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16 SMT Magazine • April 2014 trochemical corrosion mechanisms and gaseous corrosion. Some important failure mechanisms on outer surfaces of the PCBA are discussed lat- er, while the conductive anodic filament (CAF) failure mode usually found inside the laminate is not discussed. Gas phase corrosion Various gases such as sulphur, ozone, ammo- nia, etc., can cause corrosion of parts on a PCBA, depending on the material content. A detailed discussion is beyond the scope of this article; only a generalized discussion of corrosion of sil- ver due to sulphuric gases is described. The presence of low levels of sulphur gases can create severe corrosion problems on Ag and Cu parts. Moisture from the humidity can assist gaseous corrosion due to the dissolution of gases into the water layer generating various acid spe- cies. Therefore, at ambient temperatures, it is difficult to distinguish between chemical corro- sion due to sulphur gases and the electrochemi- cal corrosion. Figure 3 shows corrosion of silver parts on a PCBA due to sulphur-containing gases from a pig farm environment. Formation of sil- ver sulphide dendrites can be seen all over the surface [6] . Usually sulphur corrosion on PCBAs is found with sulphur gas levels below 50ppb. Anodic corrosion and Electrochemical migration Electrochemical migration is a typical form of corrosion found on electronic systems. Elec- CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues trochemical migration is caused by the pres- ence of a potential gradient between two closely spaced conductors on a PCBA (terminals of a ca- pacitor, resistor, or legs of an IC, for example), connected by a thin layer of water due to hu- midity. Under such conditions, metal ions dis- solve into the solution layer from the positive electrode (anode) which, due to the effect of high electric fields, migrate towards the oppo- sitely charged negative electrode (cathode), de- positing there in the form of a dendrite depend- ing on the local chemistry of the solution. Therefore, whether a specific metal ion de- posits on the cathode or not depends on the stability of the ion in the aqueous solution and local chemistries generated between the elec- trodes at high potential levels. As a result, only a few metals such as Sn, Pb, Cu, Ag, Au, etc., are susceptible to electrochemical migration, while other metals just precipitate as hydroxides or other compounds as corrosion products. A typical example of a non-migrating metal used in electronics is aluminium. In humid envi - ronments with chlorides, aluminium dissolves and forms hydroxide (or hydroxy chlorides) instead of migrating to the cathode regions. On the other hand, metals like Cu, Ag, Sn, Pb, etc., migrate upon dissolution and deposit at the cathode at least over a range of potentials and pH under which a particular ion is stable as predicted by the Pourbaix diagram [7] . A de- tailed discussion on the mechanism of ECM is beyond the scope of this article, but can be fEATurE Figure 3: Sulphur corrosion on silver parts on a PcBA (a) and formation of silver dendrites (b).