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20 SMT Magazine • April 2014 as dreadful for connectors and switches. For connectors and switches, the corrosion prod- ucts formed by the galvanic corrosion induce high resistance in the circuit. Usually the con- nectors are made of multilayer metallic coatings as described earlier. The metallic layers have dis- tinctly different electrochemical properties. A typical example is the ENIG parts on the PCB. Figure 6 depicts a mobile phone keypad made by the ENIG process, with corrosion due to galvanic coupling between layers. As shown in Figure 6d, the immersion gold (IM Au) layer is porous, exposing part of the EL Ni layer. The large difference in electrochemical potential be- tween EL Ni and IM Au causes corrosion of EL Ni (Figure 6c), while the Au layer acts as power- ful cathode. As the corrosion proceeds, pitting of EL Ni layer exposes Cu at deep pit areas. Even in the absence of porosity on the top coating, the gap between metallic component and resist edge can be a point where all the metallic lay- ers get exposed to the solution. Palladium has been introduced as a substitute for gold, but Pd has faster cathodic reaction kinetics than gold. Therefore, use of Pd can enhance the galvanic corrosion problems. Metals in the various PCB finishes (Cu, Sn, Ag, Ni, Au, and Pb) have large difference in the electrochemical potential for galvanic corrosion (Figure 6e). The bonding process for ICs uses different metals, which can generate galvanic corrosion problems if moisture ingress occurs due to any damage in the encapsulation material. Often a gold/aluminium bonding is used for IC chips and the combination is seldom safe from cor- rosion because of the large difference in elec- trochemical potentials. The lead frame is also coated with solderable coatings, which are gen- erally nobler than the base material. Therefore, any mechanical defects in the coating can cause corrosion problems. Fretting corrosion The most reliable material for electronic switches is gold. However, gold is expensive, so from time to time the electronic indus- try has been trying to use tin-coated copper switches. The tin-coated copper surface is sol- derable as well. The most important problem with electronic switches and contacts is the relative motion between the contacting parts due to small amplitude vibrations. The vibra- tory motion can be relatively low (10–200 mi- cron) and caused by the floor vibration, the thermal stresses, etc. The relative movement of contacting parts due to the vibration destroys the oxide film on the contacting surface. The new surface generated reacts with the environ- CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues fEATurE Figure 5: Schematic of the anodic and cathodic corrosion process.