Source: Polar Instruments Application Note AP171
"Recently a number of customers approached us because impedance controlled tracks displayed higher losses than expected. This has been evident as the impedance test trace has sloped upwards across the tested area. While on fine line traces it is quite normal to see a gently sloping trace, the level of slope (and hence loss) was much higher than predicted — so much so that Polar decided to investigate further. Eventually we discovered that the entire length of the trace had been nickel plated in the cases we investigated. While nickel is acceptable on short lengths of pad to accommodate gold plating, plating the whole trace length is generally not a good idea. This application note explains the effect that nickel will have on high frequency transmission lines."
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