Revisiting the Effect of Nickel Characteristics on High-Speed Interconnect Performance

It has been observed that some surface finish like nickel-gold plating has exhibited significantly higher losses than expected on high-speed interconnects like printed circuit board (PCB) trace and packaging. The investigation has revealed that this extra loss results from the ferromagnetic characteristics of nickel material with large resistivity and nonunit frequency-dependent relative permeability. As a result, a high-speed PCB design must take into account the board surface finish effect, solder-mask coating, and the physical geometry deformation of copper related to the etching process when modeling the PCB trace, which can be accomplished by using 3-D electromagnetic field simulation tools. Simulation results are correlated with vector network analyzer (VNA) measurements on several test cases and also on the actual application boards. In our experimental studies on PCB test structures with electroless nickel/immersion-gold (ENIG) and electroplated nickel-gold plating, there is no observation of resonating anomalies in the insertion loss and group delay related to nickel permeability. Both simulation and measurement results have demonstrated that PCB board with ENIG and solder mask over bare copper finish has less signal loss than that with ENIG or hard-gold finish.