If someone asks us to design a high speed board, one of the consideration we take is to select a low loss dielectric. The reason to select a low loss dielectric is to reduce the losses in the channel when the signal travels along. The below graph shows the signal loss over frequency with FR4 and low loss dielectrics.
Due to the losses in the dielectric, the signal rise and fall times get affected there by causing timing related issues. We need to understand that it is the loss tangent parameter of the dielectric that is to be considered here. The loss tangent represented by tan δ quantifies energy loss within a dielectric material when subjected to a varying electric field, indicating how "lossy" the material is. It's also known as the dissipation factor.
While this is part of dielectric which we need to understand, if a requirement comes or there is unavailability of low loss dielectric constant and we have to go with FR4 only, then we need to have various considerations in place to avoid losses.
- Thicker traces have lower losses. This meant using 2 oz or more thickness helps. Note that this is a costly approach
- The copper foil used on the dielectric can have various roughness which determines the resistivity of the trace. A less rougher copper trace will have lesser resistivity and hence lesser loss. So, we can give this as a fabrication input.
- Even the surface finish roughness behaves in the similar way and this can be controlled as well to minimize losses
- Stack up has to be designed such that trace to reference plane distance can be reduced (which needs trace width increase to maintain impedance). This ensures lower signal transition in the dielectric and less loss
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