What is Dissipation Factor (Loss Tangent) in PCBs?

The Dissipation Factor (Df), also known as the loss tangent (tan(δ)), is a measurement of signal attenuation as it travels down the transmission line. This attenuation, also referred to as dielectric loss, is caused by electromagnetic waves being absorbed by the dielectric material. The dielectric loss increases proportionally with frequency. 

In an alternating electric field, the hysteresis effect of dielectric conductivity and polarisation causes a phase difference between the current and voltage phasors flowing in the dielectric, resulting in the formation of a phase angle. Df represents the phase angle's tangent value. Mathematically, the Dissipation factor is the reciprocal of the ratio of the capacitive reactance to the resistance of an insulating material at a specific frequency.

The dissipation factor(Df) of a PCB board material shows how a particular material will handle complex signal routing while transporting RF/microwave signals, high-speed digital signals, and a combination of different technologies as part of a mixed-signal circuit design. Df measures a material's propensity to absorb some of the AC energy from an electromagnetic (EM) field that is passing through it. Choosing a PCB material with a low Df value usually ensures that high-frequency circuits fabricated on that material will have minimal losses. 

It should be noted that when sorting PCB materials by their Df values; temperature effects such as the heat generated by active components and integrated circuits (ICs), may have an impact and lead to a rise in Df at high temperatures. This behavior explains why the Df of a circuit material is so closely related to its thermal conductivity. Because active devices generate heat in their vicinity, even the best PCB and cabinet thermal designs will struggle to dissipate all of the heat produced by an active device. The effective Df of a PCB material may be slightly higher at elevated environmental temperatures and power levels for a given circuit design. The effects of temperature on Df should be taken into account in any circuit simulations for any CAE (Computer-aided engineering) circuit models that use PCB Df as an input to obtain a more precise estimate of dielectric loss for a material over the operating temperature range.

The dissipation factor is a crucial parameter to consider in RF circuits and high-speed digital circuits where loss and signal integrity act as important factors. The Df of most PCB materials ranges from 0.02 for most commonly used materials to 0.001 for very low-loss high-end materials.