Discrepancy in Microstrip Impedance Calculation: 2.08mil/4.06mil/5.4mil/3.84 Dielectric

A discrepancy was reported in microstrip impedance calculations using an online calculator with parameters: trace thickness 2.08 mil, substrate height 4.06 mil, trace width 5.4 mil, and dielectric constant 3.84, yielding 36.3 ohms instead of the expected ~50.6 ohms. Multiple online calculators and references, including Mantaro, Saturn, and others, consistently calculate around 50.6 ohms for similar stackups. The discrepancy was attributed to possible use of incorrect calculators or simplified approximation formulas. The EEWeb calculator is based on IPC-2141A standard formulas, which are approximations and may differ from high-end simulation tools like Polar’s impedance solver. Discussions highlighted that embedded microstrip impedance calculations can be lower than expected due to additional copper layers increasing capacitance and thus lowering impedance, consistent with transmission line theory (Z0 = sqrt(L/C)). However, some EEWeb embedded microstrip calculators showed physically inconsistent results compared to embedded stripline calculators, suggesting potential errors in the formulas or implementation. The IPC-2141A standard and its errata were referenced as sources of the formulas used, but differences remain between calculators. Users noted that these calculators provide approximations and recommended caution, especially since PCB fabrication results sometimes align better with alternative calculators like Mantaro. Overall, the consensus is that microstrip impedance calculators vary in accuracy due to differing formula implementations, assumptions, and approximations, and high-fidelity simulation or measurement is advised for critical designs.
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