This study investigates the full-wave numerical electromagnetic analysis of edge-coupled differential microstrip lines on a printed circuit board (PCB), when conductor surface roughness is taken into account. Mixed mode parameters of such lines are analyzed. The imbalance in the electric lengths of the traces causes the appearance of common-mode currents, as well as mode conversion: differential mode (DM) to common-mode (CM) and CM to DM. The presence of the solder mask on top of the PCB with microstrip traces is also taken into account. The differential lines modeled in this work are of three types: straight, bent, and curved. The signal traces are modeled with rectangular cross-section, as well as with the more realistic trapezoidal. The effect of surface roughness on the mode conversion is studied numerically in this work.