigahertz frequencies, skin effect forces current to flow on the outer surface of the trace, where copper roughness increases the path length. This surface profile increases the transmission loss by forcing the signal to travel along the irregularities of the copper-dielectric boundary.
Designers specify very-low-profile copper foils to mitigate this conductor loss.
Substrate Selection
High-frequency laminate materials are chosen for their ability to minimize signal attenuation. Laminates with low dissipation factors significantly reduce the transmission loss of microstrip and stripline structures. This reduction allows for longer routing lengths and preserves the eye diagram width at the receiver.
Testing trace patterns on test coupons verifies that the fabricated board meets the specified loss budget.
