Praveen,
Skin effect is the tendency of High speed currents to flow more on the "skin" on the wire and lesser and lesser deep inside it. This is due to increase of resistance of the wire at higher frequencies.
Skin Effect depends on lot of factors such as frequency of operation, resistivity of the material (yes- this is constant at 4.458 E0–9 for Copper), medium of operation and surface finishing of the copper etch (rougher the finishing, more is the resistance of copper). Skin effect is proportional to the Square root of Frequency. And another point to be noted is- what is this frequency - no it’s not the signal frequency as many have misunderstood. It's the switching frequency of the signal. Even a slow 5MHz signal with a Rise/Fall time of 10% of its pulse width (20 u Sec) will switch at 50MHz !!, so classifying signals that are only above 80MHz as high speed signals is not appropriate IMO.
Experts please add to this..
Best Regards,
Subbu