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76 PCB007 MAGAZINE I MAY 2020 Immersion gold deposition is a displacement reaction where one atom of nickel metal is oxidized to the nickel ion giving up two elec- trons. The two electrons are picked up by two positively charged gold ions in solution and, in turn, are reduced to the metal and deposited on the surface of the nickel substrate. The driving force of the oxidation-reduction displacement reactions can be derived from the electromotive series. The electromotive series is a listing of chemical species (atoms, molecules, and ions) in the order of their ten- dency to gain or lose electrons (be reduced or oxidized, respectively), expressed in volts and measured with reference to the hydrogen elec- trode—which is taken as a standard and arbi- trarily assigned the voltage of 1.0 v. The driving force for the Ni/Au exchange is 3.4 times greater than that of the Pd/Au ex- change. If the immersion gold ion has access to the underlying nickel, this would be the path of least resistance for the reduction of the gold. The gold will be reduced at the expense of the nickel, allowing nickel to be oxidized or cor- roded and gold to be deposited on the top pal- ladium layer. Investigation and failure mecha- nism analysis showed that the immersion gold, under certain conditions (compromised nickel deposit coupled with extended dwell time in an aggressive immersion gold bath), could get access to the underlying nickel. The IPC Amended ENEPIG Specification 4556-A specifies the immersion gold thick- ness at 1.2–2.8 µins (0.03–0.07 µm). Although data shows that this thickness of gold is ad- equate for gold wire bonding, thicker gold (> 3.0 µins, 0.075 µm) is often spelled out in the board design for the purpose of opening the wire bonding window. There are three approaches to mitigate nick- el corrosion in ENEPIG deposits. The first approach is to control the electroless nickel deposition process in order to produce an even deposit with minimum crevices, to in- crease the phosphorous content of the deposit by one or two percentage points, and not to attempt to deposit thicker gold in an immer- sion gold bath. The second approach is to deposit a thicker (6–8 µins, 0.15–0.20 µm) electroless palladium layer that could cover any imperfections in the nickel deposit and act as a barrier between the immersion gold and the underlying nickel. This is a relatively costly proposition as the price of an ounce of palladium now exceeds the price of an ounce of gold. The third approach is to replace the immer- sion gold (IG) with reduction-assisted immer- sion gold (RAIG). RAIG is a mixed reaction bath. Both immersion and autocatalytic reac- tions start simultaneously. As the substrate be- comes less accessible, the immersion reaction will diminish, and the autocatalytic reaction will dominate. RAIG is non-aggressive and will not produce substrate corrosion and is capable of depositing 4–6 µins, (0.10–0.15 µm) of gold in a single step. The choice of which mitigation approach is best is left to the manufacturer and their sup- plier to determine availability and that the process choice fits in the production floor and is within the manufacturer's budgetary con- straints. It is worth mentioning here that the RAIG solution is gaining popularity and is be- coming the choice for ENEPIG, particularly if thicker gold is specified in the design of the board. PCB007 George Milad is the national accounts manager for technology at Uyemura. To read past columns or contact Milad, click here.

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