Line impedance on two side PCB?

[Astrid]

For example PCB FR4, with 1.5mm on 915MHz microstrip line with Zo=50 Ohm will have a with 2.8mm. Power 500mW.
How will it work in reality when using SMD 0402?
It is quite unrealistic to associate such a small part as a thick line.
Actually you can use a line width of 0.5 mm, which will have an impedance of 108 Ohm.
Length of this micro strip is tenths mm, my longest micro strip have 5mm.
I know the solution is 4 layer PCB, but it is expensive, what will be the real impact of such unmatched impedance PCB?

 

[BigBoss]

Why ?? 0402 package will be soldered to the edges of the Microstriplines.
There isn't any issue with it..

 

[vfone]

The best option when use smaller SMD components with thicker microstrip lines is to use tapering technique applied to reduce the discontinuity effects associated with a step width junction. Especially at microwave frequency this would make a big difference.
A discontinuity in a microstrip (which is caused by an abrupt change in geometry of the strip conductor), can change the electric field distribution and rise its capacitance, and also it will change the magnetic field distribution and changing the inductance.