Microstrip Impedance (IPC-2141):

$$ Z_o = \frac{87}{\sqrt{E_r + 1.41}} \ln\left(\frac{5.98 \times H}{0.8 \times W + T}\right) $$

$$ Z_{diff} = 2 \times Z_o \times \left(1 - 0.48 \times \exp\left(-0.96 \times \frac{S}{H}\right)\right) $$

Note: These equations are first-order approximations. Field solvers provide superior accuracy for actual manufacturing.