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February 2015 • The PCB Magazine 47 THE FUTURE OF NICKEL IN NICKEL/PALLADIUM/GOLD FINAL FINISHES continues called "skin effect." Initially proposed by Horace Lamb in 1883 for spherical con- ductors, it was then applied to conductors of any shape by Oliver Heaviside in 1885 (Wikipedia). In laymen's terms, the theory promul- gates that induced electro- magnetic fields (eddy cur- rents) caused by alternating current will orientate the highest current density flow to the outside of the con- ductor. This is referred to as the skin effect. In turn, dif- ferent conductor properties (electrical and magnetic) in conjunction with different frequencies will exhibit dif- ferent skin depths (δ). With a given conductor property the skin-depth decreases exponentially with the in- crease of frequency. The inclusion of Ni at 4–7 µm ensures that the primary signal will pass through the outer Ni skin. Not only is nickel a poorer conductor than copper, the effective area for the signal to pass through is greatly reduced. This is rather like placing your thumb over the end of a hose pipe, the resultant water flow is reduced and the directional flow is greatly disturbed. The skin effect is one of the reasons why the electronics industry needs to entertain the notion of nickel-free surface finishes. feature Figure 3: the potential for spreading when using a ni inclusive finish. Figure 4: the impact of frequency on skin depth (δ) with reference to process application.