A 1oz copper foil thickness (1.37mil or 34.8um) is generally sufficient and recommended for most applications.

For certain applications, more copper can improve the Q factor of coils, especially those operating at lower frequencies. However, since it is not possible to manufacture a thicker copper foil with the same shape, the number of turns is reduced in terms of space, and the reduced number of turns reduces the inductance value.

Reducing the copper allows the use of smaller coils operating at higher frequencies (above 3MHz), but reduces the Q factor.