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February 2015 • The PCB Magazine 29 This is highlighted further through Figure 5, where the average thickness and standard de- viation of the intermetallic layer has been mea- sured from the microscope images. The HASL in- termetallic layer grows rapidly at approximately 2 µm/week, with considerable height variation due to the formation of the larger nodules. In contrast, the thickness of the HASLEN in- termetallic layer has barely changed, showing very little alteration in the composition through the coating. This has very exciting implications to the properties of a HASLEN PCB compared to one that is HASL coated: 1) The PCBs should in theory have an almost infinite shelf life as it would take an extremely long time for any intermetallic growth to make its way through to the surface of the pad. 2) As intermetallic growth is very slow then any resulting solder joint should be less suscep- tible to brittle fracture causing a more reliable electronic after manufacture. This is particular- ly true where HASL PCBs are used in high tem- perature situations. 3) The electroless Ni layer should help to mitigate tin whisker formation. Because the intermetallic layer is thin and uniform then stresses within the coating should be lower reducing the likelihood that tin whiskers will form. The results above demonstrate the im- proved behaviour of PCBs with an elec- troless nickel barrier layer versus conven- tional HASL, or in fact any other surface finish which consists of a thin layer of a protective metal directly onto copper such as immersion tin, immersion silver and OSP as once soldered the nature of the solder join should be similar for all of these surface protection methods. This additional benefit of having an electroless nickel barrier layer should be consistent for HASLEN, ENIG and any other protec- tive surface finish in which such a barrier layer is present. ENIG and Black Pad Due to the presence of gold in the sur- face finish, ENIG presents considerable cost implications and often is only used where absolutely necessary or for examples where high reliability is key. However, ENIG also pos- sesses its own challenges and reliability prob- lems, the most common of which is referred to as black pad. The ENIG surface finish is a combination of two processes to provide a coating system with two distinct layers. First, a Ni layer is depos- ited followed by a thin immersion gold layer to prevent passivation of the underlying nickel substrate. With an immersion process, a metal on a surface is exchanged for one which is more "noble" from solution. In this case, nickel on the surface is oxidised into solution with gold being reduced onto the surface. As such, this is an inherently corrosive process which, in ad- dition to the immersion gold bath being nec- essarily acidic to prevent the nickel layer from passivation, can lead to corrosion problems. An example of this is shown in Figure 6 where excessive reaction has occurred down an elec- troless nickel grain boundary. In areas where excessive reaction has occurred, the Ni surface becomes oxidised, meaning that it is rendered HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature Figure 6: Cross-section of an EnIG-coated PCB showing attack down electroless ni grain boundary during immersion gold deposition.