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24 The PCB Design Magazine • April 2015 ent frequencies will exhibit different skin depths (δ). With a given conductor prop- erty the skin-depth decreases exponen- tially with the increase of frequency. The inclusion of Ni at 4–7 µm en- sures 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 great - ly disturbed. The skin effect is one of the reasons why the electronics industry needs to en- tertain the notion of nickel-free surface finishes. ENIG Thousands of words have already been writ- ten about the workhorse, ENIG, and many pan- els have been processed successfully using it. This is an established process that fulfils the re- quirements of many existing and past applica- tions but falls short of the requirements prom- ised in the future. As such this process will not feature further in this article unless to make a comparison. ENEPIG with Thin Ni Although in terms of words and produc- tion, the above is equally true for the "the all- purpose solution" (ENEPIG) process, this is the most promising of the Ni inclusive processes. In addition to performing well generically there a frequent question is raised within the industry: Why can't this process fulfil future re- quirements by employing a thin Ni layer? Ni is employed as a barrier layer due to its dense crystal formations. The surface morphol- ogy, in turn is created by thicker Ni plating. This is the background for the IPC (4552 – ENIG and 4556 – ENEPIG), minimum thickness require- ment of 3µm. Higher Ni thicknesses positively influence the integrity of solder joints. By reducing the Ni layer to conform to dimensions dictated by EPAG, dramatic quality issues are encountered. Early indications of this can be found by ex- amining the intermetallic compound (IMC) by SEM. Whilst the IMC created using EPAG has no detrimental impact to the solder joint, the IMC created using EN (low thickness) EPIG dis- plays evidence of demarcation lines. Demarcation lines are cosmetically alarm- ing and create concerns for the end-user. This is not an acceptable situation. The impact of demarcation lines, in terms of quality, can be demonstrated using a drop test. The drop test is a simulation of a handheld device being dropped. According to the drop test, the performance of the EPAG system is superior to that of the thin Ni ENEPIG process. In addition to the poor drop test performance, the thin Ni layer becomes su- perfluous as a protection layer due to the low thickness and resultant poor morphology. Silver Wire Bonding Wire bonding is a field that is evolving like any other. Bonding speed, wire hardness and cost are all in the mix. Some 3D packaging philosophies mean that wire bonding is here to stay unless it is replaced completely by flip- chip derived bonding technology. Reduction in cost without yield and capacity loss is a ma - THE FUTURE oF NICkEL IN NICkEL/PALLADIUM/GoLD FINAL FINISHES continues feature Figure 5: how the Ni morphology is influenced by the layer thickness.