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38 The PCB Design Magazine • June 2014 The Nickel Doesn't Make Cents! PCB surface finishes vary in type, price, availability, shelf life, assembly process and reli- ability. While each treatment has its own merits, electroless nickel immersion gold (ENIG) finish has traditionally been the best fine pitch (flat) surface and lead-free option for SMT boards over recent years. But unfortunately, nickel is a poor conductor with only one third the con- ductivity of copper. Also, nickel has a ferromag- netic property that can adversely affect electro- magnetic fields in the high-frequency domain. The PCB industry has addressed the issue of the ferromagnetic properties of nickel by in- troducing a nickel/gold (NiAu) alloy. Gold is slightly less conductive than copper, and has no ferromagnetic properties, so it has relatively little impact on the conductor's loss characteris- tics at high frequencies. Microstrip (outer) layers of a multilayer PCB suffer from wide variations in both trace width and thickness. This is due to the additional fab- rication process of electroplating the through- holes. Copper barrel thickness is generally spec- ified as a minimum of 1 mil (25.4 µm), and so extra copper plating is applied to the surface in order to produce the correct barrel wall thick- ness. This, unfortunately, is also added to the traces. But as the thickness and width varies, so does the impedance. This is one of the reasons why routing controlled impedance signals, on the microstrip layers, should be avoided. It is also very important not to pour copper fills on the signal layers of the board, as these will dramatically change the impedance of the traces rendering the impedance control ineffec- tive. A surface finish can be defined as a coating, either metallic or organic in nature, which is applied to a PCB in order to assure solderability of the metal underneath after a certain time in storage and under different environmental con- ditions. The surface finish protects the copper mounting pads from corrosion in order to en- sure good soldering. There are several choices of protective coatings: • Lead-free hot air solder leveling (HASL) • Organic solderability preservative (OSP) such as Entec 106 or Shikoku • Electroless nickel immersion gold (ENIG) • Electroplated gold over nickel (NiAu) • Immersion silver (IAg) • Immersion tin (ISn) Electroplated NiAu can be applied as a full body finish, prior to solder mask application. This is because for that finish, the NiAu layer is typically used as a resist to etch the outer lay- ers. Therefore, nickel ends up covering the en- tire length of the outer layer traces. But as men- tioned, nickel is a ferromagnetic material and is not desirable from a loss perspective. Alternatively, ENIG can be applied as a full body finish, or the more common (and recom- mended) way as a selective finish, because it is BEYoND DESIgN column by Barry olney in-CirCuiT DeSign PTY lTD auSTralia Surface Finishes for High-Speed PCBs

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